Variational transition state theory vibrator

B. C. Garrett and D. G. Truhlar, WKB approximation for the reaction-path Hamiltonian Application to variational transition state theory, vibrationally adiabatic excited-state barrier heights, and resonance calculations,/. Chem. Phys. 81 309 (1984). 

Variational Transition State Theory, Vibrationally Adiabatic Potential Curves, and Tunneling... 

A. W. Magnuson, and J. N. L. Connor, Variational transition state theory, vibrationally adiabatic transmission coefficients, and the unified statistical model tested against accurate quantal rate constants for collinear F 4- H2, H 4- F2, and isotopic analogs, J. Chem. Phys. 73 1721 (1980). 

Both approaches include tunneling corrections and provide approximately the same accuracy. However, the variational transition state theory is computationally quite demanding, and at least 40 points on the path of the proton transfer should be available. In contrast, the instanton approach uses only vibrational frequencies calculated for local minima and transition states and corresponding values of energy. 

The inclusion of s(v) in the definition of separatrix involves the assumption that, as in adiabatic variational transition state theory, the diatom remains in the same vibrational state throughout the slow van der Waals bond stretching and breaking process. That is, the vibrational quantum number v of the diatom in the... 

The variationally optimized transition state geometries were found to be different for transfer of a proton or a deuteron, the first indication of such a difference for an enzyme reaction [67]. Quantum treatment of vibrations was found to be important for the calculation of the rate constant, and variational transition state theory was important for calculating kinetic isotope effects. The... 

The RBU model can be used to study the effect of exciting the vibrational modes treated within the model. For the reactions X (X=C1, 0 and H) + CH4 HX + CH3 we find that exciting a vibrational inode results in a lower threshold to reaction. It was also found that exciting the reactive C-H stretch enhances the reactivity more than exciting the CH4 umbrella mode. Vibrational enhancements for the umbrella and C-H stretch vibrations have also been found in other studies of the dynamics[75, 80] and in canonical variational transition state theory (C T) calculations [84]. Enhancement of the Cl + CH4 reaction due to vibrational excitation of the H-CH3 stretch has also been confirmed b experimental measurements by Zare and coworkers[85]. 

Computational methods now exist that include contributions from all vibrational modes to the H/D-transfer process, thus eliminating the need to introduce any empirical parameters, e.g., variational transition state theory with semiclassical tunneling corrections (Truhlar, D. G. Garett, B. C. Klippen-stein, S. J.J. Phys. Chem. 1996, 100, 12771) and the approximate instanton method (Siebrand, W Smedarchina, Z. Zgierski, M. Z. Femandez-Ramos, A. Int. Rev. Chem. Phys. 1999, 18, 5). 

The partition functions and numbers of accessible states can all be calculated from the reaction path data, (j) and F(s), assuming harmonic vibrations and separation of vibration and rigid-body rotation. The vibrational partition function may be improved in accuracy by accounting for anharmonicity in some modes. This has been done simply in a separable mode approximation (e.g., Morse stretches and quartic terms in the bending potentials) [129-131]. There are now a number of examples of applications of various forms of canonical variational transition-state theory using ab initio reaction path calculations [15,106,108,112,131-142]. 

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