Mechanism of the bimolecular(?) hydrogen‐iodine reaction

A combined phase‐space/trajectory study of the hydrogen‐iodine exchange reactions (overall, H₂+I₂→HI+HI) was carried out for a LEPS potential energy surface modified to favor reaction through the collinear reaction configuration. The results show reaction to occur by both bimolecular and termolecular mechanisms involving pairs of iodine atoms with energies slightly below (bimolecular) or slightly above (termolecular) the dissociation limit. Together with other considerations this finding suggests that in stating the reaction proceeds by the bimolecular mechanism the old textbooks may have been correct. A detailed comparison with Sullivan's photochemical experiments requires a knowledge of the iodine‐atom recombination process which is not presently available. Sullivan's primary conclusion, i.e., the iodine species participating in the exchange reaction are the same as those in the slow step of recombination, is upheld; but, the experimental distinction between bimolecular and termolecular mechanisms depends on the question of the relative rates of recombination and relaxation of iodine.