Extended Gorin Model for Radical-Radical Recombination Reactions

Abstract

Radical-radical recombination reactions (e.g., CH3 + CH3 reversible arrow C2H6) proceed with no barrier through simple-fission transition states. The application of transition state theory (TST) to these reactions is discussed, achieving a new understanding of the dividing surface and dynamical assumption implicit in all TST treatments of these reactions. A reinterpretation of the modified Gorin model for such transition states is discussed which removes several inconsistencies from this model and greatly improves data prediction and interpretation for radical-radical recombination reactions (and the reverse unimolecular dissociations) in the gas phase. The suggested model is an extension of the basic Gorin approach, which treats the transition state as consisting of two moieties which have the same vibrational and rotational properties as the fully separated fragments. The method discussed here proposes an improvement of the modified Gorin model Hamiltonian that better describes simple-fission reaction dynamics by completely excluding trajectories occurring with unfavorable orientations of the combining moieties from the transition state theory rate coefficient. This new approach is sufficiently simple that the description is applicable to any system and thus can be routinely implemented with modest computational resources. Comparison with experiment and with more precise theoretical descriptions for ethane and neopentane decomposition reactions shows that this treatment provides quantitative agreement for ethane. It is also concluded that more sophisticated treatments of transitional modes than afforded by hindered rotor models are needed for the description of transition states with bulky moieties at elevated temperatures, such as the neopentane decomposition system described here.