Pressure and temperature effects on 2H spin-lattice relaxation times and 1H chemical shifts in tert-butyl alcohol- and urea-D2O solutions

The pressure and temperature effects of hydrophobic hydration were studied by NMR spectroscopy, The H-1 chemical shifts (delta) were measured at 7.7, 29.9, and 48.4 degrees C under high pressure up to 294() MPa for HDO contained as impurity in neat D2O, 1 mol kg(-1) tert-butyl alcohol (TBA)-D2O, and 1 mol kg(-1) urea-D2O solutions, for the methyl group of, TEA in the TBA-D2O solution, and for the amino group of urea in the urea-D2O solution. The H-2 spin-lattice relaxation times (T-1) were measured under the same conditions as the chemical shift measurements for D2O in neat D2O, TBA-D2O and urea-D2O solutions with organic contents up to 8 mol%, The following features are observed for the pressure effect on delta (HDO) and H-2-T-1 in TBA-D2O solutions: (1) The delta (HDO) ,exhibits a downfield shift relative to that in neat D2O, and the difference of delta (HDO) between TEA solution and neat D?O-2 becomes larger with increasing; pressure at lower temperature. (2)! The decrement of the rotational correlation time of water in the hydration shell of TEA (tau(c)(s),) relative to the value at atmospheric pressure is smaller than that in the bulk (tau(c)(0)). (3) The pressure coefficients of T-1 are positive in dilute solutions but are negative: in more than 4 to 5 mol% solutions, These results suggest that the hydrophobic hydration shell of TEA is different than the open structure of water present in bulk, and resists pressure more strongly than the open structure of water in the bulk, Ln solutions of 4 to 5 mol%, the hydration shell collapses, On the other hand. the tau(c)(s) in the hydration shell of urea is slightly larger than that in bulk water at lower pressure, but is obviously larger at higher pressure. Ln view of the rotational motion of water molecules, urea seems to strengthen the water structure slightly rather than weaken it, although: delta (HDO) approaches that in the hulk with pressure, It is difficult to classify urea into a structure maker or a breaker, (C) 1998 American Institute of Physics. [S0021-9606(98)50604-4] .