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  • 1
    Electronic Resource
    Electronic Resource
    Bidimensional tunneling splitting in the A˜ 1B2 and X˜ 1A1 states of tropolone (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 353-359 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The intramolecular proton transfer in tropolone has been theoretically analyzed. Ab initio calculations using a variety of basis sets have been performed for both the singlet ground state (X˜ 1A1) and the first excited singlet state (A˜ 1B2). A configuration interaction all single excitation method (CIS) has been used to deal with the excited singlet state. Tunneling splittings in both electronic states have been obtained by fitting a bidimensional surface into the ab initio results. This way, a new strategy designed to avoid calculations of the intrinsic reaction coordinate (IRC), which require a very long computer time, is proposed and shown to give accurate results. Our calculations provide a theoretical interpretation of previous extensive spectroscopical data from which the tunneling splitting for the excited A˜ 1B2 state was shown to be clearly higher than for the ground X˜ 1A1 state. Finally, the experimentally observed diminution of the splitting upon deuteration of the transferring hydrogen is also accounted for by our theoretical results. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469647
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  • 2
    Electronic Resource
    Electronic Resource
    Shear flow at liquid–liquid interfaces (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 716-724 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two nonmiscible liquids separated by planar interfaces and undergoing shear flow have been simulated with nonequilibrium molecular dynamics (NEMD) methods. A homogeneous shear scheme was used for imposing shear flow in the system. The homogeneous shear algorithm needs to be combined with a profile-unbiased thermostat (PUT) in order to assure meaningful results in our nonhomogenous system. Local values of several quantities such as viscosity, local stream velocity, temperature, shear stress, and rate of entropy production were calculated. Planar Couette flow appears in the "bulk'' regions of the system with a slip between the two streams of bulk fluid at the interfaces. The shear stress is constant across the system (PUT results) at low strain rates but at high shear rates the shear stress at the interface is lower than in the bulk region. The shear viscosity at the interfaces is lower than in the bulk region showing that the transport of momentum in the former region is less efficient than in the bulk. At high strain rates, the differences in the local rates of viscous heat production and heat removal result in strong temperature gradients. When comparing the viscosity values in the bulk region of the inhomogeneous system with values computed in independent simulations of the bulk, no important differences are found. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470105
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  • 3
    Electronic Resource
    Electronic Resource
    Energy distribution in aniline scattered from various low energy surfaces (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4921-4927 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Aniline has been scattered from three organic substrates and a LiF single crystal surface. Applying multiphoton ionization and time of flight measurements the vibrational, rotational, and translational energy distributions were measured simultaneously. The NH2 "umbrella'' like mode was found to be a very efficient accepting mode in the energy transfer process. The less rigid the surface, the greater the efficiency with which this mode is populated. The mode specificity does not exist for the rigid LiF surface. A model is presented, which explains all observations based on the collision time and kinematics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460577
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  • 4
    Electronic Resource
    Electronic Resource
    Fluid alkanes in confined geometries (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 1490-1502 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A comparative study of confined fluid films composed of three different alkanes has been carried out using molecular dynamic simulation techniques. The films were confined in thin slit pores, only a few molecular diameters thick, and the substances studied were n-butane, n-decane, and 5-butyl-nonane. The properties of the film were obtained in equilibrium conditions and under shear. All the studied films show a strong layering of the distribution of methylene subunits. Chains at the solid boundaries align with the walls and show a tendency to stretch. The diffusion parallel to the solid walls is found to be higher in the proximity of the walls than in the inner part of the pore. The molecular motion normal to the confining walls can be described as noncorrelated molecular transitions between the contact layer and the inner part of the pore. Shear flow was induced in the film by moving the solid walls. The resulting velocity profiles across the pore were computed as well as the viscosity of the films. The viscosities of the confined fluids in the three cases appear to be the same as those of the bulk, within the uncertainty of the results. No significant influence of the shear flow on the inter- or intramolecular was found.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468437
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  • 5
    Electronic Resource
    Electronic Resource
    A theoretical study of the isotope effects on the fluorescence excitation spectrum of 5-aminotropolone (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 8114-8122 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A combined ab initio+nuclear dynamics study is performed to theoretically analyze the intramolecular H-atom transfer process in 5-aminotropolone in both the ground (S0) and first excited (S1) singlet electronic states. A complete active space self-consistent field (CASSCF) method is used to optimize the geometries. Energies are then corrected through the second order Møller–Plesset perturbation theory. These results are used to build up reduced bidimensional energy surfaces so that the nuclear wave functions for the nuclear motions in both electronic states are obtained. In particular we have analyzed the six isotopomers that result from deuteration of the amino and hydroxy groups of 5-aminotropolone. It is found that for symmetric structures (−OH/−NH2, −OH/−ND2, −OD/−NH2, and −OD/−ND2), the two lowest vibrational levels in both S0 and S1 appear as a quasidegenerated tunneling doublet. The tunneling splitting in S0 is much lower so that the doublet at the origin, seen in the fluorescence excitation spectra of 5-aminotropolone, can be entirely assigned to the S1 state. In agreement with the experimental findings, this splitting greatly diminishes when the transferring hydrogen is substituted by a deuterium, whereas deuteration of the amino group produces only a modest decrease of such a splitting. A quite different result is found for the nonsymmetric isotopically substituted structures (−OH/−NHD and −OD/−NHD), as the isotope induced asymmetry, combined with the high energy barrier in the S0 potential energy surface, leads to a complete localization of the two lowest vibrational wave functions in S0. On the other hand, for S1 the asymmetry and energy barriers are low enough so that an important degree of delocalization of the two lowest vibrational wave functions is found. These results are again in agreement with the presence of an isotope induced quartet in the fluorescence excitation spectra of these species. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476251
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  • 6
    Electronic Resource
    Electronic Resource
    On the photoisomerization of 5-hydroxytropolone: An ab initio and nuclear wave function study (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 6275-6282 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we perform ab initio calculations for the stable conformations and the transition states for the isomerization processes in 5-hydroxytropolone in both the ground (S0) and first excited (S1) singlet electronic states. The Hartree–Fock self-consistent field (SCF) level and a complete active space SCF (CASSCF) level for S0 are considered, whereas the configuration interaction all single excitation method (CIS) and the CASSCF levels are used to deal with the S1 state. Energies are reevaluated at all levels through perturbation theory up to second order: Møller–Plesset for the Hartree–Fock and CIS methods, and the CASPT2 method for CAS results. The ab initio results are then used to perform different monodimensional fits to the potential energy surfaces in order to analyze the wave functions for the nuclear motions in both electronic states. Our best results predict that for the S0 state two stable conformers, syn and anti, can exist in thermal equilibrium. In accordance with experimental expectations the syn isomer is the most stable. As for the S1 state, and again in accord with experimental spectroscopical data, the order of stability reverses, the anti being the most stable. A more interesting result is that analysis of the nuclear wave functions shows an important syn–anti mixing in the S1 state that does not appear in S0. This result explains the appearance of syn–anti and anti–syn crossover transitions observed in the electronic spectra of 5-hydroxytropolone so that syn–anti reaction may take place through photoisomerization. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474344
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  • 7
    Electronic Resource
    Electronic Resource
    Simulating shear flow (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 5956-5963 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In the present article, we have analyzed to which extent the steady states produced in simulations of fluids undergoing shear flow, can truly be representations of experimental steady states. For this purpose, we have performed nonequilibrium molecular dynamics (NEMD) simulations of two different fluid systems undergoing shear flow. One system is a Lennard-Jones (LJ) fluid where the viscous heat produced by shearing the system is eliminated only in certain regions of the simulation box. The other system is a polymer immersed in an atomic solvent. In this case, the viscous heat was removed by coupling a homogeneous thermostat to different degrees of freedom in the system. The results of these simulations show that at the shear rates commonly produced in simulation, the rate of production of viscous heat is very large. This heat is eliminated by the thermostat at rates that are higher than the rates of transport of heat across the fluid. Moreover, the heat has no time to redistribute into the different degrees of freedom of the system, and different steady states are reached depending on to which degrees of freedom the thermostat is coupled. As a conclusion of this investigation we believe that the efforts of simulating fluids undergoing shear flow should be directed to simulate lower shear rates. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471327
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  • 8
    Electronic Resource
    Electronic Resource
    Chemical structure effects on the equilibrium and under shear properties of thin films in confined geometries: A molecular dynamics simulation study (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2157-2168 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have made a comparative study of confined thin fluid films, composed of either n-decane or 4-propyl-heptane. The films are studied in equilibrium and under shear using molecular dynamics (MD) simulations. The films composed of linear chains present density profiles of methylene subunits with higher degree of layering than those composed of branched molecules. There are no significant differences in the diffusion coefficients of the two molecules studied in bulk, or in confined geometries. The diffusion coefficients for the confined films are strongly dependent on the strength of the frictional forces exerted by the wall, rather than on the density of the films. They also indicate that the confined films remain in a fluidlike state in all the simulations. The bulk values of the diffusion coefficient of n-decane are in excellent agreement with the experimental data. When the confining walls move in opposite directions, the fluid films develop shear flow with a very weak shear rate. Superimposed to the shear flow, the films seem to perform an oscillatory motion, where they alternately flow following the direction of motion of either wall. The steady state values of the shear stress increase linearly with the pressure normal to the confining walls, as also found experimentally. The films composed of linear chains exhibit higher resistance to the displacement of the walls than those composed of branched chains. This is because the films composed of linear chains have higher density of methylene subunits in the region of the pore where the fluid molecules exert frictional forces on the walls. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469692
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  • 9
    Electronic Resource
    Electronic Resource
    Spinodal decomposition under shear flow (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 2342-2347 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The spinodal decomposition of a two-dimensional model binary fluid undergoing planar Couette flow has been studied by molecular dynamics simulation. The effect of the strength of the shear field on the growth of the domains was analyzed. The main effect of the shear field is the deformation of the domains which results in anisotropic structure developments. We have characterized these anisotropic structures by measuring the domain size in two different directions, the direction of the flow and the direction of the shear. We find that the dependence of the deformation of the domains on the strain applied to the system shows the same behavior as found in experiments. Moreover, we find that the shear flow can enhance the domain growth in the direction of the flow and it can restrain and even suppress this growth in the direction normal to the flow. The influence of the morphology on rheological properties was also analyzed. We find that viscosity depends on the quench time and the shear field, and is caused by the extension and direction of the interfacial area. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473788
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  • 10
    Electronic Resource
    Electronic Resource
    Characterization of proton exchange lithium niobate waveguides (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 3981-3987 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Proton exchanged samples of LiNbO3 have been profiled by micro-Raman spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering channeling, and by x-ray diffraction (XRD). Following proton exchange (PE) there are two different phases in addition to pure LiNbO3 detected by XRD. After successive annealing steps the outermost phase disappears and an interfacial region forms progressively between PE and LiNbO3. Specific vibrational bands are correlated to electro-optic and nonlinear optical properties of the system, and the recovery of these properties upon annealing is correlated to chemical bonding changes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358495
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