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  • 1990-1994  (23)
  • 1
    Electronic Resource
    Electronic Resource
    Twisted intramolecular charge transfer and the torsional potential function of dimethylaminobenzonitrile (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7558-7559 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent experimental and theoretical studies of the twisted intramolecular charge transfer process and its interdependence with the torsional potential for dimethylaminobenzonitrile (DMABN) are discussed. It is shown that while the available experimental and theoretical evidence does not effectively discriminate between an untwisted or twisted ground state for DMABN in either the gas phase or solution, strong indirect evidence exists for a twisted DMABN ground state in the presence of a polar solvent.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460190
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  • 2
    Electronic Resource
    Electronic Resource
    Variational solutions for the thermal and real time propagator using the McLachlan variational principle (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 6570-6577 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new approximation to the propagator is presented. The approximation as applied to the thermal propagator (coordinate space density matrix) is obtained by using an analog of the McLachlan variational principle for the solution of the Bloch equation. The approximation as applied to the real time propagator is obtained by using the McLachlan variational principle for the solution of the time-dependent Schrödinger equation. The approximate coordinate space density matrix has the same functional form of the high temperature limit of the density matrix, while the approximate real time propagator has the same functional form as the short time propagator. We present numerical results for the thermal propagator for several test systems and compare these results to previous work of Zhang, Levy, and Freisner [Chem. Phys. Lett. 144, 236 (1988)], Mak and Andersen [J. Chem. Phys. 92, 2953 (1990)], and Cao and Berne [J. Chem. Phys. 92, 7531 (1990)]. We also present numerical results for the approximate real time propagator for several test systems and compare to the exact results and results obtained by Gaussian wave packet propagation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467066
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  • 3
    Electronic Resource
    Electronic Resource
    Tunneling in the Presence of a Bath: A Generalized Transition State Theory Approach (1994)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 98 (1994), S. 8396-8405 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100085a021
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  • 4
    Electronic Resource
    Electronic Resource
    Reversible work based quantum transition state theory (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 8964-8971 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical basis is presented for reversible work evaluation of transition rates within the framework of transition state theory. The method involves computing statistical averages of forces without having to evaluate transition state partition functions or densities, and therefore eliminates the need for a harmonic reference system. The method can be applied to systems of high dimensionality which is particularly important in calculations on quantum systems, where each quantum particle may be represented by several images in a Feynman path integral chain. The relationship between this method and the fixed centroid method of Gillan and centroid density theories is established. The various methods are compared on a model quantum system consisting of an Eckart barrier coupled to a harmonic oscillator.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468447
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  • 5
    Electronic Resource
    Electronic Resource
    Centroid-density quantum rate theory: Variational optimization of the dividing surface (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 8525-8536 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A generalization of Feynman path integral quantum activated rate theory is presented that has classical variational transition state theory as its foundation. This approach is achieved by recasting the expression for the rate constant in a form that mimics the phase-space integration over a dividing surface that is found in the classical theory. Centroid constrained partition functions are evaluated in terms of phase-space imaginary time path integrals that have the coordinate and momenta centroids tied to the dividing surface. The present treatment extends the formalism developed by Voth, Chandler, and Miller [J. Chem. Phys. 91, 7749 (1989)] to arbitrary nonplanar and/or momentum dependent dividing surfaces. The resulting expression for the rate constant reduces to a strict variational upper bound to the rate constant in both the harmonic and classical limits. In the case of an activated system linearly coupled to a harmonic bath, the dividing surface may contain explicit solvent coordinate dependence so that one can take advantage of previously developed influence functionals associated with the harmonic bath even with nonplanar or momentum dependent dividing surfaces. The theory is tested on the model two-dimensional system consisting of an Eckart barrier linearly coupled to a single harmonic oscillator bath. The resulting rate constants calculated from our approximate theory are in excellent agreement with previous accurate results obtained from accurate quantum mechanical calculations [McRae et al., J. Chem. Phys. 97, 7392 (1992)].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464511
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  • 6
    Electronic Resource
    Electronic Resource
    Dynamic solvent effects on activated chemical reactions. I. Classical effects of reaction path curvature (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 9116-9137 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In gas phase reactions, dynamical recrossings across a phase space dividing surface induced by nonlinear reaction path curvature coupling leads to the breakdown of the fundamental dynamical approximation of classical transition state theory (TST). In the following study, we examine the nature of this breakdown for chemical reaction dynamics occurring in solution. As a model system, we consider the collinear A+BC reaction where reaction path curvature increases as the mass of B becomes small compared to the mass of A and C. We use a London–Eyring–Polanyi–Sato (LEPS) potential to describe the solute interaction and model the influence of the solvent by using a generalized Langevin equation that is further represented by a system of coupled harmonic oscillators. Exact classical rate constants are compared to those obtained from conventional TST and canonical variational transition state theory (CVT) as a function of solvent friction coupling. A harmonic TST analysis at the saddle point of the full system (solute plus solvent) with an optimum dividing surface containing both solute and solvent degrees of freedom returns the Grote–Hynes expression for the rate. For the case of no solvent coupling, both TST and CVT are identical and fail to account for the dynamical recrossings induced by reaction path curvature. At intermediate couplings, CVT provides an improvement to the TST estimate and agrees with dynamical simulation results. All estimates of the rate constant approach each other asymptotically at large couplings. The results are interpreted in terms of recrossings in the extended system (solute and solvent coordinates).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463970
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  • 7
    Electronic Resource
    Electronic Resource
    Critical comparison of approximate and accurate quantum-mechanical calculations of rate constants for a model activated reaction in solution (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 7392-7404 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accurate quantum-mechanical calculations of rate constants for a model of reaction in solution are used as benchmarks for two approximate methods: variational transition-state theory with semiclassical corrections for reaction coordinate motion, and the path-integral centroid density method. The reaction model corresponds to a single solute coordinate coupled to a harmonic bath mode. When the harmonic frequency of the bath oscillator is sufficiently high, the results of the approximate methods agree well with the accurate quantum-mechanical ones. For the lowest-frequency bath oscillator considered, the agreement is not as good, but still satisfactory; the worst discrepancies are a factor of 2.0 for the centroid density methods and a factor of 3.3 for variational transition-state theory with semiclassical tunneling corrections. Applications of the approximate methods to models including up to ten bath oscillators indicate that a single bath oscillator provides a reasonable model of a converged harmonic bath.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463511
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  • 8
    Electronic Resource
    Electronic Resource
    Centroid-density quantum rate theory: Dynamical treatment of classical recrossing (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 1674-1684 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method is presented for the calculation of quantum mechanical rate constants for activated processes. This method is a hybrid approach involving Feynman path integrals and classical dynamics that is an extension of previous work of Messina, Schenter, and Garrett [J. Chem. Phys. 98, 8525 (1993)]. We make an ansatz for the quantum mechanical analog to the classical flux correlation function expression for the rate constant. This expression involves an imaginary-time, phase-space Feynman path integral, with the dividing surface and characteristic function expressed as a function of the phase-space centroid variables. The reactive flux correlation function is obtained from a classical-like expression in which the characteristic function is evaluated by evolving the phase-space centroid variables as if they were classical dynamical variables. We show that the theory gives exact analytic results in the high temperature and harmonic limits. The theory is further tested on a model anharmonic two-dimensional system of an Eckart barrier coupled to a harmonic oscillator. The results of the theory compare favorably to accurate numerical calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465284
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  • 9
    Electronic Resource
    Electronic Resource
    Quantum activated rate theory: Variational optimization of planar dividing surfaces (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 8644-8653 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A variational procedure is presented for finding the optimal planar dividing surface within a centroid-density based quantum rate theory for the model of a general reaction coordinate coupled to a harmonic bath. The approach described here is a limiting form of the method for choosing the best coordinate and momentum dependent dividing surfaces that was previously presented by the authors [J. Chem. Phys. 98, 8525 (1993)]. The present approach can also be considered a direct quantum mechanical generalization of the classical variational method of Berezhkovskii, Pollak, and Zitserman [J. Chem. Phys. 97, 2422 (1992)]. We also relate this method to the analytical approach of Voth [Chem. Phys. Lett. 170, 289 (1990)] that incorporates a transmission coefficient in the centroid-density based quantum rate theory. The variational procedure is also applicable to systems coupled to a continuum of oscillators, and it is shown that this procedure can be efficiently implemented for an arbitrary number of oscillators in the bath. Numerical results are presented for an Eckart barrier coupled to a bath of harmonic oscillators. Numerical results show that a strict variational optimization of the planar dividing surface offers some improvement for the rate constants relative to those of the analytic theory of Voth, thus justifying the extra work needed for the variational search.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465588
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  • 10
    Electronic Resource
    Electronic Resource
    Inclusion of nonequilibrium continuum solvation effects in variational transition state theory (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 5756-5770 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We define a general set of effective solvent coordinates for reactions in solution that should be especially well suited for polar solvents. No specific functional form is assumed for the variation of charges with motions along or orthogonal to the reaction coordinate, and as a consequence the formalism allows the treatment of arbitrary reaction types using electronic structure calculations for the solute charge distribution. A critical assumption of the theory is that the nonequilibrium polarization field may be represented in terms of the adiabatic polarization fields that arise for linear geometric displacements. The results are combined with the Cartesian-coordinate canonical variational transition state theory presented previously to provide a general method for including both equilibrium and nonequilibrium solvation effects of a nondissipative, continuum-dielectric solvent in practical calculations of rate constants.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464868
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