Pressure and temperature effects on H2 spin-lattice relaxation times and H1 chemical shifts in tert-butyl alcohol- and urea-D2O solutions

The pressure and temperature effects of hydrophobic hydration were studied by NMR spectroscopy. The ${}^1\mathrm{H}$ chemical shifts (δ) were measured at 7.7, 29.9, and 48.4 °C under high pressure up to 294 MPa for HDO contained as impurity in neat ${\mathrm{D}}_{\mathrm{2}}\mathrm{O}$ , $\text{1\hspace{0.17em}}{\mathrm{mol\hspace{0.17em}kg}}^{\mathrm{-1}}$ tert-butyl alcohol (TBA)-${\mathrm{D}}_{\mathrm{2}}\mathrm{O},$ and $\text{1\hspace{0.17em}}{\mathrm{mol\hspace{0.17em}kg}}^{\mathrm{-1}}$ urea-${\mathrm{D}}_{\mathrm{2}}\mathrm{O}$ solutions, for the methyl group of TBA in the TBA-${\mathrm{D}}_{\mathrm{2}}\mathrm{O}$ solution, and for the amino group of urea in the urea-${\mathrm{D}}_{\mathrm{2}}\mathrm{O}$ solution. The ${}^2\mathrm{H}$ spin-lattice relaxation times ${\mathrm{(T}}_1)$ were measured under the same conditions as the chemical shift measurements for ${\mathrm{D}}_{\mathrm{2}}\mathrm{O}$ in neat ${\mathrm{D}}_{\mathrm{2}}\mathrm{O},$ TBA-${\mathrm{D}}_{\mathrm{2}}\mathrm{O}$ and urea-${\mathrm{D}}_{\mathrm{2}}\mathrm{O}$ solutions with organic contents up to 8 mol%. The following features are observed for the pressure effect on δ (HDO) and ${}^2\mathrm{H}{\mathrm{-T}}_1$ in TBA-${\mathrm{D}}_{\mathrm{2}}\mathrm{O}$ solutions: (1) The δ (HDO) exhibits a downfield shift relative to that in neat ${\mathrm{D}}_{\mathrm{2}}\mathrm{O},$ and the difference of δ (HDO) between TBA solution and neat ${\mathrm{D}}_{\mathrm{2}}\mathrm{O}$ becomes larger with increasing pressure at lower temperature. (2) The decrement of the rotational correlation time of water in the hydration shell of TBA ${\mathrm{(\tau }}_c^s)$ relative to the value at atmospheric pressure is smaller than that in the bulk ${\mathrm{(\tau }}_c^0\mathrm{).}$ (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 TBA 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. In 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. In view of the rotational motion of water molecules, urea seems to strengthen the water structure slightly rather than weaken it, although δ (HDO) approaches that in the bulk with pressure. It is difficult to classify urea into a structure maker or a breaker.