MINDO/3 calculation of the potential energy surface for C3H5+ → C3H3+ + H2 as applied to understanding energy partitioning accompanying fragmentation

The semi-empirical molecular orbital method MINDO/3 was used to investigate the potential energy surface describing the loss of H2 from the allyl cation C3H5 +. The two lowest energy processes found are the 1,2- and 1,3-eliminations and they commence by 2 → 1 and 3 → 1 hydrogen atom migrations, respectively. In the 1,2-elimination this involves isomerization to the 2-propenyl ion while in the 1,3-elimination, isomerization is to an intermediate corner-protonated cyclopropene. The reactions then proceed by H2 elimination from the 2-propenyl ion and from protonated cyclopropene to yield the propargyl and cyclopropenyl ions, respectively. The results of the calculations are compared to previous experimental studies on metastable ions to examine the partitioning of reverse activation energy between kinetic energy of separation, and internal vibrational energy. In making such a comparison, the limitations of MINDO/3, in particular the overestimation of the stability of small ring compounds, are recognized. It is found that the reverse activation energy appears predominantly in the released kinetic energy, for both reactions. Such findings are in accord with qualitative expectations based on reaction dynamics and are discussed in relation to them. In particular, the activated complexes for both the 1,2- and 1,3-eliminations are “tight” and the potential energy surfaces are of the “repulsive” type for which it is predicted that a high percentage of the reverse activation energy will be partitioned as kinetic energy. The activated complexes are well described as species in which the daughter ion is solvated by the H2 leaving. The incipient products are close to their equilibrium geometries, which is in accord with the observation that energy is partitioned predominantly into translation.