We have investigated pressure-enhanced C−H···O interactions in tert-butyl alcohol (TBA)/D2O mixtures. On the basis of its responses to changes in pressure and concentration, TBA appears to be the ideal candidate to study the variations in structural and dynamical properties of C−H···O interactions. For dilute aqueous TBA, the pressure dependence of the C−H bands yielded blue frequency shifts at pressures below 0.3 GPa, but an increase in pressure leads to a red frequency shift at pressures above this value. This discontinuity in frequency shift is related to enhanced C−H···O interactions. The frequency of the C−H stretching modes that characterize C−H···O hydrogen bonding undergoes a blue shift with pressure. This behavior is in contrast with the general trend of red shifts observed in strongly hydrogen-bonded systems that occur through O−H···O and CO···H interactions. We discuss the results of density functional theory calculations that predict the frequency shift of the C−H stretching vibrations.

Charge-enhanced C–H–O interactions are investigated for imidazole hydrochloride/D2O mixtures. Based on the concentration-dependence results, the infrared spectra reveal a strong C–H band and a weak shoulder at ∼3065 cm−1. The pressure-dependence measurements also exhibit considerable spectral changes as the mixtures transform to high-pressure ices. The absorption intensity of the weak shoulder, located in the region of 3060–3090 cm−1, drastically increases as the pressure is elevated. Observation of this low frequency band provides the experimental evidence of C2–H–O hydrogen bonds in the aqueous imidazolium solutions. Ab initio calculation results, predicting the frequency shift of the C–H stretching vibrations as C–H–O is interacting via hydrogen bonding, are discussed. Structural identification of the hydrophobic isomers in infrared spectra seems to be complicated by the presence of more than one stable isomeric form.

The hydrophobic hydration of dimethylsulfoxide (DMSO)/D2O was explored using a combination of the high-pressure method and ab initio calculations. The frequencies of the C–H stretching vibration of DMSO increase as the mole fraction of D2O increases, while no appreciable changes in spectral shapes are observed upon dilution. Interestingly, the infrared spectra of DMSO/D2O observed under high-pressure exhibits dramatic changes, while the new spectral features locating at ∼2942 and ∼3033 cm−1 appear. The spectral changes were attributed to the combined effect of C–H–O hydrogen bonding between C–H in DMSO and oxygen atom in D2O, a phase change, pressure increase, etc. Ab initio calculation results, forecasting the frequency shift of the C–H stretching vibration as C–H–O is interacting via hydrogen bonding, are discussed. The reorganization of the hydrogen-bond network or geometry may be responsible for spectral changes as the pressure was elevated.

The in fra red spec tra of DMSO/D2O and CH3COOD/D2O were mea sured un der var i ous con cen tra tions and pres sure. The fre quen cies of the C-H stretch ing vi bra tion bands for DMSO/D2O mix tures in crease with in creas ing the mole frac tion of D 2 O, im ply ing the con trac tion of the C-H bonds. As the pres sure was el e vated to 1.5 GPa, the C-H vi bra tions of DMSO/D 2 O were al tered into four bands at ~2927 cm -1 , ~2941 cm -1 , ~3007 cm -1 , and ~3025 cm -1 . Fur ther con cen tra tion-dependent stud ies in di cated that the spec tral fea tures at ~2941 cm -1 and ~3036 cm -1 arise from pres sure-induced C-H---O hy dro gen bond ing be tween C-H in DMSO and oxy gen atom in D2O. Ab in itio cal cu la tions were per formed, while the cal cu lated re sults are con sis tent with the in ter pre ta tions based on the high-pressure in fra red ex per i ments. Anal o gous to DMSO/D2O, the pressure-dependent stud ies on CH 3 COOD/D 2 O mix tures pro vide the ev i dence of the C-H---O hy dro gen bond ing as the pres sure was el e vated. IN TRO DUC TIONThe ef fect of sol utes on the struc ture of wa ter has been in ten sively in ves ti gated to un der stand how mol e cules align them selves in aque ous so lu tions.1,2 From the past in ves ti gations, sev eral pro pos als have evolved to as sess the struc tural al ter ations of wa ter caused by the sol utes in terms of struc ture mak ing and break ing prop er ties. 1,[3][4][5] The in ter ac tion of wa ter with the hy dro pho bic parts of mol e cules is also dis cussed in the lit er a ture, 2,5-8 but in most cases in ter pre ta tion has been com pli cated by the in her ent weak ness of hy dro pho bic hydration. Wa ter and dimethyl sulfoxide (DMSO) in ter act with each other and with other sol utes through hy dro gen bonding, 3-11 but de tails of such in ter ac tions are not well char ac terized. Vi bra tional spec tra of aque ous DMSO, spe cif i cally of the S=O stretch ing vi bra tion, 3,7,12-14 has been stud ied at wide con cen tra tion in ter vals, while DMSO has been de scribed as en hanc ing wa ter struc ture. 1,5 The en hanced hy dro gen bonding be tween wa ter mol e cules in aque ous DMSO com pared to bulk wa ter was at trib uted by Soper et al 5 to the pres ence of a strong hy dro gen-bonding site on the DMSO ox y gen and not to any spe cial or der ing around the methyl head groups. 5 Never the less, some stud ies in di cated that the hydration of the C-H groups may also arise in low con cen trated DMSO so lution, 3,7,8 while the pres ence of anom a lous po lar ized wa ter mol e cules, i.e., hy dro pho bic hydration, was in di cated in the con cen tra tion re gion of X wa ter > 0.8. 7 The con tro versy is mainly caused by the dif fi culty to ob tain the ev i dence of hydro pho bic in ter ac tions, since the C-H---O in ter ac tion is typ ically weak or less than 1 kcal/mol. 2Anal o gous to DMSO/wa ter, ace tic acid mixed with water has been stud ied as a model sys tem for C-H---O in ter action. Ace tic acid mol e cules are hy dro gen-bonded pairwise to form cy cl...