EXTENDED GORIN-MODEL FOR RADICAL-RADICAL RECOMBINATION REACTIONS

Radical-radical recombination reactions (e.g., CH3 + CH3 reversible ar row C2H6) proceed with no barrier through simple-fission transition st ates. The application of transition state theory (TST) to these reacti ons is discussed, achieving a new understanding of the dividing surfac e and dynamical assumption implicit in all TST treatments of these rea ctions. A reinterpretation of the modified Gorin model for such transi tion states is discussed which removes several inconsistencies from th is model and greatly improves data prediction and interpretation for r adical-radical recombination reactions (and the reverse unimolecular d issociations) in the gas phase. The suggested model is an extension of the basic Gorin approach, which treats the transition state as consis ting of two moieties which have the same vibrational and rotational pr operties as the fully separated fragments. The method discussed here p roposes an improvement of the modified Gorin model Hamiltonian that be tter describes simple-fission reaction dynamics by completely excludin g trajectories occurring with unfavorable orientations of the combinin g 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 computat ional resources. Comparison with experiment and with more precise theo retical 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 transition al modes than afforded by hindered rotor models are needed for the des cription of transition states with bulky moieties at elevated temperat ures, such as the neopentane decomposition system described here.