Nuclear spin relaxation and molecular motion in liquid HBr and HCl

Abstract

Measurements of proton magnetic spin-lattice relaxation times T 1 are reported for liquid solutions of anhydrous HBr in DBr, under their own vapor pressure, over the entire liquid range and slightly above the critical temperature. Our previous measurements of proton T 1 for liquid solutions of anhydrous HCl in DCl at low temperatures are revised and reinterpreted. The proton relaxation includes contributions from intra- and intermolecular nuclear dipole-dipole interactions and spin-rotational interaction. The various contributions to the relaxation rate T 1 −1 have been separated by a graphical method, using the values of molecular reorientation correlation times τ d , determined for HCl and HBr from chlorine nucleus and deuteron resonance, respectively. The analysis allowed the determination of the spin-rotation correlation times τ sr and gave an estimate of the self-diffusion coefficients D for the liquids. It is found for HBr that τ sr increases linearly with temperature and becomes almost equal to τ d at the critical temperature. In contrast, τ sr for HCl remains constant up to about −50°C and then increases with temperature and is proportional to T 3 above 0°C; τ sr is approximately equal to τ d at the critical temperature as for HBr. Neither the magnitude nor the temperature dependence of the product τ d τ sr corresponds to I 0/6kT. The values of D estimated for liquid HCI are, except for the lowest temperatures, in reasonable agreement with those measured by O'Reilly and also with those calculated from the cubic cell model of Houghton. By analogy with HCl, values of D are estimated for liquid HBr and are also compared with those obtained from the model; the agreement is satisfactory except at high temperatures. The results for τ sr and D suggest a significant degree of hydrogen bonding in liquid HCl at low temperatures.