NMR relaxation study of disorder in condensed matter  – low temperature studies in the mixed system  − BPI(1−x)BP x  – transition from classical to quantum rotation

1H NMR spin-lattice relaxation time (T1 ) studies have been carried out in the mixed system, betaine phosphite–betaine phosphate (BPI(1−x)BP x ), to study the effects of disorder. The T1 data in the temperature range 100 K to 4 K, at two Larmor frequencies 11.4 and 23.3 MHz, has been analysed following Lourens’ model, which shows a gradual transition from classical reorientations to quantum tunnelling motions. At lower temperatures, (when thermal motions become too slow) differences (due to disorder) in the local environments of the reorienting groups result in a distribution in the activation energy (Ea ) and the energy gap of the ground to the first excited torsional level (E01 ). Below 50 K, the system moves into the quantum tunneling regime and the magnetisation recovery shows biexponential behaviour which is another signature of disorder. These results are compared with those of the parent compounds BP and BPI.

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