REACTIONS AND THERMOCHEMISTRY OF PROTONATED METHANOL CLUSTERS PRODUCED BY ELECTRON-IMPACT IONIZATION

Protonated methanol clusters H+(CH3OH)n for n in the range 1-17 have been formed by electron impact ionization of neutral methanol clusters. The formation of H+(H2O)(CH3OH)n clusters is observed to occur only for n greater-than-or-equal-to 6. In CD3OH clusters, both H+(CD3OH)n and D+(CD3OH)n were formed. The D+/H+ ratio showed a strong dependence on cluster size, which is apparently related to neutral boiloff sequences. Reduced data for production of H+(H2O)(CH3OH)n reveals two distinct size-specific manifolds. The results also suggest that the ion H+(CH3OH)3 is particularly stable and corresponds to the filling of the first solvent shell around a protonated methanol molecule. New thermochemical data are presented for the H+(CH3OH)n series. These data show that the limiting attachment energy of CH3OH molecules to the large clusters (9 kcal/mol) is similar to the condensation energy of liquid methanol, and therefore the complex intermolecular forces in the liquid are also present in the clusters.