Energy partitioning accompanying fragmentation of protonated methanol

The semi-empirical molecular orbital method MINDO/3 is used to investigate the potential energy surface for the H2 and H2O elimination reactions of protonated methanol. The energy profiles for these reactions are prototypical of the rearrangement and simple bond cleavage types. The calculated surface for the reaction CH3OH2 + → CH3 + + H2O accurately reproduces the experimental thermochemistry. The reaction has no reverse activation energy and the kinetic energy release increases with increasing energy in the activated complex, being larger for collision-induced dissociations than for ions undergoing metastable reactions. The calculated energy surface for H2 loss differs significantly from experiment. However, both experimental and calculated results show a large reverse activation energy, a substantial fraction of which appears as kinetic energy release. This energy partitioning pattern is considered in terms of orbital symmetry arguments but is accounted for in terms of the calculated transition state geometry which includes an H2 moiety with a bond length close to its equilibrium bond length leaving orthogonal to the (shortening) CO bond. The kinetic energy release decreases on increasing the internal energy of the ion, as shown when collision-induced dissociations are compared to those of metastable ions. This is suggested to result from the loosening of the activated complex with less effective partitioning of the reverse activation energy into translational energy. Other examples of this phenomenon are presented.