Effect of CH3-reorientation on spin-lattice relaxation time in ketones

Abstract

The influence of CH₃-reorientation on the spin-lattice relaxation time T₁ in 6 solid ketones has been investigated. The results of the T₁ measurements can be explained by assuming that a sufficiently rapid reorientation of a methyl group takes place only when this group is separated from the keto group by at least one methylene group. At about –100°C, a T₁ minimum is observed which is a measure of the ratio between the total number of protons and the number of reorienting methyl protons. Deuteration of the methyl group adjacent to the keto group gave for [1,1,1,3,3-²H₅]-2- pentanone a T₁ minimum which is within 10% of that expected on the ground of the above-mentioned assumption. The T₁ minimum for [1,1,1,3,3-²H₅]-2-butanone deviated 30 %, which is probably due to contaminations and the absence of methyl-methylene interactions.

The activation energy of the CH₃-reorientation appears to increase according as the reorienting methyl group approaches the CH₃CO group. It varies from 2.8 ± 0.2 kcal/mole for 2-tridecanone to 3.5 ± 0.2 kcal/mole for 2-pentanone. The asymmetry in the T₁-curves can be explained by assuming an asymmetric correlation time distribution, which causes a variation of 3 % at most in the T₁ minimum.