Spin isomerisation of para-substituted phenyl cations

The singlet and tripler potential energy surfaces of a series of p-X-substi tuted aryl cations (X = H, CN, CH3, F, OH, NH,) are investigated computatio nally at the B3LYP/6-31G(d) level of theory. The first four species are fou nd to be ground state singlets, the last has a tripler ground state, and th e spin states of the OH derivative are almost isoenergetic. The minimum ene rgy crossing points (MECPs) between the two surfaces are found to lie very little above the higher of the two minima in all cases, and the spin-orbit coupling is significant at those points. Therefore, it is expected that ary l cations will rapidly convert to their most stable spin state, and that in cases of near degeneracy such as for p-HO-C6H4+, the states may interconve rt rapidly enough to both be accessible in thermal reactions.