Proton-Assisted Hydration at Hydrophobic Sites in Protonated Ether and Keto Dimers

摘要

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 [(CH_3)_2O]_2H~+ 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 [(CH_3)_2O]_2H~+ ions are first synthesized by corona discharge of dimethyl ether-H_2O mixtures seeded in a continuously operated H_2 beam. Through supersonic expansion, the protonated hydrophobe (CH_3)_2O-H~+-O(CH_3)_2 forms complexes with H_2O, leading to hydrophobic hydration. Two types of isomers, hydrophobic and H_3O~+-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 [(CH_3)_2O]_2H~+-H_2O, water clusters containing the hydrophobes of protonated methyl ethyl ether dimers [(CH_3)(C_2H_5)O-H~+-O(CH_3)(C_2H_5)], protonated acetone dimers [(CH_3)_CO-H~+-OC(CH_3)_2], and protonated acetaldehyde dimers [(CH_3)HCO-H~+-OCH(CH_3)] 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-.