Hydrophobic hydration of acetone probed by nuclear magnetic resonance and infrared: Evidence for the interaction C–H⋯OH2

The concentration and the temperature dependencies of H1 and C13 chemical shifts in NMR of aqueous acetone mixtures were studied, together with the concentration dependence of the frequency of the C–H stretching vibration of acetone in IR spectra. H1 and C13 chemical shifts were measured at 1 °C, 23 °C, and 48 °C by the external double reference method using a capillary with a blown-out sphere at the bottom for tetramethylsilane as the external reference substance. By this method, it is possible to determine the volume magnetic susceptibility of a sample solution at each temperature, for which the observed chemical shifts may be corrected exactly. Thus, we revealed the detailed electronic polarization in acetone as well as water as functions of concentration and temperature. On diluting acetone with water, the chemical shift of water protons, δH2O, is 3 ppm at the mole fraction of water Xw=0,05 and increases to the value for pure water, ca. 5 ppm, at Xw=0.96, with increasing Xw. In the region of Xw>0.96, δH2O is slightly larger than the value, indicating the presence of more polarized water species than pure water. The chemical shifts of C–H proton, δCH_3, and C–H carbon, δC_H3, also increase slightly with increasing Xw up to Xw=0.96. The frequency for the C–H vibration of acetone, νC–H, increases from the value for pure acetone, 3005 cm−1, to 3013 cm−1 at Xw=0.96, while it decreases sharply with further increase in Xw. These results of IR and NMR measurements show that the hydration of acetone accompanies electronic redistribution in the C–H bonds in cooperated with the change in the polarization of the surrounding water molecules, and that two different types of hydration of acetone are predominant in different concentration regions, Xw<0.96 and Xw>0.96. In the region of Xw<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–H hydrogen and water oxygen. In the region of Xw>0.96, we can interpret the results well by considering that the pushing by the water oxygen becomes strong enough to induce the polarization of the C–H bonds compared to the pushing at Xw⩽0.96. Since the polarization of the C–H bond was found to increase with decreasing temperature, the repulsive interaction seems to have the property of hydrogen bonding and to be denoted as C–H⋯OH2(⋯OH2)n, where OH2(⋯OH2)n expresses water molecules hydrogen-bonded cooperatively and responsible for the more polarized water than pure water. The ratio of water to acetone seems to be a predominant factor to cause the transition of the hydration state from the repulsive interaction to hydrophobic hydration of acetone.