Isotope Effects on Deuterium Spin-Lattice Relaxation

The deuterium spin-lattice relaxation time, 7",, of H 2 0 /D 2 0 mixtures is measured at 298.2 K. The relaxation rate, Ff 1 , is found to increase with increasing deuterium atom fraction, n, the plot of T\ 1 vs. n exhibiting a small depature from linearity. A general equation Tf 1 (n) for the H 2 0 / DiO system is formulated. The temperature dependence of Tx is investigated in the temperature range 278.2 K to 298.2 K for n = 6.8 x 10", 6.8 x IO, 0.244, 0.500, and 0.997. On the assumption that the electric field gradient parameters (e q Q/h and (5) are independent of n and temperature, an effective correlation time, rc_ eff, is derived from the T} data. Relatively large isotope effects on rc 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 (A* H) and entropy (zf*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 rc efT. Both activation parameters are found to increase linearly with n: A* H/kJ mol 1 = 18.2 + 2.46 n, A* S/J K" mol 1 = 37.5 + 6.77 n.