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We derive a closed form analytical expression for the non-adiabatic transition probability for a distribution of trajectories passing through a generic conical intersection (CI), based on the Landau-Zener equation for the non-adiabatic transition probability for a single straight-line trajectory in the CI's vicinity. We investigate the non-adiabatic transition probability's variation with topographical features and find, for the same crossing velocity, no intrinsic difference in efficiency at promoting non-adiabatic decay between peaked and sloped CIs, a result in contrast to the commonly held view. Any increased efficiency of peaked over sloped CIs is thus due to dynamical effects rather than to any increased transition probability of topographical origin. It is also shown that the transition probability depends in general on the direction of approach to the CI, and that the coordinates' reduced mass can affect the transition probability via its influence on the CI topography in mass-scaled coordinates. The resulting predictions compare well with surface hopping simulation results. [ABSTRACT FROM AUTHOR]

The connection between diffusion and solvent exchanges between first and second solvation shells is studied by means of molecular dynamics simulations and analytic calculations, with detailed illustrations for water exchange for the Li+ and Na+ ions, and for liquid argon. First, two methods are proposed which allow, by means of simulation, to extract the quantitative speed-up in diffusion induced by the exchange events. Second, it is shown by simple kinematic considerations that the instantaneous velocity of the solute conditions to a considerable extent the character of the exchanges. Analytic formulas are derived which quantitatively estimate this effect, and which are of general applicability to molecular diffusion in any thermal fluid. Despite the simplicity of the kinematic considerations, they are shown to well describe many aspects of solvent exchange/diffusion coupling features for nontrivial systems. [ABSTRACT FROM AUTHOR]

Constrained molecular dynamics simulations are carried out to compare the adsorption/incorporation mechanism of two (nonionizable) hydrogen halide acid molecules HF and HCl on/in ice at typical stratospheric temperatures (190 and 235 K). From the determination of the free energy profiles, it is shown that the free energy barrier to molecular HF incorporation is higher than that for molecular HCl. This difference is interpreted as resulting from the formation of two strong H bonds between HF and its water neighbors, while a single, more flexible, H bond with water favors the penetration for HCl. [ABSTRACT FROM AUTHOR]

Molecular Dynamics simulation is used to study the vibrational relaxation of the first excited state of the O–H stretch for HOD dissolved in D2O. The technique applied is based on a Landau–Teller type formula, in which the solvent contribution is computed classically, while the quantum nature of the solute enters through the transition moments of the molecular normal modes. The experimental result for the relaxation time (≊8 ps) is accounted for, and the pathway to the ground state is determined. The relaxation proceeds through a sequence of intramolecular transitions initially facilitated by the solute internal anharmonicities. In particular, the anharmonicity allows an initial and rate-determining transfer to the first overtone of the HOD bend; a corresponding harmonic force field calculation in which this step is precluded yields a relaxation time that is three orders of magnitude larger. The excess energy is removed by the bath modes, which include rotations and translations of all molecules, including the solute. Relaxation by Coriolis coupling plays a minor but non-negligible role, while the centrifugal coupling contribution to the relaxation is negligible. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A theory for the description of electronic structure in solution for solution phase chemical reactions is formulated in the framework of a dielectric continuum solvent model which takes solute boundary effects into account. This latter feature represents a generalization of the Kim–Hynes theory, in which the solute boundary was treated in the dielectric image approximation. The electronic structure of the molecular solute, embedded in a cavity of the dielectric, is described by a manifold of orthogonalized diabatic—e.g., valence bond (VB)—states. The polarization of the dielectric solvent is partitioned into an electronic (fast) and an orientational (slow) component. The formulation encompasses both nonequilibrium and equilibrium regimes of the orientational polarization with respect to the solute charge distribution. The analysis is carried out in the general case of quantized solvent electronic polarization, but with reference to two limits in terms of which the general results can be most readily comprehended: with the electronic polarization much slower than the solute electronic motions and equilibrated to a delocalized solute charge distribution—the self-consistent limit; with the electronic polarization fast enough to equilibrate to components of the solute electronic distribution rather than to the average distribution—the Born–Oppenheimer limit. The general results depend on the relative time scales of the resonant interconversion between the VB states and the solvent electronic polarization. With the ansatz that the nonequilibrium orientational polarization is a linear combination of equilibrium terms with nonequilibrium coefficients, the solute–solvent system free energy is obtained together with a nonlinear Schrödinger equation for the solute electronic structure. A procedure is given for the natural definition of the set of solvent coordinates which describe the nonequilibrium regime necessary... [ABSTRACT FROM AUTHOR]

The electronic structure in solution theory developed in the preceding article is applied to the molecular ion I2-[larger_closed_square]I+I- reaction system in the dipolar, aprotic solvent acetonitrile, which illustrates in detail the implementation of the general theory. A two-dimensional, nonequilibrium free energy surface in the nuclear separation and a difference solvent coordinate is constructed via solution of a nonequilibrium solvation, nonlinear Schrödinger equation. The reduction to a single important solvent coordinate—from a manifold of three solvent coordinates—is motivated by an examination of the equilibrium solvation path and an analysis of the harmonic nonequilibrium fluctuations around this path. The evolving solute electronic structure over the basis of two orthogonal valence bond diabatic states—approximately corresponding to -II and II-—is discussed. Comparisons with the limiting Born–Oppenheimer and self-consistent approximations for the solvent electronic polarization are made, with the former proving to be more accurate, and the latter giving a qualitatively inaccurate picture of the electronic structure near the equilibrium geometry. The validity of the dielectric image approximation is also examined. The polarization force associated with the charge shift in the reaction system and important for the system vibrational relaxation is also calculated. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A theory is presented for the rate constant k for electron transfer between a metal electrode and a redox couple solute in solution, in or near the electronically adiabatic regime. The departure of k from its electronically adiabatic transition state theory limit kTST is described via Grote–Hynes theory, and includes two sources of friction. The electronic friction arises from excitation of electron hole pairs in the metal, i.e., electronic nonadiabaticity effects. The solvent friction arises from solvent dynamical effects. Both features can result in significant reduction of k below kTST, and their interplay can lead to interesting nonmonotonic variations with reaction overpotential. [ABSTRACT FROM AUTHOR]

The electronic structure of a solute in a polar and polarizable solvent depends on the nonequilibrium (or equilibrium) state of the solvent. Here we present a theory for this phenomenon, at the level of a dielectric continuum description of the solvent, characterized by an orientational polarization Por and an electronic polarization Pel. The entire range of electronic coupling between solute electronic states is considered. The present theory supersedes, in important respects, our earlier work [H. J. Kim and J. T. Hynes, J. Phys. Chem. 94, 2737 (1990); J. Chem. Phys. 93, 5194 (1990); 93, 5211 (1990)] by including a full quantization of Pel; this is a feature recently shown in a model study for weak electronic coupling [J. N. Gehlen, D. Chandler, H. J. Kim, and J. T. Hynes, J. Phys. Chem. (to be published)] to be necessary for, e.g., a correct description of electron transfer activation free energies and transition states. The quantization of Pel is effected via a coherent state formulation, coupled with a multiconfiguration self-consistent representation of the solute-Pel wave function. Nonequilibrium free energies and solute electronic structure are found and depend explicitly on the comparative time scales of a transferring electron in the solute and of Pel. The corresponding equilibrium relations are also found.For activated electron transfer, the current theory recovers (and generalizes) the conventional expression for the activation free energy, in both the electronically nonadiabatic and adiabatic regimes, for small electronic coupling. For larger electronic coupling—but still within the regime of activated electron transfer—the theory predicts an increased activation free energy compared to conventional results, associated with the (partial) solvation by the electronic polarization of delocalized solute electronic structure of the transition state. This trend is the same as that previously reported by us, but is smaller in magnitude due... [ABSTRACT FROM AUTHOR]

The results of a molecular dynamics (MD) computer simulation are presented for the solvation dynamics of an ion pair instanteously produced from a neutral pair, in a model polar aprotic solvent. These time-dependent fluorescence dynamics are analyzed theoretically to examine the validity of several linear response theory approaches, as well as of various theoretical descriptions (e.g., Langevin equation) for the solvent dynamics per se. It is found that these dynamics are dominated for short times by a simple inertial Gaussian behavior, a feature which is absent in many current theoretical treatments, and which is related to the approximate validity of linear response theory. Nonlinear aspects, such as an overall spectral narrowing, but a transient initial spectral broadening, are also discussed. A model photochemical charge transfer process is also briefly considered to elucidate aspects of the connection between solvation dynamics and chemical kinetic population evolution. [ABSTRACT FROM AUTHOR]

It is shown how a dynamical theory for proton transfer rates in solution can be implemented in a molecular-dynamics simulation for a model reaction system. The reaction is in the nonadiabatic limit, in which the transfer occurs via quantum tunneling of the proton. The importance of the coupling of the proton to the solvent and to an intramolecular vibration is illustrated, and the simulation results are successfully compared with analytic rate-constant expressions in several limiting regimes. [ABSTRACT FROM AUTHOR]

A theoretical formulation is developed to describe the electronic structure of an immersed solute, electrostatically coupled to a polar and polarizable solvent. The solvent is characterized, in the dielectric continuum approximation, by electronic and orientational polarizations. Starting from a general free-energy expression for the quantum solute–solvent system, a time-independent nonlinear Schrödinger equation is derived. The nonlinearity arises from the assumed equilibration of the solvent electronic polarization Peqe, to the solute electronic wave function Ψ and the solvent orientational polarization Por. When Por is arbitrary, there is nonequilibrium solvation. When Por is equilibrated to Peqe and Ψ, equilibrium solvation obtains. The theory is illustrated for a model symmetric electron donor–acceptor solute system in a two state basis set description. Solution of the nonlinear Schrödinger equation in the presence of arbitrary Por yields nonequilibrium solvation stationary states (NSS) for the solute–solvent system, including the solvent-dependent solute electronic structure, and the associated free energies. When Por=Peqor, the corresponding equilibrium solvation states (ESS) and their characteristics are obtained. The NSS are classified into three distinct regimes, according to the relative strengths of the electronic coupling, which tends to delocalize the solute electronic distribution, and the solvent polarization, which tends to localize it. The ESS stability characteristics are also important in this classification. Two of the regimes correspond to activated electron transfer processes, and differ according to whether there is or is not a continuous free-energy path leading from localized reactants to localized products. The third regime, in which the electronic coupling dominates the solvent polarization, corresponds to stable delocalized states between which spectroscopic transitions are of interest.... [ABSTRACT FROM AUTHOR]

The formulation developed in the preceding paper [H. J. Kim and J. T. Hynes, J. Chem. Phys. 93, 5194 (1990)] is applied to describe the electronic structure and spectroscopic features of a model symmetric electron–donor–acceptor solute system D-A⇋DA- in solution in the strong coupling limit. In this limit, the electronic coupling is sufficiently strong to overcome the localizing influence of the solvent polarization, and two stable delocalized solute electronic states are found in the presence of either nonequilibrium or equilibrium solvation. The nonlinear influence of the equilibrated solvent electronic polarization and of exchange contributions to the solute electronic distribution incorporated in the theory lead to several consequences absent in standard descriptions. Among these are the necessity of two solvent coordinates to describe the system, and the prediction of solvent-dependent spectral shifts and the appearance of solvent relaxation dynamics after a Franck–Condon transition between the delocalized electronic states. Estimates of the magnitude of these new effects are provided, and the possibility for their experimental observation is briefly discussed. [ABSTRACT FROM AUTHOR]

The atom–atom encounter model developed in the accompanying paper [M. G. Sceats, J. Chem. Phys. 91, 0000 (1989)] is applied to the collisional deactivation of highly vibrationally excited triatomic molecules CS2 and SO2 by the monatomic colliders He, Ne, Ar, Kr, and Xe at 300 K. The molecular inputs are a crude normal mode analysis, vibrational frequencies and effective anharmonicities, while the collisional inputs are parameters of the atom–atom potentials. The results for CS2 are compared with the simulations of Bruehl and Schatz and the experiments of Dove, Hippler, and Troe, while those for SO2 are compared with the simulations of Schranz and Troe and the experimental results of Heymann, Hippler, and Troe. Excellent agreement is found with experiment, and the superlinear energy dependence of the average energy transfer is attributed to anharmonicity of the triatomic molecule. [ABSTRACT FROM AUTHOR]

The resonance Raman spectrum of the chlorine molecular anion Cl-2 has been obtained using pulsed laser photolysis as a radical production method. The fundamental and nine overtone transitions are observed. Analysis of the vibrational progression gives the vibrational constants ωe =277.0±0.3 cm-1 and ωeχe =1.83±0.05 cm-1; these constants give an upper limit dissociation energy of 1.3±0.1 eV (all errors are 1σ). Time-resolved resonance Raman spectroscopy has been employed to study the kinetics of Cl-2 disproportionation. A disproportionation rate coefficient of (11±7)×109 M-1 s-1 is obtained where the reported error includes 1σ precision and estimates of systematic errors. A resonance Raman detection limit for Cl-2 of 5×10-6 M has been obtained. [ABSTRACT FROM AUTHOR]

A solution reaction path Hamiltonian (SRPH) is constructed for heavy particle charge transfer reactions in a nondissipative polar solvent. This formulation includes anharmonicities arising from, e.g., potential anharmonicity and reaction path curvature. The SRPH is used in conjunction with variational transition state theory (VTST) to obtain an expression for the reaction rate constant. This expression generalizes the van der Zwan–Hynes rate theory to include the influence of anharmonic effects as well as nonequilibrium solvation. A discussion of the reaction path in solution and its departure from an equilibrium solvation perspective is given. [ABSTRACT FROM AUTHOR]

The solution reaction path Hamiltonian (SRPH) developed in the previous paper is applied to model SN2 and ionic dissociation reactions in water solvent. The solution reaction paths are determined and show marked deviations from a standard equilibrium solvation picture. The impact of potential anharmonicities, reaction path curvature, and varying solvent mass on the rate constant is calculated via the variational transition state theory approach of I, and the deviations from harmonic van der Zwan–Hynes (ZH) theory are calculated. Typically only minor deviations from ZH theory are found. The reasons for this are discussed. [ABSTRACT FROM AUTHOR]

A dynamical theory for the rates of unimolecular dissociations in polar solvents is constructed. Two classes of dissociation reactions, with dipolar and ionic transition states, are considered, and the theory is illustrated for a generalized continuum model water solvent. The rate of charge variation along the reaction coordinate is found to play a central role. Deviations from equilibrium solvation transition state theory predictions are found and discussed. Two nonequilibrium solvation regimes—nonadiabatic solvation and polarization caging—occur, and their appearance is connected to whether the solvated transition state has a reactant-like or product-like charge distribution. [ABSTRACT FROM AUTHOR]

A detailed classical trajectory study of the overtone-induced dissociation of hydrogen peroxide HO2H and its isotopic variant HO2D is presented. The factors affecting intramolecular energy flow, such as nonlinear resonances and the various couplings, are examined in detail. The dissociation lifetimes are found to be on the order of picoseconds and comparable with statistical lifetimes, although the intramolecular energy redistribution is not complete within the lifetime of the molecules. Lifetime broadening contributes very little to the rather large width of the overtone excitation feature, which we conclude is in the main inhomogeneously broadened by rotational structure instead. The implications of our results are discussed. [ABSTRACT FROM AUTHOR]

Examines the vibrational phase and energy relaxation of cyanide ion in liquid water. Adoption of molecular dynamics simulations and semiclassical perturbation theory; Role of Coulomb and non-Coulomb forces; Relevance of intermolecular terms in the vibration-solvent coupling on frequency shift.

We consider dynamical curve crossing behavior for one dimensional intermolecular potentials of arbitrary curvature. Our focus is threefold. (1) A semiclassical WKB formalism yields simple analytic expressions for time dependent transition probabilities throughout the crossing region. For low velocities v, our results reduce to the required quasistatic unitary transformation between diabatic and adiabatic representations. At higher v, the WKB solutions exhibit time dependent oscillations (‘‘beating’’ between the stationary adiabatic states) in excellent quantitative agreement with exact numerical solutions. (2) The WKB analysis indicates that an oscillatory dependence of curve crossing probability Pcr on v occurs for potential crossings of sufficient curvature, in qualitative disagreement with the extensively used Landau–Zener expression. These probability oscillations can be understood and predicted from simple considerations of potential curvature and coupling strengths. (3) We explore this oscillatory behavior for realistic curve crossing geometries. The results indicate that a nonmonotonic dependence of Pcr on velocity may occur in real molecular systems and could be experimentally observed in favorable albeit special circumstances. Even in the absence of oscillations, however, pronounced departure from Landau–Zener predictions can be anticipated. [ABSTRACT FROM AUTHOR]

The vibrational energy relaxation of a model methyl chloride molecule in water is studied through equilibrium and nonequilibrium molecular dynamics simulations. Previous work [Whitnell, Wilson, and Hynes, J. Phys. Chem. 94, 8625 (1990)] has demonstrated the validity of a Landau–Teller formula for this system in which the relaxation rate is equal to the frequency-dependent friction that the solvent exerts on the bond. In the present work, an analysis of this friction is used to test the isolated binary interaction (IBI) approximation for vibrational energy relaxation. In this system, where long-range electrostatic Coulomb forces dominate the interaction between the water solvent and the CH3Cl molecule, we show that the binary approximation to the friction only partially accounts for the rapid relaxation of the vibrational energy. We attribute the importance of cross correlations between different solvent molecules to the overlap of the CH3Cl vibrational frequency with the librational band of the water solvent. The dominance of the long-range Coulomb forces is further explored in nonequilibrium simulations. The vibrational energy relaxation is effected by a hysteresis in the Coulomb forces that the solvent exerts on the solute such that the force as the CH3Cl bond compresses is different from that as it expands. The non-Coulomb forces do not show this hysteresis to any significant extent. This hysteresis is reflected in the spatial distributions for the average dipole moment of the water solvent molecules. These spatial distributions also show that a large number of solvent molecules participate in the energy flow out of the CH3Cl molecule and that most of these important molecules are at positions perpendicular to the CH3Cl bond. The overall picture we develop here is of a process that is more complex than a simple binary interaction description can accurately portray. [ABSTRACT FROM AUTHOR]

Results are presented from a computer simulation of the dynamics of a model SN1 reaction in water, very loosely based on the reaction t-BuCl→t-Bu++Cl-. Two diabatic electronic states are considered, covalent and ionic, which cross in the presence of the polar solvent. The curve crossing is treated in the electronically adiabatic limit, which gives rise to coupled reagent and solvent dynamics involving a mixed covalent/ionic adiabatic potential surface. The reaction dynamics are analyzed in terms of a simple solute reaction coordinate defined to be the t-Bu to Cl separation distance. By employing constraint dynamics techniques, the potential of mean force is determined as a function of this reaction coordinate. The time evolution of the reaction is followed in terms of the full molecular dynamics of all reagent and solvent atoms. Beginning with the largely covalently bound reactant t-BuCl, the following was observed: (i) how energy flows out of the water solvent bath into a solvent–reactant fluctuation driving the system to the top of the barrier, (ii) how barrier recrossings lower the reaction rate below the transition state theory prediction, and (iii) how the products slide down the barrier toward separated t-Bu+ and Cl-, dissipating their excess energy back into the solvent. The calculated transmission coefficient measuring the departure of the rate constant from its transition state theory value is 0.53±0.04. This is found to agree with the Grote–Hynes theory prediction, and also with its nonadiabatic solvation, frozen solvent limit, to within the estimated error bars. By contrast, Kramers’ theory incorrectly predicts a much smaller value. [ABSTRACT FROM AUTHOR]

The reaction rate and mechanism of the interconversion between a contact ion pair and solvent separated ion pair in model polar solvents is investigated by molecular dynamics (MD) simulation. The full rate constant for the model reaction is estimated from the product of the transition state theory (TST) rate constant, determined from the potential of mean force between the ions in an equilibrium solvent, and the transmission coefficient, which depends on the details of the dynamics. The collective character of the solvent motion in the reaction barrier crossing is examined in some detail, and the important role of solvent dynamics in causing the reaction rate to markedly deviate from the TST rate is discussed. The MD results are compared with the predictions of Kramers and Grote–Hynes theories. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations of the model SN2 reaction Cl-+CH3Cl→ClCH3+Cl- in water, and variants thereof, are presented. The resulting transmission coefficients κ, that measure the deviations of the rates from the transition state theory (TST) rate predictions due to solvent-induced recrossings, are used to assess the validity of the generalized Langevin equation (GLE)-based Grote–Hynes (GH) theory. The GH predictions are found to agree with the MD results to within the error bars of the calculations for each of the 12 cases examined. This agreement extends from the nonadiabatic regime, where solvent molecule motions are unimportant and κ is determined by static solvent configurations at the transition state, into the polarization caging regime, where solvent motion is critical in determining κ. In contrast, the Kramers theory predictions for κ fall well below the simulation results. The friction kernel in the GLE used to evaluate the GH κ values is determined, from MD simulation, by a fixed-particle time correlation function of the force at the transition state. When this is expressed as a (Fourier) friction spectrum in frequency, marked similarities to the pure solvent spectrum are observed, and are used to identify the water solvent motions that determine the transmission coefficient κ. The deviations of κ from unity, the TST value, are dominated by solvent motions (translational and reorientational) which on the time scale of the recrossings are essentially static configurations. The deviations from the frozen solvent, nonadiabatic limit values κNA are dominated by the hinderd rotations (librations). Finally, the underlying assumptions of the GLE and the GH theory are discussed within the context of the simulation results. [ABSTRACT FROM AUTHOR]

The technique of time-resolved infrared–ultraviolet double resonance (IRUVDR) spectroscopy is used to characterize the rate and mechanism of state-to-state rotational energy transfer (RET) in D2CO/D2CO collisions. The investigations employ CO2-laser irradiation to prepare a D2CO molecule in the v4=1, (J,Ka) =(18,11) rovibrational level of its X 1A1 electronic ground state. Vapor-phase collisions with other D2CO (v=0) molecules then induce RET, with IRUVDR-monitored quantum-number changes ΔJ for the state-selected molecule ranging between +3 and -7. Kinetic modeling of the resulting experimental data shows that the inelastic cross sections for such J-changing rotational relaxation can be described adequately by simple scaling laws based on the rotational energy change |ΔE| for the state-selected molecule, with a power-gap fitting law proving marginally superior to an exponential-gap fitting law. The range of |ΔJ| monitored in these experiments is sufficiently extensive to discredit a simple propensity-rule fitting law, comprising consecutive collision-induced processes with individual changes |ΔJ| confined to values of 1 or 2. The microscopic rate constants derived reflect the dominance of ΔJ=±1 contributions for J-changing RET in D2CO/D2CO collisions, owing to long-range dipole/dipole interactions. These results elucidate RET in collisions between a pair of dipolar polyatomic (D2CO) molecules at a level of detail usually confined to studies of dipolar diatomic molecules, such as HF. Less detailed IRUVDR results, for RET in self-collisions of HDCO and for D2CO colliding with a variety of foreign-gas molecules, are also presented. [ABSTRACT FROM AUTHOR]

Molecular dynamics are computed for a model SN2 reaction Cl-+CH3Cl→ClCH3+Cl- in water and are found to be strongly dependent on the instantaneous local configuration of the solvent at the transition state barrier. There are significant deviations from the simple picture of passage over a free energy barrier in the reaction coordinate, and thus, a marked departure from transition state theory occurs in the form of barrier recrossings. Factors controlling the dynamics are discussed, and, in particular, the rate of change of atomic charge distribution along the reaction coordinate is found to have a major effect on the dynamics. A simple frozen solvent theory involving nonadiabatic solvation is presented which can predict the outcome of a particular reaction trajectory by considering only the interaction with the solvent of the reaction system at the gas-phase transition barrier. The frozen solvent theory also gives the transmission coefficient κ needed to make the transition state theory rate agree with the outcome of the molecular dynamics trajectories. This theoretical κ value, which is the implementation for the SN2 reaction of the van der Zwan–Hynes nonadiabatic solvation transmission coefficient, is in good agreement with the trajectory results. In contrast, a Kramers theory description fails dramatically. [ABSTRACT FROM AUTHOR]

An analytic theory for SN2 reactions in polar solvents in the nonadiabatic solvation limit is presented and used to interpret the computer simulation results of the preceding paper by Bergsma et al. The theory is based on the nonadiabatic solvation limit of previous studies by van der Zwan and Hynes and incorporates the solvent approximately but explicitly via a coordinate additional to the intrinsic reaction coordinate. Central results include: an explicit expression for the reaction transmission coefficient κ, the dependence of reaction probability on kinetic energy, the interpretation of κ in terms of nonequilibrium solvation entropy effects, and the deviation of the reaction coordinate from that assumed in the standard equilibrium solvation transition state theory view of the reaction. [ABSTRACT FROM AUTHOR]

Molecular dynamics are computed for model atom transfers A+BC→AB+C in rare gas solvents at liquid densities. We find that the reaction dynamics can be understood in terms of a simple picture which consists of three stages: (1) activation of reactants, (2) barrier crossing, and (3) deactivation of products. The effects seen in stages (1) and (3) can be largely interpreted in terms of existing models of energy and phase decay in solution, while the effects seen in stage (2) can be largely interpreted in terms of gas phase A+BC barrier crossing dynamics. We find that transition state theory is in perfect agreement with the simulations for the 20 and 10 kcal/mol barrier reactions and is a very good description for a 5 kcal/mol reaction barrier. At low barrier curvature, dynamical effects due to the solvent are shown to induce some recrossings of the transition state barrier, thus causing rate constants calculated by simple transition state theory to be slightly too high. The Grote–Hynes modification of transition state theory, which considers the effect of the time dependent friction of the solvent on the dynamics at the transition state, predicts corrections to the rate constants in good agreement with the results from the simulations. [ABSTRACT FROM AUTHOR]