Pressure and temperature dependence of the longitudinal proton relaxation times in supercooled water to -87 °C and 2500 bar

The longitudinal proton relaxation times T₁ of the water protons have been determined at 100.1 MHz in the temperature range from +20 to -87 °C and up to pressures of 2500 bar. At temperatures below +10 °C, the T₁ isotherms exhibit a maximum at pressures between 1.5 and 2 kbar. While at +10 and 0 °C, T₁ rises by ∼10% from its atmospheric pressure value, this maximum becomes much more pronounced between -20 and -45 °C. In this region application of pressure increases T₁ by ∼100%. The isobars at 2 kbar and above run through a minimum at -76 °C, indicating that at this temperature ωτ_ϑ?1 and that the proton relaxation rate cannot be described by the extreme narrowing condition below ∼-40 °C. The experimental T₁ data and the τ_ϑ values derived at 2 kbar could be treated by a sum of two exponentials. While the smaller activation energies derived from this fit of 3.44±0.17 kcal mole^-1 is independent of pressure, the higher activation energy decreases from 13±0.65 kcal mole^-1 at atmospheric pressure to 9.3±0.5 kcal mole^-1 around 1 kbar and then remains independent of pressure to 2.5 kbar. The data are qualitatively discussed in terms of a random hydrogen-bond network.