The mechanism of unimolecular 1,l-elimination of H, from carbocations has been investigated by the semiempirical MNDO method in view of its very good performances in the analogous elimination from H,CQ. Contrary to previous suggestions, the critical configuration obtained at the MNDO level is characterized by a reduced symmetry with respect to the reacting molecule and by a very short H-H distance. RRKM computations of the rate constants and isotope effect employing MNDO results for the activation energies and vibrational frequencies indicate also that the present, nonsynchronous mechanism is compatible with all the available experimental data.
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