Pressure and Temperature Dependence of the Longitudinal Proton and Deuteron Relaxation Rates

T h e mo l ecu la r m o b i l i t y o f l i q u i d a m m o n i a is der i ved f r o m the d e t e r m i n a t i o n o f the l o n g i t u d i n a l r e l a x a t i o n t imes of the p ro tons and deuterons. T h e exper iments were pe r f o rmed i n the t empera tu re in te rva l between the m e l t i n g p r e s su r e curve a n d 467 K for N H 3 a n d to 351 K for N D 3 at pressures u p to 250 M P a . A t t empera tures be l ow 3 5 0 K the m o l e c u l a r m o b i l i t y c a n be descr ibed by the i so t r op i c s m a l l step di f fus ion mode l . T h e a c t i v a t i o n energies at cons tant pressure are de r i v ed for the r o t a t o r i c d i f f u s i o n to 7.0 + 0.5 k J • m o l 1 and for the t rans la to r i c d i f fus ion to 6 ± 1 k J • m o l " 1 f r o m the t empera ture dependence o f the r e l a x a t i o n rates. I n a d d i t i o n the ac t i va t i on energy at constant v o l u m e for the r o t a t o r i c d i f fus ion has been de t e rm ined to 5.7 ± 0.5 k J • m o l " l . T h e i s o t h e r m s for a l l r e l axa t i on rates are l inear i n a l ogU/Ti ) versus 1/7" p lo t , y i e l d i n g A K i n* l r a = 2.5 ± 0.6 c m 3 • m o l " 1 a n d A V*x^ = 5 ± 1 c m 3 m o l " 1 .