EXISTENCE OF NEGATIVE ACTIVATION-ENERGIES IN SIMPLE BIMOLECULAR METATHESIS REACTIONS AND SOME OBSERVATIONS ON TOO-FAST REACTIONS

A large number of reactions of the type R-. + HX and R-. + X-2 have be en reported as having negative activation energies (X = I, Br, Cl). Th ese reactions have none of the behavior of reactions that are expected to have negative activation energies. It is shown that they must be s imple metathesis reactions having a single transition state, (R . H . X)double dagger or (R . X . X)double dagger. It is concluded that the negative activation energies must be artifacts of the experimental tec hniques employed. Some of what appear to be simple metathesis reaction s but which proceed via atom + radical recombination have had rate con stants reported, close to the collision limit. When examined from a co llisional point of view, it is shown that they require collision diame ters from 8 to 25 Angstrom, far in excess of any known long-range inte raction at these distances between neutral species. Again, artifacts o f the experimental methods may be responsible.