Multi-path variational transition state theory for chemical reaction rates of complex polyatomic species: ethanol + OH reactions

Abstract

Complex molecules often have many structures (conformations) of the reactants and the transition states, and these structures may be connected by coupled-mode torsions and pseudorotations; some but not all structures may have hydrogen bonds in the transition state or reagents. A quantitative theory of the reaction rates of complex molecules must take account of these structures, their coupled-mode nature, their qualitatively different character, and the possibility of merging reaction paths at high temperature. We have recently developed a coupled-mode theory called multi-structural variational transition state theory (MS-VTST) and an extension, called multi-path variational transition state theory (MP-VTST), that includes a treatment of the differences in the multi-dimensional tunneling paths and their contributions to the reaction rate. The MP-VTST method was presented for unimolecular reactions in the original paper and has now been extended to bimolecular reactions. The MS-VTST and MP-VTST formulations of variational transition state theory include multi-faceted configuration-space dividing surfaces to define the variational transition state. They occupy an intermediate position between single-conformation variational transition state theory (VTST), which has been used successfully for small molecules, and ensemble-averaged variational transition state theory (EA-VTST), which has been used successfully for enzyme kinetics. The theories are illustrated and compared here by application to three thermal rate constants for reactions of ethanol with hydroxyl radical--reactions with 4, 6, and 14 saddle points.

Measurements of Rate Coefficients for Reactions of OH with Ethanol and Propan-2-ol at Very Low Temperatures

“…6 Theoretical studies on this system have shown that this pathway has the lowest barrier to hydrogen abstraction and R2c has the highest. [18][19][20][21] Kinetic studies of the reaction of OH with various deueterated analogues of propan-2-ol were performed by Dunlop and Tully between 293-502 K. 4 Analysis of the observed kinetic isotope effects demonstrated that abstraction of the alpha hydrogen (R3b) dominated, but with a significant contribution from the beta abstraction channel (R3a) that increases with temperature.…”

Transition state theory for enzyme kinetics

“…For reactions of complex molecules in the gas-phase, there are usually many transition states and a proper treatment involves a multi-faceted dividing surface that passes through them all. 6,7 If it passes through all the saddle points oriented normally to their imaginary-frequency normal modes, such a treatment is called multi-structural conventional transition state theory or multi-path conventional transition state theory (where the difference is in how the transmission coefficient is calculated). If the multi-faceted transition state is optimized, such a treatment is called multi-structural variational transition state theory or multi-path variational transition state theory (where the difference again is in how the transmission coefficient is calculated).…”

“…6,7 We can then carry out a complete analysis of the nuclear motion and configurations by classical mechanics (for example, vibrations might be treated by the classical mechanical harmonic oscillator approximation) or, if the system is not too large, by quantum mechanics (for example, vibrations might be treated by the quantum mechanical harmonic oscillator approximation). Clearly that is impossible for a liquid or an enzyme in solution, where it is not practical to even think about all the structures, and we are forced to use statistical mechanical sampling rather than full enumeration of structures.…”

A Theoretical Study of the X-Abstraction Reactions (X = H, Br, or I) from CH2IBr by OH Radicals: Implications for Atmospheric Chemistry

“…We have assumed in the complex mechanism that a thermal equilibrium distribution of rovibrational energy levels is maintained, which corresponds to the high-pressure limiting behavior, and that all these levels, from the bottom of the well of the complex up to the top of the barrier, contribute to tunneling [100]. On the contrary, at low pressure limit, the lack of collisional stabilization causes that none of reactant complexes can reach energies below reactants, and the Γ (T ) value is the same as when tunneling is calculated from the isolated reactants.…”