Migration of an excess proton upon asymmetric hydration:: H+[(CH3)2O](H2O)n as a model system

An excess proton can migrate from a solute to solvent molecules upon asymmetric solvation. The migration depends sensitively on solvation number, solvation structure, and proton affinity differences between solute and solvent molecules. The present study demonstrates this intriguing solvation-induced effect using protonated dimethyl ether-water clusters as the benchmark system. An integrated examination of H+[(CH3)(2)O](H2O)(n) by vibrational predissociation spectroscopy and ab initio calculations indicates that the excess proton is (1) localized on (CH3)(2)O at n = 1, (2) equally shared by (CH3)(2)O and (H2O)(2) at n = 2, and (3) completely transferred to (H2O)(n) at n greater than or equal to 3. The dynamics of proton transfer is revealed by the characteristic free- and hydrogen-bonded-OH stretching vibrations of the water molecules in direct contact with the excess proton. Both hydrogen bond cooperativity and zero-point vibrations have crucial influences on the final position of the proton in the clusters. Further insight into this remarkable phenomenon of intracluster proton transfer is provided by a comparison between H+[(CH3)(2)O](H2O)(n) and its structural analogues, H+(H2O)(n+1) and H+[(C2H5)(2)O](H2O)(n).