TY - JOUR
T1 - Interpolated variational transition-state theory
T2 - Practical methods for estimating variational transition-state properties and tunneling contributions to chemical reaction rates from electronic structure calculations
AU - Gonzalez-Lafont, Angels
AU - Truong, Thanh N.
AU - Truhlar, Donald G
PY - 1991
Y1 - 1991
N2 - In many cases, variational transition states for a chemical reaction are significantly displaced from a saddle point because of zero-point and entropic effects that depend on the reaction coordinate. Such displacements are often controlled by the competition between the potential energy along the minimum-energy reaction path and the energy requirements of one or more vibrational modes whose frequencies show a large variation along the reaction path. In calculating reaction rates from potential-energy functions we need to take account of these factors and - especially at lower temperatures - to include tunneling contributions, which also depend on the variation of vibrational frequencies along a reaction path. To include these effects requires more information about the activated complex region of the potential-energy surface than is required for conventional transition-state theory. In the present article we show how the vibrational and entropic effects of variational transition-state theory and the effective potentials and effective masses needed to calculate tunneling probabilities can be estimated with a minimum of electronic structure information, thereby allowing their computation at a higher level of theory than would otherwise be possible. As examples, we consider the reactions OH + H2, CH3 + H2, and Cl + CH 4 and some of their isotopic analogs. We find for Cl + CH4 → HCl + CH3 that the reaction rate is greatly enhanced by tunneling under conditions of interest for atmospheric chemistry.
AB - In many cases, variational transition states for a chemical reaction are significantly displaced from a saddle point because of zero-point and entropic effects that depend on the reaction coordinate. Such displacements are often controlled by the competition between the potential energy along the minimum-energy reaction path and the energy requirements of one or more vibrational modes whose frequencies show a large variation along the reaction path. In calculating reaction rates from potential-energy functions we need to take account of these factors and - especially at lower temperatures - to include tunneling contributions, which also depend on the variation of vibrational frequencies along a reaction path. To include these effects requires more information about the activated complex region of the potential-energy surface than is required for conventional transition-state theory. In the present article we show how the vibrational and entropic effects of variational transition-state theory and the effective potentials and effective masses needed to calculate tunneling probabilities can be estimated with a minimum of electronic structure information, thereby allowing their computation at a higher level of theory than would otherwise be possible. As examples, we consider the reactions OH + H2, CH3 + H2, and Cl + CH 4 and some of their isotopic analogs. We find for Cl + CH4 → HCl + CH3 that the reaction rate is greatly enhanced by tunneling under conditions of interest for atmospheric chemistry.
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U2 - 10.1063/1.461221
DO - 10.1063/1.461221
M3 - Article
AN - SCOPUS:0345116888
SN - 0021-9606
VL - 95
SP - 8875
EP - 8894
JO - The Journal of chemical physics
JF - The Journal of chemical physics
IS - 12
ER -