Unimolecular reaction tunneling effect

The effects of QMT at cryogenic temperatures can be quite spectacular. At extremely low temperatures, even very small energy barriers can be prohibitive for classical overbarrier reactions. For example, if = Ikcal/mol and A has a conventional value of 10 s for a unimolecular reaction of a molecule, Arrhenius theory would predict k = 2 X 10 ° s , or a half-life of 114 years at lOK. But, many tunneling reactions of reactive intermediates have been observed to occur at measurable rates at this and lower temperatures, even when energy barriers are considerably higher. Reactive intermediates can, thus, still be quite elusive at extremely low temperatures if protected only by small and narrow energy barriers. 

Classical trajectories are the only feasible means to explicitly treat all atoms in a dynamical study of a unimolecular reaction. Trajectories have been used extensively to interpret A + BC bimole-cular reactions and a considerable amount of literature exists with respect to these studies. Excitation functions, scattering angles, product energy distributions, and other dynamical properties are usually quantitatively determined by the trajectory calculations. The semiclassical studies of Marcus and Miller have in general confirmed the accuracy of classical trajectories in calculating dynamical properties for bimolecular reactions. However, the trajectories do not describe quantum mechanical effects such as interferences, tunneling, and nonadiabatic electronic transitions. 

The first application made of this approach has been to tunneling effects in unimolecular rate constants. Specifically, the microcanonical rates k(E) for the reactions... 

Very recently, some attempts were undertaken to uncover the intimate mechanism of the cation radical deprotonation. Thus, the reaction of 9-methyl-10-phenylanthracene cation radical with 2,6-lutidine was studied (Lu et al. 2001). The reaction takes place by a two-step mechanism that involves the intermediate formation of a cation radical/base complex prior to unimolecular proton transfer and separation of products. Based on the value of the kinetic isotope effect observed, it was concluded that extensive proton tunneling is involved in the proton-transfer reaction. The assumed structure of the intermediate complex involves 77-bonding between the unshared electron pair on nitrogen of the lutidine with the electron-deficient 77-system of the cation radical. 

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