Nuclear Magnetic Relaxation in Camphor

Abstract

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.