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.