Proton-assisted hydration at hydrophobic sites in protonated ether and keto dimers

Charge-enhanced hydrophobic hydration is investigated for protonated hydrophobe-containing water clusters synthesized by a supersonic expansion. The investigation 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 initio calculations based on density functional theory. The [(CH3)(2)O](2)H+ ions 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 and non-hydrogen-bonded OH stretches of the solvent water molecules. The two isomers display distinctly different OH stretching spectra and, furthermore, 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 protonated acetaldehyde dimers [(CH3)HCO-H+-OCH(CH3)] are also studied. These clusters together represent four test systems for the understanding of the nature of hydrophobic interactions in the unusual form of -C-H ... O-.