HYDROPHOBIC HYDRATION OF ACETONE PROBED BY NUCLEAR-MAGNETIC-RESONANCEAND INFRARED - EVIDENCE FOR THE INTERACTION C-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-OH2

The concentration and the temperature dependencies of H-1 and C-13 che mical shifts in NMR of aqueous acetone mixtures were studied, together with the concentration dependence of the frequency of the C-H stretch ing vibration of acetone in IR spectra. H-1 and C-13 chemical shifts w ere measured at 1 degrees C, 23 degrees C, and 48 degrees C by the ext ernal double reference method using a capillary with a blown-out spher e at the bottom for tetramethylsilane as the external reference substa nce. By this method, it is possible to determine the volume magnetic s usceptibility of a sample solution at each temperature, for which the observed chemical shifts may be corrected exactly. Thus, we revealed t he detailed electronic polarization in acetone as well as water as fun ctions of concentration and temperature. On diluting acetone with wate r, the chemical shift of water protons, (H2O)-H-delta, is 3 ppm at the mole fraction of water X-w = 0,05 and increases to the value for pure water, ca. 5 ppm, at X-w =0.96, with increasing X-w. In the region of X-w>0.96, (H2O)-H-delta is Slightly larger than the value, indicating the presence of more polarized water species than pure water. The che mical shifts of C-H proton, (CH3)-C-delta, and C-H carbon, (CH3)-C-del ta, also increase slightly with increasing X-w up to X-w = 0.96. The f requency for the C-H vibration of acetone, v(C-H), increases from the value for pure acetone, 3005 cm(-1), to 3013 cm(-1) at X-w = 0.96, whi le it decreases sharply with further increase in X-w. These results of IR and NMR measurements show that the hydration of acetone accompanie s electronic redistribution in the C-H bonds in cooperated with the ch ange in the polarization of the surrounding water molecules, and that two different types of hydration of acetone are predominant in differe nt concentration regions, X-w<0.96 and X-w>0.96. In the region of X-w< 0.96, the results can be explained satisfactorily if we consider that a part of the electron about the C-H proton is pushed out into the C-H bond due to a repulsive interaction between the C-K hydrogen and wate r oxygen. In the region of X-w>0.96, we can interpret the results well by considering that the pushing by the water oxygen becomes strong en ough to induce the polarization of the C-H bonds compared to the pushi ng at X(w)less than or equal to 0.96. Since the polarization of the C- H bond was found to increase with decreasing temperature, the repulsiv e interaction seems to have the property of hydrogen bonding and to be denoted as C-H ... OH2(... OH2)(n), where OH2(... OH2)(n) expresses w ater molecules hydrogen-bonded cooperatively and responsible for the m ore polarized water than pure water. The ratio of water to acetone see ms to be a predominant factor to cause the transition of the hydration state from the repulsive interaction to hydrophobic hydration of acet one. (C) 1998 American Institute of Physics. [S0021-9606(98)01645-6].