Oxonium ions. Solvatin by single acetonitrile molecules in the gas phase and by bulk solvents

The standard free energies of formation of gaseous complexes between monoprotonic oxonium ions and acetonitrile have been obtained by using the ion cyclotron resonance equilibrium constant method for exchange of acetonitrile between the ions. The results show that the "solvation" by a single molecule of acetonitrile in the gas phase reproduces the important diverse effects of molecular structure on oxonium ion solvation by bulk water. It is estimated from the present results that the effects of solvation by bulk water are only about three times as great as the corresponding effects of "solvation" by a single water molecule in the gas phase. Internal charge delocalization from the protonic site of the oxonium ion strongly reduces solvation. Proton transfer between charge-localized and charge-delocalized oxonium ions may be reversed by differential solvation, both with single H-bonding molecules in the gas phase and with bulk solvent. Further evidence for the site of preferred protonation of esters and amides is provided.