Effects of Neutral Bases on the Isomerization of Conventional Radical Cations CH3X•+ to Their Distonic Isomers •CH2X+H (X = F, OH, NH2): Proton-Transport Catalysis and Other Mechanisms

Abstract

High level ab initio theory is used to investigate the effects of the neutral bases HF, H2O, and NH3 on the mechanisms and barriers for isomerization of the conventional radical cations CH3X•+ (X = F, OH, and NH2) to their corresponding distonic isomers •CH2X+H. It is found that the isomerization mechanism is determined largely by the relative proton affinities of the base and the parent radical •CH2X. If the proton affinity of the base is substantially lower than the proton affinity at either C or X of •CH2X, the barrier is lowered but remains positive relative to separated base plus CH3X•+. If the proton affinity of the base lies between that at C and X, the barrier becomes negative and the base successfully catalyzes the isomerization of CH3X•+ to •CH2X+H. In fact, the barrier is found to be negative even in cases where the proton affinity of the base is lower than the lower proton affinity site of •CH2X, provided that this proton affinity difference is not too large. If the proton affinity of the base is higher than that at both C and X, the barrier to rearrangement is lowered even further. However, intermolecular proton transfer from the ion to the base rather than intramolecular proton migration is then the lower energy process. An alternative isomerization mechanism for the [CH3OH/base]•+ systems is also detailed in which the base remains bound to the hydroxyl hydrogen throughout. The barriers for this so-called “spectator” mechanism are found to be higher than those for the interconversion of the isolated conventional and distonic ions. A rationalization based on the nature of the intervening ion−base complexes is presented.