Charge-enhanced hydrophobic hydration is investigated for protonated hydrop
hobe-containing water clusters synthesized by a supersonic expansion. The i
nvestigation begins with clusters consisting of a protonated dimethyl ether
dimer [(CH3)(2)O](2)H+ and one water unit, which can be considered as the
prototype of ionic hydrophobe-water systems. The methodologies involved in
this investigation are vibrational predissociation spectroscopy and ab init
io calculations based on density functional theory. The [(CH3)(2)O](2)H+ io
ns are first synthesized by corona discharge of dimethyl ether-H2O mixtures
seeded in a continuously operated H-2 beam. Through supersonic expansion,
the protonated hydrophobe (CH3)(2)O-H+-O(CH3)(2) forms complexes with H2O,
leading to hydrophobic hydration. Two types of isomers, hydrophobic and H3O
+-centered, are identified by a close examination of both hydrogen-bonded a
nd non-hydrogen-bonded OH stretches of the solvent water molecules. The two
isomers display distinctly different OH stretching spectra and, furthermor
e, a drastic change of the spectra with the variation of beam temperature.
In this work, in addition to [(CH3)(2)O](2)H+H2O, water clusters containing
the hydrophobes of protonated methyl ethyl ether dimers [(CH3)(C2H5)OH+-O(
CH3)(C2H5)], protonated acetone dimers [(CH3)(2)CO-H+-OC(CH3)(2)], and prot
onated acetaldehyde dimers [(CH3)HCO-H+-OCH(CH3)] are also studied. These c
lusters together represent four test systems for the understanding of the n
ature of hydrophobic interactions in the unusual form of -C-H ... O-.