Potential energy surfaces variational transition state theory

In this contribution, we present a preliminary account of our recent work on the Cl + H2 and Cl + D2 reactions, which includes a new potential energy surface, variational transition state theory and semiclassical tunneling calculations for both reactions and for other isotopomeric cases, and accurate quantum dynamical calculations of rate constants and state-to-state integral and differential cross sections. 

Variational transition state theory (VTST) is formulated around a variational theorem, which allows the optimization of a hypersurface (points on the potential energy surface) that is the elfective point of no return for reactions. This hypersurface is not necessarily through the saddle point. Assuming that molecules react without a reverse reaction once they have passed this surface... 

R. Sayos, J. Hernando, J. Flijazo, M. Gonzalez, An analytical potential energy surface of the HFCL(2A ) system based on ab initio calculations. Variational transition state theory study of the H+C1F F+HC1,C1+HF, and F+HCl Cl+HF reactions and their isotope variants, Phys. Chem. Chem. Phys. 1 (1999) 947. 

A further advance occurred when Chesnavich et al. (1980) applied variational transition state theory (Chesnavich and Bowers 1982 Garrett and Truhlar 1979a,b,c,d Horiuti 1938 Keck 1967 Wigner 1937) to calculate the thermal rate coefficient for capture in a noncentral field. Under the assumptions that a classical mechanical treatment is valid and that the reactants are in equilibrium, this treatment provides an upper bound to the true rate coefficient. The upper bound was then compared to calculations by the classical trajectory method (Bunker 1971 Porter and Raff 1976 Raff and Thompson 1985 Truhlar and Muckerman 1979) of the true thermal rate coefficient for capture on the ion-dipole potential energy surface and to experimental data (Bohme 1979) on thermal ion-polar molecule rate coefficients. The results showed that the variational bound, the trajectory results, and the experimental upper bound were all in excellent agreement. Some time later, Su and Chesnavich (Su 1985 Su and Chesnavich 1982) parameterized the collision rate coefficient by using trajectory calculations. 

In chemical dynamics, one can distinguish two qualitatively different types of processes electron transfer and reactions involving bond rearrangement the latter involve heavy-particle (proton or heavier) motion in the formal reaction coordinate. The zero-order model for the electron transfer case is pre-organization of the nuclear coordinates (often predominantly the solvent nuclear coordinates) followed by pure electronic motion corresponding to a transition between diabatic electronic states. The zero-order model for the second type of process is transition state theory (or, preferably, variational transition state theory ) in the lowest adiabatic electronic state (i.e., on the lowest-energy Bom-Oppenheimer potential energy surface). 

In our recent work [89], the reaction of HO2 with CIO has been investigated by ab initio molecular orbital and variational transition state theory calculations. The geometric parameters of the reaction system HO2 + CIO were optimized at the B3LYP and BH HLYP levels of theory with the basis set, 6-311+G(3df,2p), which can be found in Ref. [89]. Both singlet and triplet potential energy surfaces were predicted by the G2M method, as shown in Fig. 24. 

Joseph, T.R., Steckler, R. and Truhlar, D.G. (1987) A new potential energy surface for the CH3 + H2 CH + H reaction Calibration and calculation of rate constants and kinetic isotope effects by variational transition state theory and semi-classical tunneling calculations, J. Chem. Phys. 87, 7036-7049. 

D. G. Truhlar, K. Runge, and B. C. Garrett, Variational transition state theory and tunneling calculations of potential energy surface effects on the reaction of 0( P) with H2, Twentieth Symposium (International) on Combustion, Combustion Institute, Pittsburgh, 1984, p. 585. 

G. C. Lynch, R. Steckler, D. W. Schwenke, A. J. C. Varandas, D. G. Truhlar, and B. C. Garrett, Use of scaled external correlation, a double many-body expansion, and variational transition state theory to calibrate a potential energy surface for FH2, J. Chem. Phys. 94 7136 (1991). 

Chapter 2, Michael L. McKee and Michael Page address an important issue for bench chemists how to go from reactant to product. They describe how to compute reaction pathways. The chapter begins with an introduction of how to locate stationary points on a potential energy surface. Then they describe methods of computing minimum energy reactions pathways and explain the reaction path Hamiltonian and variational transition state theory. 

In this Section we discuss and compare the results obtained using the various theories of Section 3, and a variational transition state theory with a tunnelling correction [9]. The reaction we concentrate on is H+BrH - HBr+H. We emphasize the rate constants, but also discuss reaction cross sections. The potential energy surface we used in the H+BrH computations is of a semiempirical diatomics in molecules type, the form of which (called DIM-3C) is due to Last and Baer [ll]. The surface contains a three-centre integral term that has been parameter-ised [33] by comparing ESA-CSA calculations with an experimental [3] room temperature rate constant for the D+BrH -) DBr+H reaction. The minimum potential energy path is collinear, and there is a strong... 

In the present work, information about the potential energy surfaces for these systems is obtained by the BAC-MP4 method [28-33]. This method has been very successful for predicting the thermochemistry of molecules and radical species, and has been extended to calculating the potential information along reaction paths needed for the variational transition state theory calculations. In the latter case, the method has been shown to be capable of quantitative predictions for a gas phase chemical reaction [33]. In the present study our interests are in estimates of the order of magnitude of reaction rates, and in studies of qualitative trends such as the effect of cluster size on the magnitude of quantum tunneling. The methods employed here are more than adequate for these types of studies. 

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