Hc. Chang et al., Migration of an excess proton upon asymmetric hydration: H+[(CH3)(2)O](H2O)(n) as a model system, J AM CHEM S, 121(18), 1999, pp. 4443-4450
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).