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 been reported as having negative activation energies (X = I, Br, Cl). These reactions have none of the behavior of reactions that are expected to have negative activation energies. It is shown that they must be simple 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 techniques employed. Some of what appear to be simple metathesis reactions but which proceed via atom + radical recombination have had rate constants reported, close to the collision limit. When examined from a collisional point of view, it is shown that they require collision diameters from 8 to 25 Angstrom, far in excess of any known long-range interaction at these distances between neutral species. Again, artifacts of the experimental methods may be responsible.