Spin isomerisation of para-substituted phenyl cations

Abstract

The singlet and triplet potential energy surfaces of a series of p-X-substituted aryl cations (X = H, CN, CH₃, F, OH, NH₂) are investigated computationally at the B3LYP/6-31G(d) level of theory. The first four species are found to be ground state singlets, the last has a triplet ground state, and the spin states of the OH derivative are almost isoenergetic. The minimum energy 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 aryl 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 interconvert rapidly enough to both be accessible in thermal reactions.