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  • Articles  (11,976)
  • American Institute of Physics (AIP)  (11,976)
  • 1995-1999  (11,976)
  • 1965-1969
  • Chemistry and Pharmacology  (11,976)
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  • Articles  (11,976)
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  • 1
    Electronic Resource
    Electronic Resource
    Effect of temperature on the infrared and sum-frequency generation spectra of adsorbates (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 5920-5927 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dephasing and energy relaxation contributions to the line width in infrared (IR) and sum-frequency generation (SFG) spectra of adsorbates are derived from the generalized master equation approach. Expression for the line shift is also obtained. The anharmonic interaction between the adsorbate and the substrate is expanded in a polynomial in terms of the adsorbate and phonon coordinates, and the dephasing is shown to be mainly due to two-phonon processes, while two-phonon, three-phonon or four-phonon processes can contribute to energy relaxation, depending on the relative values of the adsorbate vibrational and the phonon frequencies. The temperature-dependence data of the IR absorption for C(111):H is found to be consistent with the theory, and the large line width for C(111):D can be accounted for by the efficient two-phonon energy relaxation process which is not available for C(111):H due to the higher adsorbate vibrational frequency for C(111):H. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473257
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  • 2
    Electronic Resource
    Electronic Resource
    The mobilities of ions and cluster ions drifting in polar gases (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 5937-5942 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The mobility of ions drifting in polar gases is explored both theoretically and experimentally. New experimental results are presented for (i) NO+ ions drifting in H2O (the reduced zero-field mobility K0(0) is 0.66±0.07 cm2 V−1 s−1), (ii) H3O+(H2O)3 ions drifting in H2O (K0(0)=0.43±0.06 cm2 V−1 s−1), and (iii) NO+(CH3COCH3)n ions (n=2,3) drifting in CH3COCH3 (K0(0)=0.041 ±0.010 cm2 V−1 s−1 for n=2 and K0(0)=0.050±0.015 cm2 V−1 s−1 for n=3). A number of theoretical models for ion mobilities in polar gases are described. The models are compared with the available experimental data and a reasonable agreement is obtained. For larger cluster ions the measured mobilities are considerably smaller than the calculated values. Some possible reasons for the discrepancies are discussed, including momentum transfer outside the capture cross section, dipole–dipole interactions, ligand exchange, inelastic collisions, and the validity of Blanc's law. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473609
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  • 3
    Electronic Resource
    Electronic Resource
    Electronic to vibrational energy transfer between Rb(5 2PJ) and H2 (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 5972-5978 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have investigated the electronic-to-vibrational (E-V) energy transfer between electronically excited rubidium atoms (Rb 5 2P1/2,3/2) and hydrogen molecules (H2). We have used the CARS (Coherent Anti-Stokes Raman Scattering) spectral technique to probe the internal state distribution of collisionally-populated H2 molecules. Both scanned CARS spectra and activated CARS spectra reveal that during E-V energy transfer processes H2 molecules are produced only at the v=1 and v=2 vibrational levels. From scanned and activated CARS spectral peaks two possible population ratios (n1/n2) are obtained. Through shape simulations of the time-resolved CARS profiles under a simple kinetic model, the actual population ratio n1/n2 is unambiguously determined to be of 0.59 (σ=0.05). This n1/n2 ratio indicates that the H2 molecules produced by the E-V energy transfer process are 37% populated at the v=1 vibrational level and 63% at v=2, and that the efficiency of the E-V energy transfer is 0.489 for the Rb 5 2P1/2−H2 system and 0.481 for the 5 2P3/2−H2 system, coincident with the highest E-V transfer fraction 0.489 under the impulsive model and a collinear collision geometry. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473244
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  • 4
    Electronic Resource
    Electronic Resource
    A quasiclassical trajectory study of product state distributions from the CN+H2→HCN+H reaction (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6001-6015 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An extensive quasiclassical trajectory study of the dynamics of the CN+H2→HCN+H reaction has been undertaken on two of the potential energy surfaces reported by ter Horst, Schatz, and Harding [J. Chem. Phys. 105, 558 (1996)] with the goal of converging product state distributions. The effect of zero-point energy violations on the behavior of the reactive cross section near threshold has been examined leading to an improved estimate of the thermal rate constant on ter Horst–Schatz–Harding potential energy surface 3 (3.01±0.24×10−14 cm3/s at 300 K). The calculated HCN vibrational product state distribution is not statistical and exhibits a systematic over population in the stretching vibrations of the ground state bend manifold indicating that the –C–N does not behave like a "spectator bond" in this reaction. There is also significant population in modes with bending excitation, but these vibrations are under populated relative to prior statistical expectations. The sensitivity of the distribution on the size of the barrier and its location in the entrance channel has been undertaken by comparing results on the ter Horst–Schatz–Harding potential energy surfaces 2 and 3. Similar to the case of exoergic atom-diatom reactions, it is found that the earlier barrier on ter Horst–Schatz–Harding potential energy surface 3 gives rise to more excitation in the –C–H stretching vibration. The rotational distributions of the HCN product appear similar to the thermal distribution of CN reagents from which they are born indicating that the abstraction of the light H atom perturbs the rotational motion of the cyano radical very little. The dependence of the average HCN rotational quantum number, 〈J〉, on the bending quantum number, v2, exhibits an interesting alternation such that the points for even values of v2 are larger than those for odd. There is a corresponding alternation in the dependence of the average scattering angle, 〈θ〉, on v2 in the opposite sense. These observations suggest that for the odd bending states (which are primarily l=1) the energy diverted into exciting motion perpendicular to the reaction path at the transition state is not available to excite product rotation or to produce reactive trajectories with large impact parameters which lead to small scattering angles. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473264
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetic shielding surface in molecules. Neutron as a probe in the hypothetical magnetic resonance spectroscopy (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6061-6067 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is proposed to analyze the magnetic shielding tensor as a continuous function of space point coordinates in atoms and molecules. Such a three-dimensional magnetic shielding surface should be very helpful for better understanding of nuclear magnetic shieldings, which is not a trivial problem. The magnetic shielding surface provides more information about molecular electronic structure than nuclear magnetic shieldings alone. A connection between magnetic shielding at an arbitrary point and nuclear magnetic shielding can be made by "performing" a hypothetical NMR experiment using a neutron as a virtual probe, instead of nuclei with nonzero magnetic moments. The theory is illustrated by the numerical results of the magnetic shielding surface calculations for atoms and small linear molecules. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473268
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  • 6
    Electronic Resource
    Electronic Resource
    The splitting of excited electronic states in optically inactive molecules due to the parity-violating electron-nuclear interaction (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6045-6050 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper considers optically inactive molecules possessing a symmetry plane. Degenerated excited electronic states in such molecules may, in principle, differ in symmetry with respect to mapping onto the symmetry plane. Should this prove the case, the parity-nonconserving electron-nuclear interaction (PNI) causes the degenerated electronic level to suffer a splitting linear in the Weinberg constant. The paper analyzes from this standpoint the lowest-lying excited states in the ten-electron HF, NH3, B2, and H2O molecules. Two of them, namely HF and NH3, possess the necessary and sufficient symmetry properties for such a linear splitting to occur. Factors are discussed that augment the PNI-induced splitting of the excited states under consideration in comparison with the splitting of the ground state in left- and right-handed modifications of optically active molecules. Computations confirm the occurrence of a great ((approximate)10−13 eV) splitting of the levels being considered due to the PNI effect. A similar effect can also occur in the electronic ground state of paramagnetic molecules, such as NO. The computation uses the consistent multiple-electron perturbation theory with a model single-electron central field bare potential. The computer code used is a modification of the original code developed for precision atomic calculations. All the computations boil down to the solution of a single set of ordinary differential equations, i.e., a unidimensional procedure. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473267
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  • 7
    Electronic Resource
    Electronic Resource
    Hydrated proton clusters: Ab initio molecular dynamics simulation and simulated annealing (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6086-6094 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An ab initio molecular dynamics simulation technique is developed employing the Born–Oppenheimer (BO) approach in the framework of a Gaussian implementation of Kohn–Sham density functional theory (DFT). Simulation results for H5O2+ at 200 K are reported. The density profiles, autocorrelation functions and power spectra are presented. The anharmonic frequencies at 200 K are found to be close to the harmonic frequencies calculated directly from quantum methods at 0 K. Structures of large hydrated proton clusters are optimized. Simulated annealing techniques were employed to search for low energy structures and found to be very useful for clusters with 7–8 water molecules. A few very different structures with ground state energy 1–2 kcal/mol apart are shown. H3O+ is found to be the central unit of a few structures optimized. The ionic hydrogen bond was responsible for the stability of the H9O4+ unit in the large hydrated proton clusters. We also find structures with nascent H5O2+ units at the center whose energy is close to, sometimes even lower than that of the H3O+ centered structures. This can be used to explain the solvation facilitated proton transfer in clusters and in solution. The vibrational frequencies of the structures we optimized are tabulated and compared with the experimental results of Price et al. Questions are raised regarding their prediction of a new feature due to water molecules in the third solvation shell. Some new features have been observed for large clusters with heretofore unpredicted structures. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473607
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  • 8
    Electronic Resource
    Electronic Resource
    A general mixture theory. II. Mixtures of nonspherical molecules (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6116-6125 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A recently suggested mixture theory for the prediction of mixture properties is formulated for nonspherical molecules. The theory is capable of handling equations of state that are not conformal (do not obey the corresponding states principle). The exact composition dependence of the density expansion of the mixture equations, plus other features of the original theory, are retained. Two methods for evaluating the cross molecular parameters for nonspherical molecules are suggested. They become identical for spherical molecules, and give similar predictions for mixture properties of nonspherical molecules at high densities. The two methods predict the properties of mixtures of convex bodies and fused hard spheres within simulation uncertainty. Properties of diatomic Lennard-Jones mixtures, and ethane–carbon dioxide are predicted fairly accurately. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473233
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  • 9
    Electronic Resource
    Electronic Resource
    Threading dynamics of a polymer through parallel pores: Potential applications to DNA size separation (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6152-6159 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: DNA orientation measurements by linear dichroism (LD) spectroscopy and single molecule imaging by fluorescence microscopy are used to investigate the effect of DNA size (71–740 kilo base pairs) and field strength E (1–5.9 V/cm) on the conformation dynamics during the field-driven threading of DNA molecules through a set of parallel pores in agarose gels, with average pore radii between 380 Å and 1400 Å. Locally relaxed but globally oriented DNA molecules are subjected to a perpendicular field, and the observed LD time profile is compared with a recent theory for the threading [D. Long and J.-L. Viovy, Phys. Rev. E 53, 803 (1996)] which assumes the same initial state. As predicted the DNA is driven by the ends into a U-form, leading to an overshoot in the LD. The overshoot-time scales as E−(1.2−1.4) as predicted, but grows more slowly with DNA size than the predicted linear dependence. For long molecules loops form initially in the threading process but are finally consumed by the ends, and the process of transfer of DNA segments, from the loops to the arms of the U, leads to a shoulder in the LD as predicted. The critical size below which loops do not form (as indicated by the LD shoulder being absent) is between 71 and 105 kbp (0.5% agarose, 5.9 V/cm), and considerably larger than predicted because in the initial state the DNA molecules are housed in gel cavities with effective pore sizes about four times larger than the average pore size. From the data, the separation of DNA by exploiting the threading dynamics in pulsed fields [D. Long et al., CR Acad. Sci. Paris, Ser. IIb 321, 239 (1995)] is shown to be feasible in principle in an agarose-based system. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473237
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  • 10
    Electronic Resource
    Electronic Resource
    Measurement of the splay-bend elastic constant in lyotropic ferronematic liquid crystals: The influence of the bounding surfaces (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6187-6193 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the first experimental investigation devoted to analyze the influence of a ferrofluid on the surface properties of lyotropic liquid crystals doped with magnetic particles. By means of optical methods, the bulk orientation of the director vs the temperature is determined. The theoretical analysis shows that near the nematic–isotropic transition, the thermodynamical model for the temperature surface transition works reasonably well. On the contrary, in the low temperature region near the lamellar–nematic transition, important deviations from the theoretical behavior are observed. In order to interpret our experimental data, we propose a simple phenomenological extension of the thermodynamical model that takes into account the different contributions to the surface energy and of the residual lamellar order in the nematic phase. We show, furthermore, that the effective splay-bend elastic constant depends on the ferrofluid doping concentration, in agreement with a recently proposed theory. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473241
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