Isotope Effects on Deuterium Spin-Lattice Relaxation in H2O/D2O Mixtures

Abstract

The deuterium spin-lattice relaxation time, T1, of H2O/D2O mixtures is measured at 298.2 K. The relaxation rate, T1 -1, is found to increase with increasing deuterium atom fraction, n, the plot of T1 -1 vs. n exhibiting a small depature from linearity. A general equation T1 -1 (n) for the H2O/D2O system is formulated. The temperature dependence of T1 is investigated in the temperature range 278.2 K to 298.2 K for n = 6.8 x 10-3, 6.8 x 10-2, 0.244, 0.500, and 0.997. On the assumption that the electric field gradient parameters (e2 q Q/h and δ) are independent of n and temperature, an effective correlation time, τc,eff, is derived from the T1 data. Relatively large isotope effects on τc,eff are found; possible reasons for the existence of such isotope effects are discussed in terms of a simple Debye model. The mean activation enthalpy (Δ≠ H) and entropy (Δ≠ S) for the relaxation process within the temperature range studied are derived on the basis of Eyring's absolute rate theory and the temperature dependence of τc,eff. Both activation parameters are found to increase linearly with n: Δ≠ H/kJ mol-1 = 18.2 + 2.46 n, Δ≠ S/J K-1 mol-1 = 37.5 + 6.77 n.