A density functional theory study of the catalytic role of Ar, Kr, Xe, and N2 in the CH3OH•+ to CH2OH2•+ isomerization reaction

Density functional theory has been employed to investigate the influence of the neutral bases Ar, Kr, Xe and N-2 on the isomerization reaction of the methanol radical cation (CH3OH.+) to its distonic isomer (CH2OH2.+). Results of all electron, BP86/DN** calculations predict that these neutral bases do catalyze this isomerization reaction. It is expected that, in the presence of Xe or N-2, this intramolecular H-atom transfer could be catalyzed efficiently, competing with the unimolecular dissociation channel which produces a hydrogen atom and the hydroxy methylene cation. Using theoretical data for four different isomerization reactions and numerous different catalysts, a linear relationship is found between the difference in proton affinities of the catalyst and the H-leaving site and the difference between the barrier heights of the uncatalyzed and catalyzed reactions, i.e. the lowering of the barrier. With this relationship, the decrease in the barrier height associated with a given base can be estimated, and the likely consequence of the coexistence of neutral base and hydrogen-containing cations in experiments can be assessed. It is further established that the density functional theory (DFT) methods and basis sets employed in this study do not predict very accurately the relative thermodynamic properties of the ionic species studied in this work. They do, however, give reasonable estimates of the decrease in energy because of association of a catalyst with the transition state. (Int J Mass Spectrom 190/191 (1999) 181-194) (C) 1999 Elsevier Science B.V.