Base-catalyzed hydrogen/deuterium exchange between water and acetonitrile in anionic water clusters

An H/D-exchange reaction involving the methyl group of acetonitrile is observed in the small anionic water cluster OH^-(CD ₃CN)(H₂O)₂. In this cluster, a randomization of H and D atoms takes place, and in subsequent collisions with CD ₃CN, the potentially partially deuterated acetonitrile is replaced by the fully deuterated species. In sequential collisions, fully deuterated clusters OD^-(CD₃CN)(D₂O)₂ are obtained as final products. The H/D-exchange does not occur in OH _-(CD₃CN)(H₂O)₃ or larger clusters. Density functional calculations show that in these larger species, the OH ^- no longer directly attacks the methyl group, and the barriers for H/D-exchange become higher than those for ligand loss reaction channels. This is a very subtle influence of solvation on an H/D-exchange reaction in the gas phase, where a relatively acidic methyl group encounters a strongly basic reaction partner.