Close interactions of the C(alpha)[bond]H- - -O type have been analyzed via X-ray crystallography and high-pressure infrared spectroscopy. The results demonstrate that the C(alpha)[bond]H- - -O interactions can offer an additional stability to the beta-sheet formation. X-ray structural data suggest that while 1-acetamido-3-(2-pyrimidinyl)-imidazolium bromide exhibits a bilayer stacking, the PF(6)(-) salt reveals a beta-sheetlike pattern. The appearance of the free-NH infrared absorption indicates that the conventional N[bond]H- - -O or N[bond]H- - -N hydrogen bonds do not fully dominate the packing for the PF(6)(-) salt. The high-pressure infrared study suggests that the C(alpha)[bond]H- - -O hydrogen bonds are the important determinants for the stability of the PF(6)(-) salt. This study also verifies that the imidazolium C[bond]H stretching frequency shifts to a longer wavelength upon the formation of the C[bond]H- - -O hydrogen bonds.

This study provides the evidence for hydrophobic hydration of CH3COOD as its aqueous solution was compressed by high pressure. On going from neat CH3COOD to diluted CH3COOD/D2O mixtures, the C–H vibration absorption bands do not reveal the solvent shift in frequency upon dilution, while the C–H absorption bands are altered into one prominent band at ∼2955 cm−1 as the pressure was applied above 0.3 GPa. The possible explanation is the enhancement of C–H–O interactions between C–H and D2O under the condition of high pressure. Ab initio calculations predict that the CH3 stretching frequencies and intensities of acetic acid are hardly affected by the presence of water, if the methyl group does not participate in the complex formation. Nevertheless, the strongest absorption peak at ∼2928 cm−1 is predicted in several low-energy (acetic acid)–(water)x complexes with the participation of C–H–O hydrogen bond. The reorganization of hydrogen-bond network or geometry is suggested to be responsible for the formation of C–H–O hydrogen bond.

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...