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  • Articles  (25,731)
  • American Chemical Society  (23,030)
  • American Institute of Physics (AIP)  (2,701)
  • 1995-1999  (25,731)
  • 1990-1994
  • 1999  (25,731)
  • Chemistry and Pharmacology  (25,731)
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  • Articles  (25,731)
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  • 1995-1999  (25,731)
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  • 1
    Electronic Resource
    Electronic Resource
    Hard core Yukawa fluid with temperature and density dependent interaction: Phase diagram of the AOT/water/decane microemulsion (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2151-2156 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A fluid with an interaction potential consisting of a hard core and an attractive Yukawa tail is considered. The strength of the attraction is taken to depend both on density and temperature in order to take into account the state dependence of the effective interaction suggested by the analysis of structural data of several colloidal suspensions, in particular microemulsions of AOT reverse micelles. The thermodynamics of this fluid is investigated starting from the inverse temperature expansion of the free energy in the mean spherical approximation. The temperature and density dependence of the interaction is incorporated in consistent expressions for the pressure, isothermal compressibility, and chemical potential. The phase diagram predicted by this method is in agreement with experimental data on AOT reverse micelles analyzed in the effective one component approach. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479485
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  • 2
    Electronic Resource
    Electronic Resource
    Melting thermodynamics of organic nanocrystals (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2176-2180 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The size-dependent melting temperature and the size-dependent melting entropy of organic nanocrystals are predicted by use of our simple model being free of any adjustable parameter. The model predictions for the size-dependent melting temperature and the size-dependent melting entropy are supported by the experimental results on benzene, chlorobenzene, heptane, and naphthalene nanocrystals. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479489
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  • 3
    Electronic Resource
    Electronic Resource
    Oxygen adsorption on Ag(111): X-ray photoelectron spectroscopy (XPS), angular dependent x-ray photoelectron spectroscopy (ADXPS) and temperature-programmed desorption (TPD) studies (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2169-2175 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The O2 adsorption on Ag(111) single crystal in the range of 300–500 K has been studied by XPS, ADXPS, and TPD. At room temperature, the O1s line observed at 530.0 eV goes down to 528.2 eV when the sample is heated to 420 K. The comparison of XPS and TPD data indicates that both lines can be assigned to chemisorbed atomic oxygen. The depth concentration profiles calculated on the basis of the angular-dependent XPS data show that the line at 530.0 eV is responsible for oxygen adsorbed at the surface, as opposed to that at 528.2 eV which is assigned to oxygen between the top and second silver layers. The formation of surface silver oxide is concluded in the latter case. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479488
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  • 4
    Electronic Resource
    Electronic Resource
    Phase behavior of gradient copolymers (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2329-2339 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Phase properties of gradient AB copolymer melts which consist of chains with the specified chemical distribution of A and B monomers have been studied within the Landau–Ginzburg model. All the melts with the linear distribution of the monomers exhibit only a direct continuous phase transition from disordered to the lamellar phase. The hexagonal, body-centered-cubic, double-gyroid (G), and lamellar-ordered structures have been found in the melts with the monotonic but nonlinear distribution of the monomers. The G structure has been also found in the gradient copolymer melts with the distribution function of monomers similar to the A–B–A triblock copolymers. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479504
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  • 5
    Electronic Resource
    Electronic Resource
    The isotropic–nematic transition and the phase separation of the tobacco mosaic virus particles with polysaccharide (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2322-2328 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The isotropic–nematic transition of the tobacco mosaic virus (TMV) particles by polysaccharide is related to a high inhibitory activity against TMV infection. We study the process of the isotropic–nematic transition of the TMV particles as a function of the polysaccharide concentration by Monte Carlo simulations in three-dimensional continuous space. In these simulations, we simplify the TMV particles and the polysaccharide molecules as the hard spherocylinders and semirigid chains, respectively, and we assume the simple interactions for the TMV particles and the polysaccharide chains. In our simulation, with increasing concentration of the polysaccharide the homogeneously dispersed TMV particles begin to segregate without orientational ordering, that is isotropic phase separation, and then transform to the nematic state of the TMV particles. The isotropic–nematic transition is caused by simple interactions such as the excluded volume effect, and the complicated biological interaction is not necessary. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479503
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  • 6
    Electronic Resource
    Electronic Resource
    Simplified model for the analysis of interaction types in two-stranded coiled-coils (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2311-2321 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate here the adequate balance between hydrophobic and electrostatic interactions in the correct folding of two-stranded, parallel coiled-coils. We have used a very reduced model of rigid straight helices, simulated by means of a Monte Carlo algorithm. The amino acid side chains are reduced to the level of β-carbons, which interact through simple Lennard-Jones-type potentials. The model is able to reproduce the association of only two chains to form dimers, when peptides with a highly regular sequence are studied. For a given value of the parameter controlling the strength of interactions between hydrophobic residues, the electrostatic interactions between charged residues are systematically analyzed. We conclude that the optimum contribution of electrostatic interactions to the global stability of the dimer is about 20%. Weaker interactions provide a substantial population of missfolded antiparallel dimers, while larger electrostatic interactions create associations between dimers. The optimized model has been tested with other simplified sequences, providing results consistent with experimental evidence. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479502
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  • 7
    Electronic Resource
    Electronic Resource
    Association of diphilic chains near the solvent critical region (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2340-2344 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By means of computer simulation the association process of diphilic macromolecules with strongly attractive head units (stickers) in the vicinity of the critical point of the solvent is studied. The calculations were carried out by means of Monte Carlo simulation with the bond-fluctuation model of polymer modified by us to mimic the hexagonal packing of monomer units. It was found that the critical concentration of micelle formation and average aggregation number depend crucially not only on the attraction between stickers but on the quality and state of solvent. The average aggregation number can demonstrate nonmonotonic behavior with change in temperature. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479505
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  • 8
    Electronic Resource
    Electronic Resource
    Erratum: "Resonance Raman intensity analysis of a dicyanovinyl-azaadamantane: Mode-specific reorganization energies for charge-transfer and locally-excited states" [J. Chem. Phys. 109, 10958 (1998)] (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2345-2346 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479509
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  • 9
    Electronic Resource
    Electronic Resource
    Erratum: "Ab initio relativistic effective potentials with spin-orbit operators. VII. Am through element 118" [J. Chem. Phys. 106, 5133 (1997)] (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2347-2347 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479506
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  • 10
    Electronic Resource
    Electronic Resource
    Physical origin of the high reactivity of subsurface hydrogen in catalytic hydrogenation (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1343-1345 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In the catalytic hydrogenation of hydrocarbons, subsurface hydrogen is known experimentally to be much more reactive than surface hydrogen. We use density functional theory to identify low-energy pathways for the hydrogenation of methyl adsorbed on Ni(111) by surface and subsurface hydrogen. The metastability of subsurface hydrogen with respect to chemisorbed hydrogen is mainly responsible for the low activation barrier for subsurface reactions. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479392
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  • 11
    Electronic Resource
    Electronic Resource
    Construction of interpolated potential energy surfaces using constrained dynamics: Application to rotational inelastic scattering (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1346-1353 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An established method for constructing molecular potential energy surfaces by interpolation of ab initio data has been coupled with a new approach to sampling molecular configurations which uses constrained classical dynamics. To illustrate this approach, model surfaces for the scattering of two rigid diatomic molecules are derived and shown to accurately predict rotational inelastic scattering cross sections. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479393
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  • 12
    Electronic Resource
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    One-dimensional arrays of oscillators: Energy localization in thermal equilibrium (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1373-1384 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: All systems in thermal equilibrium exhibit a spatially variable energy landscape due to thermal fluctuations. Thus at any instant there is naturally a thermodynamically driven localization of energy in parts of the system relative to other parts of the system. The specific characteristics of the spatial landscape such as, for example, the energy variance, depend on the thermodynamic properties of the system and vary from one system to another. The temporal persistence of a given energy landscape, that is, the way in which energy fluctuations (high or low) decay toward the thermal mean, depends on the dynamical features of the system. We discuss the spatial and temporal characteristics of spontaneous energy localization in 1D anharmonic chains in thermal equilibrium. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479396
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  • 13
    Electronic Resource
    Electronic Resource
    Resonant and antiresonant transport through a fluctuating cage (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1385-1393 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We consider an idealized model of diffusion in a cage. The boundary of the cage can be occupied by various correlated obstacles which fluctuate in time according to a dynamic of a Glauber-type. A particle enters the cage and travels through it according to a given stochastic process. Its exit may be blocked by the presence of the obstacles at the surface of the cage. We show that the probability of transmission through the cage can have a maximum and a minimum as a function of the frequency of fluctuation of the obstacles, provided a certain simple condition on the dynamics inside the cage is satisfied. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479397
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  • 14
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    Anions made of cations and dianions: [CsC9]− and [CsC7]− (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1467-1476 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recently, C92− dianions have been observed as products in the collision of 48 keV [CsC9]− ions with hydrogen or helium. Motivated by this observation we examined the structure of [CsC9]− anions in a detailed theoretical study. The geometries of the finally detected C92− dianions should be closely related to the intermediately produced [CsC9]− anions. We found stable isomers of [CsC9]− which possess "lightly embracing" structures reflecting the geometrical characteristics of the theoretically prognosticated stable C92− dianions. The examination of the smaller [CsC7]− anions confirmed these results: Lightly embracing isomers of [CsC7]− have been found to form stable anions. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479405
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  • 15
    Electronic Resource
    Electronic Resource
    Mechanism for vibrational relaxation in water investigated by femtosecond infrared spectroscopy (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1494-1500 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a study on the relaxation of the O–H stretch vibration in a dilute HDO:D2O solution using femtosecond mid-infrared pump-probe spectroscopy. We performed one-color experiments in which the 0→1 vibrational transition is probed at different frequencies, and two-color experiments in which the 1→2 transition is probed. In the one-color experiments, it is observed that the relaxation is faster at the blue side than at the center of the absorption band. Furthermore, it is observed that the vibrational relaxation time T1 shows an anomalous temperature dependence and increases from 0.74±0.01 ps at 298 K to 0.90±0.02 ps at 363 K. These results indicate that the O–H(centered ellipsis)O hydrogen bond forms the dominant accepting mode in the vibrational relaxation of the O–H stretch vibration. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479408
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  • 16
    Electronic Resource
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    High resolution molecular beam study of the origin band of the B˜ 2Σ+–X˜ 2Σ+ system of yttrium imide (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1483-1493 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The (0,0,0)–(0,0,0) band of the B˜ 2Σ+–X˜ 2Σ+ system of three isotopomers of yttrium imide (Y14NH, Y15NH, and Y14ND) has been studied by laser-induced fluorescence in a molecular beam apparatus. Rotational, fine, and nuclear magnetic hyperfine structures have been resolved and analyzed. The B˜ 2Σ+(0,0,0) state of Y14NH, Y14ND, and Y15NH is severely perturbed below J=30.5 by eight, three, and two vibronic states, respectively. Although, the nature of these perturbing states can only be speculated upon, their symmetries are either 2Σ or 2Π, and this has made it possible to deperturb the B˜ 2Σ+ state successfully. The spectra can be reproduced within 140 MHz (0.0047 cm−1). The analyses confirm that the molecule is linear in both states with the nuclear arrangement Y–N–H. The bond lengths in the ground X˜ 2Σ+ state and the B˜ 2Σ+ state have been derived to be rY–N=1.877 57(13) Å, rN–H=1.0067(10) Å, and rY–N=1.8839(43) Å, rN–H=1.242(30) Å, respectively. The results are compared with the values of ab initio calculations on YNH and YN, and the experimental data on YN and YO. The atomic character of the unpaired electron in the ground state is 58% Y + 5s and 42% Y + 5p. The electron configurations for the ground X˜ 2Σ+ state and the B˜ 2Σ+ state are discussed and compared with ab initio calculations whenever possible. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479407
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  • 17
    Electronic Resource
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    Numerical prediction of the melting curve of n-octane (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1501-1510 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We compute the melting curve of n-octane using Molecular Dynamics simulations with a realistic all-atom molecular model. Thermodynamic integration methods are used to calculate the free energy of the system in both the crystalline solid and isotropic liquid phases. The Gibbs–Duhem integration procedure is used to calculate the melting curve, starting with an initial point obtained from the free energy calculations. The calculations yield quantitatively accurate results: in the pressure range of 0–100 MPa, the calculated melting curve deviates by only 3 K from the experimental curve. This deviation falls just within the range of uncertainty of the calculations. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479409
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  • 18
    Electronic Resource
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    Pulse sequence symmetries in the nuclear magnetic resonance of spinning solids: Application to heteronuclear decoupling (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1511-1519 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We develop the average Hamiltonian theory of a class of symmetrical radio-frequency pulse sequences in the NMR of rotating solids. Theorems are presented which allow one to predict the elimination of many average Hamiltonian terms, without detailed calculation. These results are applied to the problem of heteronuclear decoupling in the presence of rapid magic angle spinning. We present sequences which minimize the number of heteronuclear terms at the same time as recoupling the homonuclear interactions of the irradiated spins. The performance of the new sequences is tested on 13C labeled calcium formate. Experimental measurements of double-quantum 1H excitation indicate a relationship between good heteronuclear decoupling of the observed spin species and efficient recoupling of the irradiated spin species. The heteronuclear decoupling performance of the new sequences is significantly better than that obtained with an unmodulated radio-frequency field. The decoupling performance is improved further by breaking the pulse sequence symmetry in a controlled fashion. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479410
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  • 19
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    Three-body dispersion contributions to the thermodynamic properties and effective pair interactions in liquid argon (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1520-1526 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The contributions of three-body triple dipole and dipole-dipole-quadrupole dispersion interactions to the thermodynamic properties of liquid argon are examined, using a recently introduced simulation scheme which contains an explicit, quantum mechanical representation of the underlying electronic structure [Mol. Phys. 94, 417 (1998)]. The experimental pressure and energy at a series of liquid densities are shown to be quite accurately reproduced by a combination of the best available pair potential (Aziz) plus these three-body terms. The extent to which these many-body effects can be encompassed by an effective pair potential is then discussed. The nonuniqueness of such an effective potential is reiterated. It is shown that in the dense liquid, the three-body contribution to the effective pair potential (φ(r)) varies approximately linearly with density and is almost temperature independent. It is shown how the addition of φ(r) to the Aziz pair potential moves the latter toward the widely used Lennard-Jones (12-6) potential. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479390
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  • 20
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    Damped oscillatory motion and internal friction in bulk polyatomic gases (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1527-1532 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A modified version of Rüchardt's experiment is used to measure the bulk moduli and relaxation times for damped oscillations in five gases with low γ=Cp/Cv (CHClF2, CHCl2F, C2Cl2F4, C2F6, C4F8). The results support the theoretical prediction that the internal friction is small for gases with γ(approximate)1 (and not too large a thermal diffusivity). Two types of damped oscillation are observed: (i) Initial oscillations which are intermediate between the adiabatic and isothermal extremes. These oscillations occur if the initial amplitude exceeds a threshold value which increases with the volume of gas. (ii) The ensuing oscillations in the tail. For the latter, the measured relaxation times are consistent with isothermal values. However, the bulk moduli are appreciably lower than the corresponding isothermal values, and the disparity increases as the atomicity of the gas increases. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479411
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  • 21
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    Molecular simulation of the phase behavior of noble gases using accurate two-body and three-body intermolecular potentials (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1533-1540 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Gibbs ensemble Monte Carlo simulations are reported for the vapor–liquid phase coexistence of argon, krypton, and xenon. The calculations employ accurate two-body potentials in addition to contributions from three-body dispersion interactions resulting from third-order triple-dipole, dipole–dipole–quadrupole, dipole–quadrupole–quadrupole, quadrupole–quadrupole–quadrupole, and fourth-order triple-dipole terms. It is shown that vapor–liquid equilibria are affected substantially by three-body interactions. The addition of three-body interactions results in good overall agreement of theory with experimental data. In particular, the subcritical liquid-phase densities are predicted accurately. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479412
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  • 22
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    Dielectric behavior of ferroelectric liquid crystals in the vicinity of the transition into the hexatic phase (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1541-1550 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Collective molecular dynamics was studied in ferroelectric smectic C* and hexatic smectic I* phases by the dielectric method. In the frequency range 1 Hz to 10 MHz one relaxation process was observed in both phases. In the SmC* phase the relaxation frequency of this process linearly decreases and its dielectric strength increases when approaching the SmC*–SmI* phase transition temperature TI. In the SmI* phase the relaxation frequency is one order of magnitude lower and further slightly decreases on cooling. A phenomenological theory has been developed which describes static and dynamic dielectric properties near TI and satisfactorily explains the experimental results. The observed relaxation has been attributed to fluctuations of the bond orientational order coupled to the molecular tilt. A parameter describing the softening of this mode is determined by fitting theoretical results to the experimental data. The model also describes an observed anomalous increase in the spontaneous polarization as a result of the change in the strength of bond orientational order when passing to the SmI* phase. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479413
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  • 23
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    Thermodynamic properties of sulfurhexafluoride in the critical region (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1551-1560 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An equation, that incorporates a crossover from singular thermodynamic behavior asymptotically close to the critical point to regular thermodynamic behavior far away from the critical point, is presented for calculating the thermodynamic properties of sulfurhexafluoride in the critical region. The equation is constructed by applying a crossover transformation to a truncated classical Landau expansion. The equation is capable of representing the thermodynamic properties of sulfurhexafluoride at temperatures from 310 to 390 K in an appreciable range of densities around the critical density. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479414
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  • 24
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    Hyper-Rayleigh scattering of centrosymmetric molecules in solution (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1561-1568 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study theoretically the harmonic light scattering (HLS) of centrosymmetric molecules in solution. Since HLS is inherently absent for centrosymmetric molecules, the intensity and line shape are obtained by taking into account two distinct physical processes: solute vibrational transition and solvent effective field contribution. The intensity is expressed in each case as the Fourier transform of the relevant time correlation functions. The implications for experiments, which use HLS to determine the first hyperpolarizability β of optically interesting molecules, are discussed. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479415
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    Perturbation theory approach to tunneling: Direct and resonance transmission in super-exchange models (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1569-1579 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we examine, within simple models, different approaches to computing tunneling probabilities in super-exchange models of electron transfer. The relationship between tunneling calculations that use scattering theory type formalisms and approaches based on standing waves, which are more closely related to electron transfer between bound donor and acceptor states, is established. Transmission probabilities computed by using truncated basis representations are compared to exact analytical or numerical results for one- and two-dimensional models. We find that while resonance tunneling is well approximated by truncated basis approaches, computing deep tunneling using such basis sets can lead to large errors. Implications for calculations of bridge assisted electron transfer are discussed. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479416
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    Ab initio molecular-dynamics simulation of K+ solvation in water (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1587-1591 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Potassium ion in water plays a very important role in chemistry and biology. In this paper, we study the solvation of this important ion using ab initio Car–Parrinello molecular dynamics. We work within the pseudopotential, density-functional approach and use the BLYP (Becke–Lee–Yang– Parr) generalized gradient approximation to the exchange and correlation potential. An analysis of the structural properties of the solvation shell shows good agreement with existing experiments, as well as with previous simulations based on classical potentials. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479418
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    On the frequency dependence of ultrasonically induced birefringence in isotropic phase of liquid crystal: 5CB (p-n-pentyl p′-cyanobiphenyl) (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1580-1586 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical expressions for the ultrasonically induced birefringence of liquids are obtained in the frame work of de Gennes' phenomenological theory. The intensity and frequency dependence of ultrasonically induced birefringence in the isotropic phase of p-n-pentyl p′-cyanobiphenyl (5CB) was measured in order to examine the usefulness of birefringence measurements for investigating dynamical properties liquids. The observed birefringence was proportional to the square root of ultrasonic intensity. The birefringence divided by the square root of ultrasonic intensity increases with increasing frequency and appears to saturate when the ultrasonic frequency approaches the relaxation frequency of molecular reorientation. The observed values of birefringence were reproduced satisfactorily by the expression derived in this paper. © 1999 American Institute of Physics.
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    Stochastic bi-resonance without external signal in the CO+O2 catalytic oxidation reaction system (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1592-1594 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The noisy dynamic behavior of a surface catalytic reaction model to describe the oxidation of carbon monoxide is investigated when the control parameter is perturbed by external noise near a supercritical Hopf bifurcation point. Noise induced coherent oscillation (NICO) is observed and the NICO strength goes through two maxima with the increment of the noise intensity D from zero, characteristic of the occurrence of stochastic multiresonance without external signal. The frequency of the NICO also increases with the increment of D. © 1999 American Institute of Physics.
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    Photodissociation dynamics of the propargyl radical (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3441-3448 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photochemistry and photodissociation dynamics of the propargyl radical, C3H3, upon UV excitation is investigated by time- and frequency-resolved detection of hydrogen atoms. From a statistical analysis of the data, we conclude that formation of cyclopropenylidene, c-C3H2, is the dominant reaction channel. Around 22% of the excess energy is released into the translational degrees of freedom. By varying the excitation energy between 265 and 240 nm, microcanonical rates for the loss of a hydrogen atom can be obtained as a function of excess energy. The experimental rates, on the order of several 106 s−1, are in good agreement with Rice–Ramsperger–Kassel–Marcus (RRKM) calculations, provided a scaling factor is used for the vibrational frequencies, to account for the effects of anharmonicity. The interpretation is confirmed in experiments using monodeuterated propargyl radicals, H2CCCD, indicating a mechanism that proceeds via an initial [1,2] H-shift, followed by cyclization. © 1999 American Institute of Physics.
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    Fourier transform emission spectroscopy and ab initio calculations on OsN (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3449-3456 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The emission spectrum of OsN has been recorded in the 3000–13 000 cm−1 region using a Fourier transform spectrometer. OsN molecules were excited in an osmium hollow cathode lamp operated with neon gas and a trace of nitrogen. Six bands observed in the 8000–12 200 cm−1 region have been classified into three transitions, a 4Π5/2–X 2Δ5/2, b 4Φ7/2–X 2Δ5/2, and b 4Φ5/2–X 2Δ5/2 with the 0–0 band origins near 8381.7, 11 147.9, and 12 127.2 cm−1, respectively. A rotational analysis of these bands provides the following equilibrium constants for the ground electronic state: ωe=1147.9492(77) cm−1, ωexe=5.4603(36) cm−1, Be=0.493 381(55) cm−1, αe=0.002 753(38) cm−1, and re=1.618 023(91) Å. Ab initio calculations have been performed on OsN and the spectroscopic properties of the low-lying electronic states have been calculated. Our assignments are supported by these calculations. The ground state of OsN has been identified as a 2Δi state consistent with the observations for the isoelectronic IrC molecule [Jansson et al., Chem. Phys. Lett. 4, 188 (1969); J. Mol. Spectrosc. 36, 248 (1970)]. The 1σ22σ21π41δ33σ2 electron configuration has been proposed for the ground state and the configurations for the other low-lying electronic states have also been discussed. This work represents the first experimental or theoretical investigation of the electronic spectra of OsN. © 1999 American Institute of Physics.
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    Inner shell excitation spectroscopy of transient molecules: HBS, HBO, and H3B3O3 (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3468-3478 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Inner shell electron energy spectroscopy (ISEELS) was used to study HBS, HBO, and H3B3O3, reactive, transient species generated in situ. The reaction of H2S with crystalline boron in a quartz tube was used to produce thioborine (HBS) at ∼1100 °C, and borine (HBO) at ∼1200 °C. The reaction of H2O vapor with crystalline boron in a quartz tube at ∼1200 °C was used to produce boroxine (H3B3O3). These species were identified from their inner shell excitation spectra and mass spectrometry. The B 1s, S 2s, and S 2p ISEEL spectra of HBS, and the B 1s and O 1s spectra of HBO and H3B3O3 are reported and analyzed with the help of GSCF3 ab initio calculations. A reaction scheme is proposed for the generation of HBO from the reaction of H2S and boron in a heated quartz tube. © 1999 American Institute of Physics.
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    Molecular constants and Rydberg–Klein–Rees (RKR) potential curve for the Na2 1 3Σg− state (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3494-3497 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Transitions into the doubly excited Na2 1 3Σg− state have been analyzed using near-dissociation expansions (NDE) to represent the vibrational energies and inertial rotational constants, while the centrifugal distortion constants were held fixed at "mechanically consistent" values calculated from the Rydberg–Klein–Rees (RKR) potential implied by those G(v) and Bv functions. The input data cover the range v=0 to 57 and N up to 47, and the fit yields vD=61.41(±0.10) and D0=3385.70(±0.2) cm−1. © 1999 American Institute of Physics.
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    Homogeneous nucleation rates of n-pentanol measured in an upward thermal diffusion cloud chamber (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3623-3629 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Homogeneous nucleation rates of n-pentanol as functions of both supersaturation and temperature were measured in two different upward thermal diffusion cloud chambers, by research groups in Prague and Baltimore. The measurements were made at temperatures between 280 K and 320 K. The nucleation rates obtained are compared to the rates measured by Luijten et al. [J. Chem. Phys. 106, 4152 (1997)], by Hrubý et al. [J. Chem. Phys. 104, 5181 (1996)], and by Strey et al. [J. Chem. Phys. 84, 2325 (1986)]. Fair agreement between our data and the data obtained by the other authors also was found. © 1999 American Institute of Physics.
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    A perturbation theory for solvation thermodynamics: Dipolar–quadrupolar liquids (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3630-3638 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The thermodynamics of solvation of a dipole in hard sphere solvents with dipoles and quadrupoles is studied by using the Padé approximation for the perturbation expansion of the solvation chemical potential and compared to Monte Carlo simulations. Solvation chemical potentials, energies, and entropies of solvation are obtained at different dipolar and quadrupolar solvent strengths. The effect of nonlinear solvation is analyzed and found not to exceed 10% in the parameter range studied. An agreement between the simulations and the analytical theory is obtained by an empirical rescaling of the triple perturbation integrals of the perturbation expansion. This rescaling does not, however, provide a quantitatively correct partitioning of the solvation free energy into the energy and entropy of solvation. © 1999 American Institute of Physics.
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    Synergy of entropy and intermolecular coupling in supercooling liquids (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3639-3643 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We show the Tg-scaled temperature dependence of the minimum number of molecules capable of undergoing a rearrangement, z*, in the Adam and Gibbs model of relaxation of glass-formers is strikingly similar to that of n and m=[d log τα/d(Tg/T)]. Here (1−n) and τα are, respectively, the exponent and the effective relaxation time in the Kohlrausch correlation function, exp[−(t/τα)1−n], of the primary α-relaxation, z* is obtained from the excess (configurational) entropy, Sc, of the Kauzmann paradox and Tg is the glass temperature. As functions of Tg/T, z*, n and m all assume their minimal values at high temperatures. On decreasing temperature they all increase monotonically with a more rapid change in the vicinity of some temperature TB above Tg. Moreover, from the data of a number of small molecule glass-formers in which the high temperature limit of Sc can be determined accurately, we find that at the glass temperature, Tg, z*(Tg) obtained from thermodynamic data correlates with the steepness index m=[d log τα/d(Tg/T)]T=Tg and the Kohlrausch exponents (1−n(Tg)). The similarity of the temperature dependencies of n, m, and z* makes plausible the explanation that the temperature dependences of the kinetic quantities, n and m, originate from that of z*, which is a pure thermodynamics quantity.© 1999 American Institute of Physics.
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    Photomanipulation of external molecular modes: A time-dependent self-consistent-field approach (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 4113-4120 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We introduce a time-dependent self-consistent-field approach for studying the coupled rotational–translational motion of molecules subject to the macroscopic potential induced by a spatially inhomogeneous, polarized laser field. A hybrid classical-quantum-mechanical variation of the scheme is employed to investigate the possibility of simultaneously focussing the center-of-mass motion of molecules and aligning their figure axis in a field-free region of space. A semiclassical-quantum-mechanical variation is used to describe the rotational–translational dynamics of cold molecules confined to an optical trap.
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    Competing reaction pathways from Y+C2H2 collisions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 4101-4112 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The crossed molecular beams method with 193 and 157 nm photoionization detection was used to study the competing reaction pathways resulting from collisions of ground state Y atoms with acetylene (C2H2). Three channels, corresponding to nonreactive decay of collision complexes, H2 elimination, and H atom elimination, were studied as a function of collision energy (〈Ecoll〉=6–25 kcal/mol). Production of YC2+H2 and decay of long-lived complexes back to reactants were observed at all collision energies studied. Product translational energy distributions for the H2 elimination channel demonstrate that a substantial fraction of excess energy available to the YC2+H2 products is channeled into relative translational energy. Analogous H2 elimination channels were studied in reactions of Zr and Nb with C2H2 at 〈Ecoll〉=6.0 kcal/mol. For these reactions, the H2 elimination product translational energy distributions were found to peak near zero kinetic energy, in contrast to the behavior observed for the YC2+H2 products. This suggests that a significant potential energy barrier exists in the exit channel of the YC2+H2 elimination step, whereas no exit channel barrier exists in forming ZrC2+H2 and NbC2+H2. The reformation of Y + C2H2 reactants following decay of long-lived collision complexes was found to transfer 40%–50% of the initial relative translational energy into C2H2 internal excitation. The YC2H+H product channel was only observed to occur above a collision energy threshold of 21.5±2.0 kcal/mol. Since YC2H+H production is fully spin-allowed and involves simple Y–H bond fission in the intermediate HYC2H complex, it is unlikely that any significant potential energy barrier is present in excess of the reaction endoergicity. Additional studies of Y+C2D2 reactions confirm that the observed collision energy threshold for the H or D atom loss channel corresponds to the energetic threshold for reaction, allowing determination of D0(Y–CCH)=110.2±2.0 kcal/mol. © 1999 American Institute of Physics.
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    Discrete dipole approximation for ultraviolet–visible extinction spectra simulation of silver and gold colloids (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1195-1208 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In order to understand more deeply the surface enhanced Raman scattering (SERS) effect, this article develops a model, based upon the simulation of the UV–visible extinction spectra which allows the determination of the morphology of metallic particles in silver and gold colloids either unaggregated or in any aggregation state. The main assumptions of this model are (i) light scattering by independent particles or clusters (objects) which enables us to express the total extinction cross section as a suitably weighted sum of cross sections of individual objects (targets); (ii) these targets are supposed to be compact and their individual cross sections are determined either from the Mie theory for spheres or from the discrete dipole approximation (DDA) for objects of any shape and size; (iii) the weight of each individual cross section is determined using a minimization process (simplex method) which looks for the best possible agreement between the experimental and calculated spectrum; (iv) lastly a simple calculation, based on the assumption of compact objects, provides the absorbance (optical density). In the case of unaggregated silver and gold colloids, this model gives a very good agreement between experimental and simulated extinction spectra thus leading to a particle size histogram which is consistent to that determined from transmission electronic microscopy (TEM) measurements. For aggregated colloids, an excellent agreement is still obtained between experimental and simulated band profiles; a slight discrepancy is observed between experimental and calculated intensities which might result from the tendency of DDA to underestimate the individual cross sections and/or from a lack of validity of the compact approximation. The clusters histogram deduced from the simulation process reveals small spheres (unaggregated particles) and elongated objects small compared to the visible light wavelength. This latter result is different from that obtained by TEM data in which the likely occurrence of a further aggregation leads to the observation of large aggregates. © 1999 American Institute of Physics.
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    Comment on "Reversible work of formation of an embryo of a new phase within a uniform macroscopic mother phase" [J. Chem. Phys. 108, 5498 (1998)] (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3769-3770 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine recently obtained expressions for the reversible work required to form a noncritical nucleus in a metastable vapor. We demonstrate that the expression obtained in by Debenedetti and Reiss [J. Chem. Phys. 108, 5498 (1998)] is identical to that in by Nishioka and Kusaka [J. Chem. Phys. 96, 5370 (1992)], thereby resolving the apparent contradiction between these two papers. © 1999 American Institute of Physics.
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    Direct observation of nonadiabatic transitions in Na+rare-gas differential optical collisions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2853-2856 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the dynamics in optical collisions of Na with Ne, Ar, Kr, and Xe in a differential scattering experiment. We report the observation of nonadiabatic transitions in the excited collisional quasimolecule based on measurements of the population ratio of the Na(3p)2P1/2 and 2P3/2 fine-structure levels. Comparison with theoretical results shows a generally very good agreement over the range of collision energies (0.01–0.3 eV) scanned in our experiment, using the best available potentials. For the heavier rare-gas systems a strong influence of the BΣ–AΠ crossing on the population ratios is observed. We further extract a universal function for the nonadiabatic transition probability for these systems. In the thermal energy range, our results are in good qualitative agreement with data from gas phase optical collision experiments. © 1999 American Institute of Physics.
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    Crossed beam reaction of the cyano radical, CN(X 2Σ+), with methylacetylene, CH3CCH (X 1A1): Observation of cyanopropyne, CH3CCCN (X 1A1), and cyanoallene, H2CCCHCN (X 1A′) (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2857-2860 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The chemical dynamics to form cyanopropyne, CH3CCCN (X 1A1), and cyanoallene, H2CCCHCN (X 1A′), via the neutral–neutral reaction of the cyano radical, CN (X 2Σ+), with methylacetylene, CH3CCH (X 1A1), is investigated under single collision conditions in a crossed molecular beam experiment at a collision energy of 24.7 kJ mol−1. The laboratory angular distribution and time-of-flight spectra of the C4H3N products are recorded at m/e=65, 64, 63, and 62. The reaction of d3-methylacetylene, CD3CCH (X 1A1), with CN radicals yields reactive scattering signal at m/e=68 and m/e=67 demonstrating that two distinct H(D) atom loss channels are open. Forward-convolution fitting of the laboratory data reveal that the reaction dynamics are indirect and governed by an initial attack of the CN radical to the π electron density of the β carbon atom of the methylacetylene molecule to form a long lived CH3CCHCN collision complex. The latter decomposes via two channels, i.e., H atom loss from the CH3 group to yield cyanoallene, and H atom loss from the acetylenic carbon atom to form cyanopropyne. The explicit identification of the CN vs H exchange channel and two distinct product isomers cyanoallene and cyanopropyne strongly suggests the title reaction as a potential route to form these isomers in dark molecular clouds, the outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan. © 1999 American Institute of Physics.
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    Frequency-dependent polarizabilities, hyperpolarizabilities, and excitation energies from time-dependent density-functional theory based on the quasienergy derivative method (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2878-2888 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A time-dependent density-functional theory for systems in periodic external potentials in time is formulated on the assumption of the existence of the Floquet states from the quasienergy viewpoint. Coupling strength integration, which connects a noninteracting system with an interacting system, is introduced by using the time-dependent Hellmann–Feynman theorem. Coupled perturbed time-dependent Kohn–Sham equations are derived from the variational condition to the quasienergy functional with respect to parameters. Explicit expressions for frequency-dependent polarizability and first hyperpolarizability are given by the quasienergy derivative method. Excitation energies and transition moments are defined from poles and residues of frequency-dependent polarizabilities, respectively. In contrast to the previous theory, our formulation has the following three advantages: (1) The time-dependent exchange-correlation potential is defined by the functional derivative of the exchange-correlation quasienergy. (2) The formal expression for frequency-dependent polarizability, which corresponds to the exact sumover-states expression, can be obtained. (3) Explicit expressions for response properties which satisfy the 2n+1 rule can be automatically obtained. © 1999 American Institute of Physics.
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    Second- and third-order spin-orbit contributions to nuclear shielding tensors (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2900-2909 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present analytical calculations of the electronic spin–orbit interaction contribution to nuclear magnetic shielding tensors using linear and quadratic response theory. The effects of the Fermi contact and the spin-dipole interactions with both the one- and two-electron spin–orbit Hamiltonians, included as first-order perturbations, are studied for the H2X (X=O, S, Se, and Te), HX (X=F, Cl, Br, and I), and CH3X (X=F, Cl, Br, and I) systems using nonrelativistic multiconfiguration self-consistent field reference states. We also present the first correlated study of the spin–orbit-induced contributions to shielding tensors arising from the magnetic field dependence of the spin–orbit Hamiltonian. While the terms usually considered are formally calculated using third-order perturbation theory, the magnetic-field dependent spin-orbit Hamiltonian requires a second-order calculation only. For the hydrogen chalcogenides, we show that contributions often neglected in studies of spin–orbit effects on nuclear shieldings, the spin-dipole coupling mechanism and the coupling of the two-electron spin–orbit Hamiltonian to the Fermi-contact operator, are important for the spin–orbit effect on the heavy-atom shielding, adding up to about half the value of the one-electron spin–orbit interaction with the Fermi-contact contribution. Whereas the second-order spin-orbit-induced shieldings of light ligands are small, the effect is larger for the heavy nuclei themselves and of opposite sign compared to the third-order contribution. © 1999 American Institute of Physics.
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    Approaches to the approximate treatment of complex molecular systems by the multiconfiguration time-dependent Hartree method (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2927-2939 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A consistent treatment of environmental effects is proposed in the framework of the multiconfiguration time-dependent Hartree (MCTDH) method. The method is extended in view of treating complex molecular systems which require an exact quantum dynamics for a certain number of "primary" modes while an approximate dynamics is adequate for a class of "secondary" modes. The latter may correspond to the weakly coupled modes in a polyatomic molecule, or the first solvent shell in a solute-solvent complex. For these modes, a description in terms of parameterized functions is introduced. The MCTDH working equations are generalized to allow for the nonorthogonality of these functions, which may take, e.g., a multidimensional Gaussian form. The formalism is developed on the level of both the wave function description and the density matrix description. Dissipative effects are accounted for in terms of a stochastic Hamiltonian approach versus master equation approach in the respective descriptions. © 1999 American Institute of Physics.
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    Physical and mathematical content of coupled-cluster equations. IV. Impact of approximations to the cluster operator on the structure of solutions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2952-2959 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The impact of approximations to the form of the cluster operator on the structure and physical significance of the complete set of geometrically isolated solutions to the coupled-cluster (CC) equations has been studied for the first time. To systematically study the correspondence of solutions obtained at various levels of the approximation process, a continuation procedure based on a set of β-nested equations (β-NE) has been proposed and applied. Numerical studies based on a homotopy method for obtaining full solutions to sets of polynomial equations have been performed for the H4 and P4 models which belong to the simplest realistic many-electron model systems. Two examples of approximation procedures have been considered. The first one involved, for the P4 model, the approximation leading from the full CC (FCC) method to the CC method based on double excitations (CCD). As a result of this approximations the number of solutions has increased from 8 to 20. In the second example, for H4, we have studied the approximation leading from the CCSD method to the CCD one. To complete these studies, we have for the first time obtained the full set of geometrically isolated solutions for a CCSD equations which consists of 60 solutions. Only a small subset of this set might have some physical significance. During the approximation process considered, the number of solution decreases from 60 to 12. This radical drop of the numbers of solutions is a consequence of the absence of the third and fourth powers of the unknowns in the CCD equations. © 1999 American Institute of Physics.
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    Electronic states of the copper silicide and its ions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2973-2977 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Potential energy curves and spectroscopic parameters of the ground and exited states of SiCu, SiCu+, and SiCu− are presented. The calculations were performed by high-level correlated methods including the relativistic correction for the lowest states. The present results are compared with recent theoretical and experimental studies of SiCu and its ions and support the earlier theoretical conclusions concerning the assignment of the electronic ground state of SiCu. According to calculations presented in this paper the lowest energy states of SiCu, SiCu+, and SiCu−, are 2Πr, 1Σ+, and 3Σ−, respectively. © 1999 American Institute of Physics.
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    Heteronuclear rare-gas dimer bonding: Understanding the nature of the Rydberg states that dissociate to the highest energy level of the Xe*(5d) manifold (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2985-2990 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: (1+1′) resonance enhanced multiphoton ionization (REMPI) spectra of jet-cooled KrXe and ArXe in the vicinity of the high energy Xe*5d[3/2]10←Xe(1S0) atomic line at 83889.99 cm−1 were obtained by exciting the neutral dimers with tunable coherent vacuum ultraviolet (VUV) radiation generated by four-wave sum mixing in mercury vapor, and then detecting the resultant ions in a time-of-flight (TOF) mass spectrometer. Precise excited state constants were derived from analyses of the resultant vibrational fine structure, while equilibrium bond lengths were estimated from Franck–Condon factor intensity simulations. Excited state symmetries were deduced from separate ultraviolet (UV) (2+1) REMPI spectra recorded with linearly and circularly polarized light. The results of this work confirm a recent model proposed by Lipson and Field, where the RgXe*(5d) states are predicted to be strongly destabilized relative to RgXe*(6p) due to strong 5d-6p Xe* l-mixing induced by the ground state Rg atom partner making up the dimer. Orbital mixing is also responsible for the observation of appreciably strong RgXe*(5d) spectra in both one- and two-photon excitation. © 1999 American Institute of Physics.
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    Microwave spectrum and molecular structure of PNO (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3012-3017 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The microwave spectra of P14N16O and its isotopomers P15N16O and P14N18O were observed in a dc glow discharge plasma of a mixture of nitric oxide and hydrogen gases over solid red phosphorus placed on the stainless steel electrode. Rotational transitions of the parent P14N16O species were measured in the ground state as well as in the vibrationally excited ν1 (PN str.), ν2 (bend), and 2ν2 states. The l=0 substate of the 2ν2 state interacts with the ν1 state through a Fermi resonance. The rotational constants determined for the ground states of the three isotopomers yield the substitution structure, rs(PN)=151.6516(87) pm and rs(NO)=119.5025(80) pm. © 1999 American Institute of Physics.
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    Torsional motion and vibrational overtone spectroscopy of methanol (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3018-3026 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An internal coordinate Hamiltonian model has been constructed to model torsional motion in the OH stretching vibrational overtone region of methanol, CH3OH. The model includes harmonic couplings between OH and CH stretching vibrations and Fermi resonance interactions between OH stretches and COH bends and between CH stretches and CH2 bends. A symmetrized basis set has been used to form block diagonal Hamiltonian matrices with strong resonance couplings off-diagonal. Observed torsional levels of the excited vibrational states have been used as data in a least squares optimization of the model parameters, some of which have been estimated by ab initio calculations. The experimentally observed increase in the effective torsional barrier in moving to highly excited OH stretching states has been explained by the model. © 1999 American Institute of Physics.
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    Electron diffraction studies of metal phthalocyanines, MPc, where M=Sn, Mg, and Zn (reinvestigation) (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3035-3041 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure of two metal phthalocyanines, MPc where M=Sn and Mg, have been determined by gas-phase electron diffraction for the first time; furthermore, the structure of ZnPc established earlier by the same technique has been reanalyzed. The focus of these studies is the position of the metal ion relative to the 16-membered C8N8 ring. Tin(II)phthalocyanine we find to be nonplanar with the Sn(II) ion 1.0(1) Å above the molecular plane in accordance with its structure in crystal established earlier; the two other molecules were found to be planar. Slight nonplanarity of ZnPc found in the previous study is now removed by a new procedure of structural analysis; this is in accordance with the results of ab initio/DFT calculations for this molecule performed by Pulay. From comparisons presented here we can conclude that there is a close structural similarity of metal phthalocyanines in the gas phase and in crystal. © 1999 American Institute of Physics.
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    Effects of vibration on the polarizability and the first and second hyperpolarizabilities of HF, HCl, and HBr (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3042-3050 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report calculations, using electron uncorrelated and correlated wave functions, of the electronic and vibrational properties which pertain to certain nonlinear optical properties for HF, HCl, and HBr. Our main focus is on vibrational effects (zero-point-vibrational averaging and pure vibration). Analysis of the results obtained at various levels of approximation indicates that first-order perturbation theory is generally adequate for finding the zero-point-vibrational-averaging corrections for these molecules and that complete second-order perturbation theory nearly always gives reliable results for the pure vibrational corrections. Attention is drawn to some differences with previously published results for these properties. © 1999 American Institute of Physics.
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    On the strongly bound B 3Π state of the CAr van der Waals complex: Bonding and predissociation (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3070-3076 
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    ISSN: 1089-7690
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    Topics: Physics , Chemistry and Pharmacology
    Notes: For the open shell van der Waals molecule, CAr, the potential energy curves (PECs) for the B 3Π and 1 5Σ− states, the B 3Π–1 5Σ− spin–orbit coupling and the B 3Π fine structure splitting are determined using multireference configuration interaction wave functions as large as ∼8 million configuration state functions. The B 3Π state is strongly bound, with De=5100 cm−1. Re(B 3Π)=3.7a0 and is considerably shorter than Re(X 3Σ−)=6.07 a0. The PEC for the repulsive 1 5Σ− state crosses that of the B 3Π state at Rx(1 5Σ−,B 3Π)=3.31a0 leading to spin–orbit induced predissociation. The B 3Π–1 5Σ− spin–orbit coupling is the result of valence-Ryberg mixing in the B 3Π state and is considerably enhanced by the heavy atom effect. The heavy atom effect is also reflected in a marked decrease in the fine structure splitting of the B 3Π state with increasing vibrational level. The implications of these results for using CAr(B 3Π) in laser induced fluorescence detection of CAr(X 3Σ−,v) are discussed. © 1999 American Institute of Physics.
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    Rotational-resolved pulsed field ionization photoelectron study of NO+(X 1Σ+,v+=0–32) in the energy range of 9.24–16.80 eV (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3058-3069 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have obtained rotationally resolved pulsed filed ionization photoelectron (PFI-PE) spectra of NO in the energy range of 9.2–16.8 eV, covering ionization transitions of NO+(X 1Σ+,v+=0–32,J+)←NO(X 2Π3/2,1/2,v″=0,J″). The PFI-PE bands for NO+(X 1Σ+,v+=6–32) obtained here represent the first rotationally resolved spectroscopic data for these states. The simulation using the Buckingham–Orr–Sichel model provides accurate molecular constants for NO+(X 1Σ+,v+=0–32), including ionization energies, vibrational constants (ωe+=2 382.997±0.122 cm−1, ωe+χe+=17.437 84±0.000 90 cm−1, ωe+ye+=0.063 209 5±3.2×10−6 cm−1, and ωe+ze+=−0.001 400 0±7.2×10−8 cm−1), and rotational constants (Be+=1.996 608±0.006 259 cm−1, αe+=0.020 103±6.3×10−5 cm−1, and γe+=−(7.22±2.26)×10−6 cm−1). For v+=0–15, the rotational branches are ΔJ=J+−J″=±1/2, ±3/2, ±5/2, ±7/2, and ±9/2, which correspond to the formation of photoelectron angular momentum states l=0, 1, 2, and 3. The ΔJ=±1/2, ±3/2, ±5/2, ±7/2, ±9/2, and ±11/2 rotational branches are observed in the spectra for v+=16–32, revealing the production of continuum photoelectron states l=0, 1, 2, 3, and 4. The maximum ΔJ value and intensities for high ΔJ rotational branches are found to generally increase as v+ is increased in the range of 0–32. This observation is attributed to an increase in inelastic cross sections for collisions between the outgoing photoelectron and the nonspherical molecular ion core as the bond distance for NO+ is increased. Thus, this observation can be taken as strong support for the electron-molecular-ion-core scattering model for angular momentum and energy exchanges in the threshold photoionization of NO. © 1999 American Institute of Physics.
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    Fifth-order two-dimensional Raman spectra of CS2 are dominated by third-order cascades (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3105-3114 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have demonstrated that fifth-order stimulated Raman spectra of the intermolecular modes in CS2 are dominated by cascading third-order processes. Previous studies have successfully discriminated against a sequential cascading process, but did not account for parallel third-order cascades. All of our measured spectra were successfully simulated considering only cascades built directly from our measured third-order spectra. Using an appropriately chosen phase matching geometry we also measured the sequential cascade, which should exist with equal probability to the parallel cascade. When employing a phase matching geometry that provided substantial discrimination against all of the third-order cascades we were not able to measure any signal. We assign an upper limit for the true fifth-order signal of 2% of the cascaded signal. © 1999 American Institute of Physics.
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    Effects of excited state mixing on transient absorption spectra in dimers: Application to photosynthetic light-harvesting complex II (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3121-3132 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The excited state mixing effect is taken into account considering the difference spectra of dimers. Both the degenerate (homo) dimer as well as the nondegenerate (hetero) dimer are considered. Due to the higher excited state mixing with the two-exciton states in the homodimer, the excited state absorption (or the difference spectrum) can be strongly affected in comparison with the results obtained in the Heitler–London approximation. The difference spectrum of the heterodimer is influenced by two resonance effects (i) mixing of the ground state optical transitions of both monomers in the dimer and (ii) mixing of the excited state absorption of the excited monomer with the ground state optical transition in the nonexcited monomer. These effects have been tested by simulating the difference absorption spectra of the light-harvesting complex of photosystem II (LHC II) experimentally obtained with the 60 fs excitation pulses at zero delay times and various excitation wavelengths. The pairs of coupled chlorophylls a and b for simulations have been taken from the best LHC II assignment model obtained by simulating the steady-state spectra and the transient absorption at various excitation wavelengths. Qualitatively the spectral peculiarities of the difference spectra are explained by means of the resonance interpigment interactions, which are responsible for the excited state mixing. © 1999 American Institute of Physics.
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    Ewald summation for systems with slab geometry (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3155-3162 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We propose a modification in the three-dimensional Ewald summation technique for calculations of long-range Coulombic forces for systems with a slab geometry that are periodic in two dimensions and have a finite length in the third dimension. The proposed method adds a correction term to the standard Ewald summation formula. To test the current method, molecular dynamics simulations on water between Pt(111) walls have been carried out. For a more direct test, the calculation of the pair forces between two point charges has been also performed. An excellent agreement with the results from simulations using the rigorous two dimensional Ewald summation technique were obtained. We observed that a significant reduction in computing time can be achieved when the proposed modification is used. © 1999 American Institute of Physics.
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    The electronic spectrum, molecular structure, and oscillatory fluorescence decay of jet-cooled germylidene (H2C(Double Bond)74Ge), the simplest unsaturated germylene (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 950-958 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electronic spectrum of germylidene (H2C(Double Bond)Ge), the simplest unsaturated germylene, has been observed for the first time. Jet-cooled H2CGe and D2CGe were produced by an electric discharge through tetramethylgermane diluted in argon at the exit of a supersonic expansion. High-resolution spectra of H2C74Ge and D2C74Ge, obtained from (CH3)4〈sup ARRANGE="STAGGER"〉74Ge prepared from isotopically enriched 74Ge metal, have been rotationally analyzed to yield the following r0 structures: r0″(CGe)=1.7908(2) Å, r0″(CH)=1.1022(5) Å, θ0″(HCH)=115.05(5)°, r0′(CGe)=1.914(4) Å, r0′(CH)=1.082(9) Å, and θ0′(HCH)=139.3(11)°. The 367–354 nm B˜1B2–X˜ 1A1 band system consists of prominent perpendicular bands involving the CGe stretching (ν3) and CH2 scissors (ν2) vibrations and a weaker series of vibronically induced parallel bands involving the CH2 rocking mode (ν6). Vibronic bands involving Δv=2 changes in ν6(b2) and ν4(b1) have also been assigned. The fluorescence decays of single rotational levels of the 000 band of H2C74Ge exhibit molecular quantum beats for about 70% of the levels surveyed. Density of states arguments reveal that most of the beats originate from interactions with high rovibronic levels of the ground state. In one case, hyperfine splittings in the Fourier transform of the beat pattern indicate an accidental coincidence with an excited triplet state level. The less frequent occurrence of quantum beats in germylidene compared to silylidene, where they are almost universal, can be attributed to the smaller density of ground state levels at the zero-point energy of the S2 state in the former. © 1999 American Institute of Physics.
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    Spectroscopic characterization of excited Ca(4s4dδ 3DJ)RG(3Δ1,2) states (RG=Ar, Kr, Xe): No "heavy-atom" mixing of RG(ndδ) character into the wave functions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 981-987 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The excited Ca(4s4dδ 3DJ)RG[3Δ1,2] states (RG=Ar, Kr, Xe) have been characterized spectroscopically by R2PI (resonance-enhanced two-photon ionization) spectroscopy. The main vibrational progressions, assigned to Ca(4s4dδ 3D1)RG[3Δ1]←Ca(4s4pπ 3P0)⋅RG[3Π0〈sup ARRANGE="STAGGER"〉−] transitions, have weak subbands 3.7±0.5 cm−1 to the blue which have been assigned to analogous transitions to the 3Δ2 upper states. For CaAr and CaKr, rotational analysis has confirmed this assignment. The 3Δ2/3Δ1 splitting is within experimental error the value expected if the molecular spin-orbit coupling constant is derived entirely from the Ca(4s4d 3DJ) atomic contribution. This indicates that there is no "heavy-atom" mixing of RG(ndδ) character into the wave functions of the CaRG(3Δ) states. © 1999 American Institute of Physics.
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    Quantum mechanical study on energy dependence of probabilities of nonreactive vibrational transitions, atom exchange reaction, and dissociation in a collinear He+H2+ collision (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 988-996 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The accurate time-independent quantum mechanical method developed by the present authors [K. Sakimoto and K. Onda, J. Chem. Phys. 100, 1171 (1994)] is applied to investigate a nonreactive vibrational transition, atom exchange reaction, and dissociation processes in a collinear H2+(vi)+He collision. The algorithm based on the three-point finite difference formula is replaced with the Numerov algorithm to improve on numerical efficiency for directly solving the Schrödinger equation represented by the hyperspherical coordinates (ρ,ω). We have employed the interaction potential surface analytically fitted by Joseph and Sathyamurthy [J. Chem. Phys. 86, 704 (1987)] for this collision system. The energy dependence of the probabilities of the nonreactive vibrational transition, atom exchange reaction, and dissociation processes is investigated at the total energy from 4 to 10 eV, and the dependence of these probabilities on the initial vibrational state of the H2+(vi)(0≤vi≤17) ion is also studied to understand deeply this collision dynamics. These probabilities are undulatory as a function of the total energy, and show that the coupling among the channels defined by the reactant and product vibrational bound and continuum states is strong. The atom exchange reaction is the dominant process for vi≤4, and the predominant process is dissociation of the H2+ for vi≥14 at the total energy investigated here. In order to clarify the sensitivity of this collision dynamics to the interaction potentials, we have investigated an effect of an additive two-body and nonadditive many-body interaction potentials on the nonreactive vibrational transition, atom exchange reaction, and dissociation processes. It is found that the collision dynamics is extremely sensitive to the short-range part of the potential energy surface. © 1999 American Institute of Physics.
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    Ground state and vertical electron detachment energies of icosahedral and D5h Al13− (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10762-10765 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Al13− clusters are studied with ab initio, many-body methods. Coupled-cluster theory places the icosahedral structure 0.54 eV lower than the D5h isomer. Electron propagator predictions on the photoelectron spectrum of Al13− are in close agreement with the observed bands and attribute shakeup character to features at higher energy. © 1999 American Institute of Physics.
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    Infrared photodissociation spectra of the C–H stretch vibrations of C6H6(plus sign)–Ar, C6H6(plus sign)–N2, and C6H6(plus sign)–(CH4)1–4 (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10754-10757 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational infrared photodissociation spectra of mass selected C6H6+–Ar, C6H6+–N2, and C6H6+–(CH4)1–4 ionic complexes are recorded in the spectral range of the C–H stretching vibrations. Transitions at 3095±15 cm−1 occur in all spectra and are assigned to C–H stretch fundamentals of the benzene cation in its 2E1g electronic ground state. In the case of the C6H6+–(CH4)1–4 complexes, additional transitions at 2904±7 and 3010±24 cm−1 are observed and attributed to the symmetric and antisymmetric C–H stretch vibrations of the CH4 ligands, ν1 and ν3. The deduced C–H stretching vibrations of C6H6+ in the 2E1g ground state are roughly 30 cm−1 higher than the corresponding frequencies in the 1A1g electronic ground state of the neutral species, indicating that the C–H bonds become stronger upon removal of an electron from the highest occupied e1g orbital of C6H6. © 1999 American Institute of Physics.
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    Grand canonical ensemble Monte Carlo simulation of a lipid bilayer using extension biased rotations (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10770-10773 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The cavity-biased grand-canonical ensemble method was applied to the simulation of a lipid bilayer using an enhanced Monte Carlo sampling technique. The enhancements include controlling the torsion and molecular rotation step size based on the lipid's conformation and controlling the order of torsion change attempts. It was found that the proposed sampling technique significantly enhances the rate of sampling of the lipid conformations while the grand-canonical ensemble implementation ensures that the water can both penetrate and escape pockets in the bilayer. The latter will be particularly important for simulating bilayers with embedded molecules. © 1999 American Institute of Physics.
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    Combining coupled cluster and perturbation theory (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10815-10826 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Single reference coupled cluster (CC) singles and doubles theory is combined with low-order perturbation theory (PT) to treat ground state electron correlation. Two variants of the general scheme are discussed that differ in the type of amplitudes that are approximated perturbatively and which are treated to infinite order. The combined CC/PT methods to include ground state correlation are merged with equation-of-motion (EOM) and similarity transformed EOM methods to describe excitation spectra of the highly correlated s-tetrazine, MnO4− and Ni(CO)4 systems. It is shown that the computationally efficient CC/PT schemes can reproduce full CCSD results even if perturbation theory by itself is a very poor approximation, as is the case for many transition metal compounds. In a second test CC/PT is applied to determine ground state equilibrium molecular structures and harmonic vibrational frequencies for a set of small molecules. Using either variant of CC/PT, full CCSD geometries are easily recovered, while vibrational frequencies can be more sensitive to details of the approximation. © 1999 American Institute of Physics.
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    A simple semiclassical approach to the Kramers' problem (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10852-10858 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We show that the Wigner–Leggett–Caldeira equation for Wigner phase space distribution function which describes the quantum Brownian motion of a particle in a force field in a high temperature, ohmic environment can be identified as a semiclassical version of Kramers' equation. Based on an expansion in powers of (h-dash-bar), we solve this equation to calculate the semiclassical correction to Kramers' rate. © 1999 American Institute of Physics.
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    Semiclassical dynamics of nonadiabatic transitions in discrete-state systems using spin coherent-state path integrals (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10843-10851 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a semiclassical method for simulating the dynamics of nonadiabatic transitions in a discrete-state quantum system coupled to a bath of explicit continuous coordinates. This method employs a coherent-state formulation of the path integrals for the discrete system whose dynamics is described by spin operators. This spin coherent-state formulation allows the discrete system to be mapped onto a continuous coordinate. Stationary approximations of the resulting coherent-state path integrals of the system plus bath lead to quasiclassical equations of motion which can be solved numerically by direct integration. This algorithm reduces the problem to a number of simple classical trajectory calculations and does not require calculating any fluctuation determinants. © 1999 American Institute of Physics.
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    Spin polarization of zero kinetic energy electrons from HBr (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10883-10886 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The range of observables in zero kinetic energy (ZEKE) electron spectroscopy of molecules, previously restricted to the total electron intensity, was extended by measuring the integral spin polarization of ZEKE electrons at the HBr Ω+=3/2 thresholds after single-photon excitation with narrow-band circularly polarized light. A comparison with calculated values from multichannel quantum defect theory underlines the importance of autoionization for the decay dynamics. © 1999 American Institute of Physics.
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    Quantum Monte Carlo simulation of intermolecular excited vibrational states in the cage water hexamer (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10866-10875 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rigid-body diffusion Monte Carlo simulations of the ground state and ten low-lying intermolecular excited vibrational states for the cage form of (H2O)6 are reported. The excited states are found by a nodal optimization procedure in which the fundamental excited-state nodes are constructed from the harmonic normal coordinates. The anharmonic effects in the excited states are found to be large. One of the states with relatively large transition intensity involves primarily flipping motions of the free OH bonds on the doubly bound monomers, and is assigned to the vibration–rotation–tunnelling band observed experimentally by Liu et al. [Nature 301, 501–503 (1996)]. © 1999 American Institute of Physics.
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    Range definitions for Gaussian-type charge distributions in fast multipole methods (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2351-2356 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Replacement of the exact analytic two electron integrals over Gaussian type orbitals by multipole–multipole interaction terms leads to discretization errors. Such errors are important in Fast Multipole Methods as well as in a number of other tree-based algorithms. We investigate the source of these errors and estimate its dependence on the angular momentum of charge distributions. Two new range schemes suitable for use in any multipole method are presented. Our best scheme allow us to achieve one and a half to two orders of magnitude higher accuracy in the total electron–electron energy than the previously proposed range scheme while requiring the same amount of CPU time. Several benchmarks are presented to illustrate the advantages of this new approach. © 1999 American Institute of Physics.
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    Path integral centroid variables and the formulation of their exact real time dynamics (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2357-2370 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A formalism is presented in this paper which, for the first time, establishes the theoretical basis for the quantum time evolution of path integral centroid variables and also provides clear motivation for using these variables to study condensed phase quantum dynamics. The equilibrium centroid distribution is first shown to be a well-defined distribution function which is specific to the canonical density operator. A quantum mechanical quasi-density operator (QDO) is associated with each value of the distribution so that, upon application of the standard quantum mechanical formalism, the QDO can be used to provide a rigorous definition of both static and dynamical centroid variables. Various properties of the dynamical centroid variables are derived, including the perspective that the centroid constraint on the imaginary time paths introduces a nonstationarity in the equilibrium ensemble which, in turn, can be shown to yield information on the correlations of spontaneous fluctuations. The analytic solution for the harmonic oscillator and a numerical solution for a double well system are provided which illustrate the various aspects of the theory. The theory contained herein provides the basis for a derivation of Centroid Molecular Dynamics, as well as the systematic improvements of that theory. © 1999 American Institute of Physics.
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    The calculation of molecular parameters for a molecule with an internal rotor (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2385-2391 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A derivation for the formulas to calculate centrifugal distortion constants, based on a recent formulation [Duan and Takagi, Phys. Lett. A (1995)] of centrifugal distortion effects for a molecule containing a threefold symmetric internal rotor, is presented. Some constants which are independent of the barrier derivatives, especially the constants representing interactions between torsion and rotation, are given in terms of molecular structural parameters and force constants. These calculated constants are helpful in the reduction of the Hamiltonian and in the analysis of observed transitions. The derived formulas are applied to numerical calculations of the centrifugal distortion constants of methanol. It is shown that most of the calculated constants are in good agreement with those obtained from the fitting to experimental data. © 1999 American Institute of Physics.
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    A derivation of centroid molecular dynamics and other approximate time evolution methods for path integral centroid variables (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2371-2384 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Several methods to approximately evolve path integral centroid variables in real time are presented in this paper, the first of which, the centroid molecular dynamics (CMD) method, is recast into the new formalism of the preceding paper and thereby derived. The approximations involved in the CMD method are thus fully characterized by mathematical derivations. Additional new approaches are also presented: centroid Hamiltonian dynamics (CHD), linearized quantum dynamics (LQD), and a perturbative correction of the LQD method (PT-LQD). The CHD method is shown to be a variation of the CMD method which conserves the approximate time dependent centroid Hamiltonian. The LQD method amounts to a linear approximation for the quantum Liouville equation, while the PT-LQD method includes a perturbative correction to the LQD method. All of these approaches are then tested for the equilibrium position correlation functions of three different one-dimensional nondissipative model systems, and it is shown that certain quantum effects are accounted for by all of them, while others, such as the long time coherence characteristic of low-dimensional nondissipative systems, are not. The CMD method is found to be consistently better than the LQD method, while the PT-LQD method improves the latter and is better than the CMD method in most cases. The CHD method gives results complementary to those of the CMD method. © 1999 American Institute of Physics.
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    Bond configurations in the Peierls–Hubbard model of small ring-shaped molecules (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2392-2400 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Peierls–Hubbard model applied to N-membered ring-shaped molecules is investigated numerically and analytically. The bond configurations minimizing the total energy at half-filling of the electronic states are determined by a self-consistent method for N≤8 in a wide range of the electron–lattice and electron–electron coupling parameters. In the even-N case, only dimerized and homogeneous configurations have been found to be stable. Odd-N rings show three types of bond configurations depending on the coupling parameters. Two of these configurations have reflection symmetry whereas the third one is irregular.© 1999 American Institute of Physics.
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    Vibrational-rotational energy transfer in H2-H2 collisions. I. Semiclassical decoupling approximation (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2401-2406 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new semiclassical decoupling procedure for rotational projection states in rovibrationally inelastic atom-diatom and diatom-diatom collisions is developed. Computed vibrational self-relaxation rate constants for para-H2 and ortho-H2 are in good quantitative agreement (within a factor of 1.5, except for the lowest temperatures) with experimental data over the investigated temperature range 50–2000 K. This allows us to hope that also more detailed (nonmeasured) rate constants for rovibrational state-to-state transitions in molecular hydrogen, calculated by our new model, are sufficiently accurate for astrophysical applications. © 1999 American Institute of Physics.
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    Ab initio calculation of the interaction potential for the krypton dimer: The use of bond function basis sets (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2407-2413 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Kr2 interaction potential is studied by ab initio calculations using several large basis sets containing high polarization functions and/or bond functions. It is shown that the addition of bond functions results in a dramatic improvement for the convergence of the calculated interaction energies. At the frozen-core MP4 level, the large atomic basis set such as [9s7p4d3f2g] recovered less than 75% of the experimental well depth. In contrast, the bond function basis set such as [9s7p4d3f]-{3s3p2d1f} produced a well depth of 617 μhartrees, over 99% of the experimental well depth. The frozen-core MP4 calculation appears to overestimate the well depth by about 25 μhartrees as compared to the calculation at the CCSD(T) level. On the other hand, the inclusion of core electron correlation at the MP4 level may contribute 13 μhartrees to the well depth. Beyond the potential minimum, the use of bond functions consistently gives significant improvement in the calculated potential from the highly repulsive wall to the attractive tail region. Final remarks are made about the counterpoise method and the use of bond functions. © 1999 American Institute of Physics.
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    Screening ionic motion in sodalite cages: A dynamical study (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2761-2769 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamical evolution of protonated helium clusters inside sodalite cages (silica-sodalite and acidic sodalite) is followed via a Car–Parrinello approach. The static simulations give framework structural and electronic features that agree with previous experimental and theoretical data. The protonated helium clusters are used to screen the positive charge borne by a "naked" proton, and are intended as simplified models of the solvation sphere of H+. The action of the framework on the solvation shell of the proton is shown to be mainly due to induction-polarization or Coulombic forces, with a minor contribution of resonant energy exchange between the framework modes and the cluster oscillators. The covalent nature of the zeolitic O–H bond is determined with the aid of the bond order conservation principle. © 1999 American Institute of Physics.
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    A transition-state theory approach to adsorbate dynamics at arbitrary loadings (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2751-2760 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: There has been much recent interest in using transition-state theory (TST) to extend the time and length scales accessible to molecular-level simulations of adsorbate transport in microsporous materials. However, the vast majority of this work has been performed on systems at infinite dilution. The objective of this paper is to obtain fundamental rate constants for adsorbate motion at nonzero loadings using multidimensional TST. More specifically, we focus on systems where the adsorption of a molecule is not highly localized in a single site, but rather distributed throughout an uncorrugated cage. We develop a theory in which high-dimensional TST integrals are approximated using exact lower-dimensional information. The evaluation of the resulting integrals is performed with an importance sampling method involving the insertion of a single molecule, thus improving the statistical quality of the results. The theory is applied to the motion of methane and xenon in the zeolite ZK4, where hopping between α cages is the rate-limiting event. Our results show that hopping rates increase with loading in the cage, which is consistent with experimental trends in the diffusivity. Agreement between our theory and corresponding molecular dynamics simulations is excellent. © 1999 American Institute of Physics.
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    Birefringence and second-order nonlinear optics as probes of polymer cooperative segmental mobility: Demonstration of Debye-type relaxation (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2779-2788 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two complementary optical techniques, second-harmonic generation (SHG), and electric birefringence are used to observe chromophore dynamics in an amorphous polymer above Tg. In particular, a random copolymer of isobutyl methacrylate and a disperse red 1-labeled methacrylate (mole ratio 94:6) is quantitatively studied over a range of time scales and temperatures. In doing so, two models of reorientation are tested: one considers the chromophores motions as small correlated diffusive steps (Debye rotation) and the other describes the motions in terms of uncorrelated instantaneous jumps. Given that SHG is sensitive to the first moment of the orientational autocorrelation function (OACF), or 〈cos θ〉 (where θ is the polar angle between the poling field and the permanent dipole moment directions), and birefringence to the second, or 〈cos2 θ〉, the ability of the models to describe chromophore motion is tested by comparing their predictions of the relationship between the OACFs and the measured properties. By investigating the same polymer system under equivalent conditions, it is shown that chromophore reorientation is explained by a modified Debye model that takes into account a distribution of relaxation times. © 1999 American Institute of Physics.
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    Self-diffusion of an asymmetric diblock copolymer above and below the order-to-disorder transition temperature (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2789-2796 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The self-diffusivity of an asymmetric poly(ethylenepropylene)–poly(dimethylsiloxane) diblock copolymer (PEP–PDMS) was investigated using pulsed field gradient NMR both in the ordered and the disordered state. In a temperature range below the order-to-disorder transition temperature, the diblock copolymer forms micelles which order in a simple cubic structure, and at lower temperatures, a noncubic structure is formed. We have found a broad distribution of self-diffusivities. The distribution width increases with decreasing temperature. The self-diffusivity as well as its distribution width changes only slightly at the order-to-disorder transition. In contrast to self-diffusion in symmetric diblock copolymers where single chains diffuse independently, our results are interpreted as a collective diffusion of chains confined to micelles which further cluster to larger aggregates. This behavior cannot be described by existing models of chain dynamics in the melt. The disordered state is characterized by a large temperature range above the order-to-disorder transition where concentration fluctuations dominate. This is revealed in the temperature dependence of the self-diffusivity and its distribution width, in the viscosity and in the inverse intensity of the first-order Bragg reflection in small angle neutron scattering (SANS). © 1999 American Institute of Physics.
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    Complexation to macromolecules with a large number of sites (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2818-2828 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper presents an approach based on the saddle-point approximation to study the equilibrium interactions between small molecules and macromolecules with a large number of sites. For this case, the application of the Darwin–Fowler method results in very simple expressions for the stoichiometric equilibrium constants and their corresponding free energies in terms of integrals of the binding curve plus a correction term which depends on the first derivatives of the binding curve in the points corresponding to an integer value of the mean occupation number. These expressions are simplified when the number of sites tends to infinity, providing an interpretation of the binding curve in terms of the stoichiometric stability constants. The formalism presented is applied to some simple complexation models, obtaining good values for the free energies involved. When heterogeneous complexation is assumed, simple expressions are obtained to relate the macroscopic description of the binding, given by the stoichiomeric constants, with the microscopic description in terms of the intrinsic stability constants or the affinity spectrum. © 1999 American Institute of Physics.
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    Wetting of a fluid interface by a homopolymer: A system with a rich prewetting behavior (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2797-2808 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A lattice-based self-consistent-field theory is used to reveal the wetting characteristics of a polymeric phase at an interface between two solvent phases. Both solvents are monomeric and the polymer chains are modeled as freely jointed chains of N=100 similar segments. From the polymer point of view we limit our investigations to the symmetric case; both solvents are equally poor for the polymer. We show that the polymer wets the interface both in the limit of strongly and weakly segregated liquids, but that at intermediate values of the L/L demixing the polymer phase can be in a lens configuration (partial wetting). The wetting transitions at strong segregation of the two solvents is always first order, the ones at weak segregation can be either first or second order, depending on the polymer/solvent interactions. For a particular value of the polymer–solvent interaction parameter the two wetting transitions as generated by changing the L/L demixing (then both of the first-order type) merge. At this point the two prewetting lines still exist and obviously merge as well. It is further possible to find detached "prewetting" lines in the regime where the polymer wets the interface at all relevant (i.e., consistent with the presence of three phase coexistence) values of the L/L demixing. Physically this means that there is a range of interfacial widths for which the formation of a new (polymeric) phase is hindered by a free energy barrier at a certain film thickness, but that there is no wetting transition associated with this barrier. © 1999 American Institute of Physics.
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    Surface-induced reactions of acetone cluster cations (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2770-2778 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The occurrence of two different chemical reactions initiated by the surface impact of acetone dimer, trimer, and tetramer cations (energy 20–70 eV) on a stainless-steel surface (covered with hydrocarbons) was observed. The reaction product is the protonated acetone ion, formed in (i) an intracluster ion–molecule reaction, and in (ii) a hydrogen pickup reaction of the cluster ion with the surface material. Only the monomer product ions (and small amounts of their dissociation products) could be observed; the spectra did not show any presence of clustered product ions. A simple model based on the Brauman double-well potential is suggested to explain the formation of the two product ions. In accordance with predictions from molecular dynamics simulations, this appears to be the first observation of competitive chemical reactions of a cluster ion driven by energy transfer in a surface collision. © 1999 American Institute of Physics.
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    Associating polyelectrolytes: Finite size cluster stabilization versus physical gel formation (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2809-2817 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The theory of phase equilibria in the solutions of associating polyelectrolyte telechelic chains proposed earlier is generalized to take into account the possibility of existence in the solutions of finite size thermodynamically stable clusters in addition to the formation of macroscopic physical gel. It is shown that in some region of polymer concentrations and energies of attraction between the end groups of the chain (stickers) the size distribution of chain clusters has a maximum, i.e., the solution is dominated by the clusters of optimum size. The stabilization of such a cluster is due to the interplay between the energy of attraction of stickers, on the one hand, and the Coulomb interaction together with the contribution from the translational entropy of counter ions, on the other hand. For small enough optimum clusters direct Coulomb interactions are more important, while for large optimum clusters the main reason for stabilization is connected with the translational entropy. This type of stabilization mechanism should be quite general for any polyelectrolyte system with attraction. © 1999 American Institute of Physics.
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    Droplet size dependence upon volume expansion rate (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2842-2843 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics of a Lennard-Jones liquid undergoing a homogeneous adiabatic expansion provides a direct numerical simulation of the atomization process. Results from a symmetric-triaxial expansion appear to be consistent with the mean droplet size found in the Knuth and Henne free-jet experiments. New simulations with axisymmetric expansions, similar to flow through an orifice, show only small departures from the symmetric expansion results. © 1999 American Institute of Physics.
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    On the nature of electronic excitations in poly(paraphenylenevinylene): A quantum-chemical investigation (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2829-2841 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Correlated quantum-chemical calculations are performed on phenylenevinylene oligomers containing up to eleven repeat units, to characterize the nature of the electronic excitations relevant for the photophysical properties of the corresponding polymer. The focus is first on the nonlinear optical response of model conjugated chains and the simulation of their frequency-dependent (third-harmonic generation, electroabsorption, and two-photon absorption) response. From the assignment of the calculated resonance features, the excited states dominating the third-order nonlinear polarizability are identified and their chain-length dependence is investigated. On that basis, we build an essential-state single-chain model (that includes the 1Bu, 2Ag, mAg, and nBu states) and apply it to the interpretation of recent experimental data reported for poly(paraphenylenevinylene) and derivatives. We then examine how the exciton binding energy, here defined as the difference between the energies of the charge-separated nBu and the strongly optically allowed 1Bu excited states, is affected by both intrachain and interchain polarization effects. © 1999 American Institute of Physics.
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    Comment on "Improving protein circular dichroism calculations in the far-ultraviolet through reparametrizing the amide chromophore" [J. Chem. Phys. 109, 782 (1998)] (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2844-2845 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Circular dichroism spectra for 23 proteins have been calculated using transition parameters from experiments on model amides and semiempirical molecular orbital (MO) calculations. The results are substantially better than those reported by Hirst. The improvements result primarily from using distributed dipoles (monopoles), rather than the point dipoles used by Hirst. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479562
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    Erratum: "An advanced dielectric continuum approach for treating solvation effects: Time correlation functions. I. Local treatment" [J. Chem. Phys. 108, 1103 (1998)] (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2848-2848 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479564
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    Site specificity in molecular hydrogen elimination from photodissociation of propane at 157 nm (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1793-1796 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Site effects on the molecular hydrogen elimination from propane at 157 nm excitation have been studied using the photofragment translational spectroscopic technique. Experimental results indicate that H2 elimination from the internal carbon of propane (2,2-elimination) is predominant while eliminations from the terminal carbon (1,1- and 1,3-elimination) and the vicinal carbons (1,2-elimination) are minor. The translational energy distributions obtained for these processes also show that the dynamics of H2 eliminations from different sites are significantly different. Relative branching ratios of the atomic hydrogen (H) and the molecular hydrogen (H2) elimination processes were also determined. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479507
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    Forces between surfaces immersed in polyelectrolyte solutions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1797-1800 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A microscopic theory is presented for the behavior of polyelectrolyte solutions between surfaces. The theory predicts oscillatory forces between hydrophobic surfaces immersed in dilute salt-free polyelectrolyte solutions. The period of the oscillation, p, scales with concentration, ρ, as p∼ρ−1/3 and p∼ρ−1/2, in the dilute and semidilute solutions, respectively. With a small amount of excess salt, the oscillatory forces are replaced by a short-ranged, predominantly attractive force. Further addition of salt causes the force to become purely repulsive. These predictions can, in principle, be tested experimentally. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479448
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    Response to "Comment on ‘Improving protein circular dichroism calculations in the far-ultraviolet through reparameterizing the amide chromophore' " [J. Chem. Phys. 111, 2844 (1999)] (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2846-2847 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In response to Woody and Sreerama's Comment that they have realized significant improvements in calculations of the circular dichroism of proteins over those reported by Hirst previously, we report the simultaneous and independent achievement of comparably accurate calculations. Our calculations are based on parameters from ab initio wave functions, in contrast to the combination of experimental data and semiempirical wave functions used by Woody and Sreerama. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479563
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    Recursion representation of gradient expansion for free fermion ground state in one dimension (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1809-1814 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We consider noninteracting Fermion system in an external field, confined to one-dimensional space. Our main objective is to take advantage of low dimension to find explicitly potential-density profile as well as kinetic energy functional in the ground state. This is accomplished by deriving the complete x-space linear response and a simple recursion representation of the gradient expansion for the kinetic energy. The results can serve as a potential test for higher dimensions, e.g., in exploring convergence properties of the gradient expansion in various regions of the energy spectrum. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479450
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    Classical mechanics and the spreading of localized wave packets in condensed phase molecular systems (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1801-1808 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The relationship between the diverging of classical trajectories in chaotic many-body systems, the spreading of quantum wave packets, and the validity and use of classical molecular dynamics is explored. This analysis, which is based on the semiclassical description of wave function propagation in terms of a weighted integration over a traveling fixed width coherent state basis, suggests that the exponential divergence of nearby classical trajectories in chaotic many-body systems should result in the rapid delocalization of an initially localized quantum wave packet describing the state of the system. Thus the justification for the use of classical molecular dynamics procedures for these supposedly classical systems cannot be based on the picture of the system wave function remaining localized as its center follows a nearly classical trajectory. The quantum evolution of the system density, on the other hand, requires two propagators, and each of these propagators is represented as an integration over trajectories in the semiclassical picture. The interference between the contributions from these two integrations over classical trajectories focuses the analysis on the most important points in this trajectory pair space, which are shown to occur when both trajectories in the pair are the same. Given reasonable assumptions for the initial density for a system that is expected to be well described by classical molecular dynamics, and given an appropriate choice for the width of the coherent state basis which is employed in the semiclassical description, it is shown that the semiclassical expressions for time dependent observables and correlation functions reduce the purely classical expressions, despite the fact that an initially localized wave packet would rapidly delocalize for the same system. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479449
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    Classical flux integrals in transition state theory: Generalized reaction coordinates (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1830-1842 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Transition state theory (TST) approximates the reactive flux in an elementary chemical reaction by the instantaneous flux passing through a hypersurface (the "transition state") which completely divides the reactant and product regions of phase space. The rigorous classical evaluation of this instantaneous flux is carried out as a trace in phase space: effectively a multidimensional integral. We present an analysis of the momentum-space component of this flux integral for the case of a generalized reaction coordinate. The classic analysis of the canonical flux by Marcus [J. Chem. Phys. 41, 2624 (1964)] is refined by reducing the determinant which appears in the transition state partition function to a very simple form, facilitating the ensuing integration over coordinate space. We then extend the analysis to provide analytic expressions for the momentum flux integrals in both the energy-resolved, and the energy+angular-momentum-resolved microcanonical ensembles. These latter expressions allow substantial gains in the efficiency of microcanonical variational implementations of Transition State Theory with generalized reaction coordinates. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479453
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    A comparison of methods for the calculation of NMR chemical shifts (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1815-1822 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theory (MPL) to compute the NMR chemical shifts in condensed matter systems using periodic boundary conditions was presented by F. Mauri, B. Pfrommer, and S. G. Louie [Phys. Rev. Lett. 77, 5300 (1996)]. The MPL method has been implemented so far within a pseudopotential formulation in which the wave functions are expanded in plane waves. In this paper, we compare analytically the MPL approach within the density functional theory to existing methods for the calculation of the chemical shifts such as GIAO (gauge-including atomic orbitals), CSGT (continuous set of gauge transformations), and IGAIM (individual gauges for atoms in molecules). To this end we apply the MPL approach to molecules since the latter methods are conceived only for finite systems. We show theoretically the equivalence between a variant of the CSGT and the MPL method applied to finite systems. Moreover, we analyze numerically the efficiency of the different methods when atomic orbital basis sets are employed, by comparing the basis-set convergence properties. We find that the CSGT and IGAIM approaches have the same convergence properties as GIAO, whereas their computational time is significantly smaller. In the MPL method, the contribution of the valence electrons to the chemical shift converges rapidly with respect to the size of the basis set, whereas the convergence properties of the core contribution are poor. We improve the convergence by separating the core and the valence contributions in a gauge-invariant manner, by applying the MPL method only to the valence contribution, and by treating the core contribution with IGAIM. The performances of the resulting approach compare favorably with those of the other methods. Finally we find that the core contribution to the chemical shift is independent of the chemical environment, in contrast to what is sometimes found in the literature. In conclusion, our results indicate that, to compute the chemical shifts in both molecules and solids, using atomic orbital basis sets, one could use the MPL method to evaluate the valence contribution and add to it a rigid core contribution as obtained, for instance, from an atomic calculation. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479451
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    Towards standard methods for benchmark quality ab initio thermochemistry—W1 and W2 theory (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1843-1856 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two new schemes for computing molecular total atomization energies (TAEs) and/or heats of formation (ΔHf&convolu;) of first- and second-row compounds to very high accuracy are presented. The more affordable scheme, W1 (Weizmann-1) theory, yields a mean absolute error of 0.30 kcal/mol and includes only a single, molecule-independent, empirical parameter. It requires CCSD (coupled cluster with all single and double substitutions) calculations in spdf and spdfg basis sets, while CCSD(T) (i.e., CCSD with a quasiperturbative treatment of connected triple excitations) calculations are only required in spd and spdf basis sets. On workstation computers and using conventional coupled cluster algorithms, systems as large as benzene can be treated, while larger systems are feasible using direct coupled cluster methods. The more rigorous scheme, W2 (Weizmann-2) theory, contains no empirical parameters at all and yields a mean absolute error of 0.23 kcal/mol, which is lowered to 0.18 kcal/mol for molecules dominated by dynamical correlation. It involves CCSD calculations in spdfg and spdfgh basis sets and CCSD(T) calculations in spdf and spdfg basis sets. On workstation computers, molecules with up to three heavy atoms can be treated using conventional coupled cluster algorithms, while larger systems can still be treated using a direct CCSD code. Both schemes include corrections for scalar relativistic effects, which are found to be vital for accurate results on second-row compounds. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479454
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    Tunneling manifold of two noninteracting NH4 torsional oscillators in polycrystalline ammonium iodide (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1335-1336 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Zeeman rotational tunneling level-crossing spectra have been interpreted on the assumption that only single noninteracting NH4 groups contribute to the spectrum. In this note, we report the results of an analysis which takes into account two noninteracting NH4 groups, in which case the inter NH4-group interactions become important. This analysis agrees with the accepted interpretation of the NH4 ground state manifold wherein the F level is triply degenerate. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479385
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    Erratum: "Comparison of thermal and photochemical behavior of O2 chemisorbed on Pt(335)" [J. Chem. Phys. 110, 6939 (1999)] (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1340-1340 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479321
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    Molecular dynamics simulations of dielectric relaxation of concentrated polymer solutions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1325-1334 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulations are conducted for concentrated solutions of flexible polymers. The results are contrasted with literature dielectric spectroscopy data, in an attempt to elucidate the observed phenomena from a molecular level perspective. A bead-spring model is used and systems with chain sizes up to N=150 beads at reduced densities 0.5≤ρ≤0.8 are studied. The dimensions of the chains follow a universal behavior with ρ/ρ*, where ρ* is the crossover density demarcating the onset of chain overlapping. All the chains are found to follow random-walk behavior. The global motion of the chains is investigated in terms of the dielectric loss E″. As in dielectric spectroscopy experiments, the motion of the chains induces prominent dielectric relaxation at low frequencies. The shape of E″ broadens with increasing density, and a normal-mode analysis indicates that overlapping of the chains with increasing density progressively renders the distribution of relaxation times more heterogeneous. For denser systems a second, smaller peak appears at the high frequency end of the spectrum. This secondary peak is not identified with segmental motion, since the simulated chains lack components of the segmental dipoles perpendicular to the chain contour. Entanglement effects are investigated calculating the mean squared displacement g1(t), and the results suggest that the topological constraints of entanglements render at least two different relaxation mechanisms with disparate time scales important. An attempt to explain the shape of the spectra in terms of a phenomenological separation of the motion of chains into a rotational and a stretching mode showed that stretching plays no important role in the relaxation function and the shape of E″. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479318
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    Nonequilibrium distribution function theory of diffusion-influenced reversible energy-transfer reactions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 804-815 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recently, we proposed a new systematic approach to evaluate the many-particle effects on the diffusion-influenced reactions. The method gives an improved result over that obtained by using the superposition approximation. In the present paper, we apply the method to treat the kinetics of reversible energy-transfer reactions of the type A*+B(arrow-right-and-left)A+B*. Until now, most theories were inapplicable when the lifetime of A* is shorter than that of B*, and a notable exception was the integral encounter theory (IET) of Burshtein et al. The present theory can be applied irrespective of the relative magnitude of the lifetimes of donor and acceptor molecules, and becomes exact for the irreversible target model. In addition, it is applicable to the system with higher reactant densities than IET; the result of IET is recovered as a limiting form in the present theory. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479198
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    A formal method for solving the stationary BBGKY hierarchy for a classical inhomogeneous fluid (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 833-841 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A methodology is developed for finding a formal solution to the entire stationary Bogoliubov, Born, Green, Kirkwood, Yvon (BBGKY) hierarchy for a classical inhomogeneous fluid in thermal equilibrium. The method is unique in that neither a closure relationship nor a restriction on the density or the temperature of the fluid is required. The decisive step in the analysis is the creation of a more general system of coupled integrodifferential equations through the introduction of a coupling parameter. Recognizing that the coupling parameter can also act as an expansion parameter, series solutions to the generalized hierarchy can be obtained through the application of regular perturbation theory. At the end of the calculation, a formal solution to the original BBGKY hierarchy can be recovered by setting the coupling parameter to unity. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479447
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    Measurement of the HD(v′=2,J′=3) product differential cross section for the H+D2 exchange reaction at 1.55±0.05 eV using the photoloc technique (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1022-1034 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe a time-of-flight apparatus that uses core extraction to determine nascent product laboratory velocity distributions from which differential cross sections may be deduced. We emphasize the characterization of the instrument, the reaction conditions, and the calibration procedure. For this purpose, we have measured H-atom velocity distributions from HBr photolysis, as well as the H2(v′=4,J′=1) velocity distribution arising from the H+HBr reaction under quasi-monoenergetic collision conditions at 1.9 eV. Collisional energy spread and reagent internal state distributions were determined from the rotational and translational temperatures of the HBr photolytic precursor and the D2 diatomic reagent. The differential cross section for H+D2→HD(v′=2,J′=3)+D at 1.55±0.05 eV is presented and found to peak near 145°±10° with an approximate full width at half maximum (FWHM) of 40°. © 1999 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.479294
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