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

An excess proton can migrate from a solute to solvent molecules upon asymme tric solvation. The migration depends sensitively on solvation number, solv ation structure, and proton affinity differences between solute and solvent molecules. The present study demonstrates this intriguing solvation-induce d effect using protonated dimethyl ether-water clusters as the benchmark sy stem. An integrated examination of H+[(CH3)(2)O](H2O)(n) by vibrational pre dissociation spectroscopy and ab initio calculations indicates that the exc ess proton is (1) localized on (CH3)(2)O at n = 1, (2) equally shared by (C H3)(2)O and (H2O)(2) at n = 2, and (3) completely transferred to (H2O)(n) a t n greater than or equal to 3. The dynamics of proton transfer is revealed by the characteristic free- and hydrogen-bonded-OH stretching vibrations o f the water molecules in direct contact with the excess proton. Both hydrog en 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 com parison between H+[(CH3)(2)O](H2O)(n) and its structural analogues, H+(H2O) (n+1) and H+[(C2H5)(2)O](H2O)(n).