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  • 1
    ISSN: 1572-9001
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Ab initio molecular orbital (MO) computations on the closed shell singlet ground state ofN, N-dimethylaminobenzonitrile (DMABN) are reported. Fully optimized structures of several conformers of DMABN were calculated at the HF/6-31G level of theory. Our results indicate that for each of these conformations the minimum energy structure has a trigonal (sp2 hybridized] amino nitrogen. The most stable DMABN conformer was found to be planar with its methyl groups eclipsed. The Koopmans ionization potentials and dipole moments of the various ground state conformers are compared. The implications for dynamical models of twisted-intramolecular charge transfer (TICT) are discussed. Moreover, the use of qualitative MO theory arguments provides an interpretation of the computational results in a simple orbital interaction framework.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7558-7559 
    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.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 5046-5057 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Small atomic or molecular clusters provide the bridge between vapor and liquid phases. Nucleation is a rare event process by which clusters of a new phase are produced. This process is inherently dynamic and as such the new phase cannot exist until an activation barrier is surmounted. Dynamical nucleation theory (DNT) utilizes variational transition state theory to provide a framework in which cluster evaporation and condensation rate constants can be determined directly. To date, the fundamental nature regarding the intrinsic instability of the kinetics of the nucleation process has eluded theoretical efforts. In this paper we present a sensitivity analysis of the homogeneous nucleation rate on kinetic parameters used in DNT. Moreover, several classical interaction potentials for water exist, most of which have been parametrized to reproduce some bulk properties of water at ambient conditions. Thus, an analysis was undertaken to explore what effects different water potentials have on the dynamical quantities relevant to nucleation. The implication of these results on future work will be discussed. © 2002 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 1674-1684 
    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.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 8644-8653 
    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.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 8525-8536 
    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)].
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 9116-9137 
    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).
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  • 8
    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.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5171-5178 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The centroid molecular dynamics technique is applied to the case of chloride–water clusters to estimate their finite temperature quantum vibrational structure. We employ the flexible RWK2 water potential [J. R. Reimers, R. O. Watts, and M. L. Klein, Chem. Phys. 64, 95 (1982)] and the parametrization of a chloride–water interaction potential of Dorsett, Watts and Xantheas [J. Phys. Chem. A 103, 3351 (1999)]. We then investigate the temperature-dependent vibrational structure (infrared spectra). We find that the centroid molecular dynamics technique is capable of recovering a majority of the red shift associated with hydrogen bonding. © 2000 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 4688-4697 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In previous work we began the description of a molecular theory of homogeneous vapor-to-liquid nucleation based on the kinetics of cluster formation and decomposition. In this work we focused on a new theoretical approach to calculating rate constants for evaporation of molecules from clusters. In the present work, we present a molecular theory for calculating condensation rate constants that are consistent with the evaporation rate constants. The new method, which uses variational transition state theory (VTST), provides an expression for the evaporation rate constant that is proportional to the derivative of the Helmholtz free energy for cluster formation with respect to the radius of the spherical volume constraining the cluster. Furthermore, the theory provides a physically justified procedure for selecting a unique value of the radius of the spherical volume for each i-molecule cluster. Since VTST obeys detailed balance, condensation rate constants can be obtained from the evaporation rate constants and the corresponding equilibrium constants. In the present work, we provide a theoretical approach to obtain the equilibrium constants that are consistent with the evaporation rate constants. Monte Carlo methods are presented for calculating the dependence of the Helmholtz free energy of cluster formation on the radius of the constraining volume, which are needed for the evaporation rate constants. In addition, Monte Carlo methods are presented for calculating the relative differences in Helmholtz free energies for clusters of different sizes, which are needed for the equilibrium constants and condensation rate constants. The volume dependent Helmholtz free energies for the water dimer up to the decamer are calculated at 243 K. © 1999 American Institute of Physics.
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