A¹H NMR study of glycerol at high pressure

Relative proton chemical shifts and spin relaxation times T₁ and T₂ have been measured at 400 MHz and room temperature, as functions of pressure using the diamond anvil cell. Hydroxyl proton chemical shifts indicate a substantial increase in hydrogen bonding with increasing pressure, to 10 kbar. From T₁ data, a minimum is inferred at a negative pressure of ∼ - 2.5 kbar, yielding an average molecular correlation time in excellent agreement with a value based independently upon extensive recent measurements of glycerol viscosity. It is concluded that over the observed pressure range NMR correlation times are essentially identical to shear (viscosity) relaxation times in glycerol, and that a single, well-defined average structural correlation time, τ, determines all relaxation phenomena in this liquid. Dielectric susceptibility and Brillouin scattering line width measurements support this conclusion, the former to very high (∼ 20 kbar) pressure. It is also found that values of the spin-spin relaxation rate T₂^-1 yield approximately 2.2 × 10¹¹ s^-2 for the coupling constant between this rate and the correlation time, τ, under slow-hopping conditions, and that differences between observed pressure dependences of viscosity and spin relaxation rates are well accounted for by the pressure dependence of the infinite-frequency shear modulus, G_∞.