Nuclear Magnetic Relaxation in Camphor

Solid solutions containing d- and l-camphor have been investigated by nuclear magnetic resonance and x-ray powder diffraction between 77° and 473°K. Striking differences are observed among the various solutions in the vicinity of the second-order phase transition. The transition temperature is found to vary with composition, passing through a minimum at the 3:1 d:l solution. The width of the λ transition, reflected in the NMR second moment, increases going from the pure isomer to the racemic mixture. Methyl group reorientation persists at 100°K, although molecular tumbling has been arrested at this temperature in all samples. The x-ray powder patterns of the pure isomer, the 1:1, and 3:1 mole ratio solids differ at 77°K, but are identical at 298°K. Plots of the NMR spin—lattice relaxation time versus reciprocal absolute temperature yield an activation energy of 2.35 kcal/mole for methyl-group reorientation in d-camphor below the λ point. Above the rotational transition, the x-ray and NMR data are independent of concentration. Activation energies of 2.8 and 14.6 kcal/mole are calculated for the processes of molecular tumbling and self-diffusion, respectively. The T1 data are quantitatively examined in terms of the BPP theory.