Spin-lattice relaxation and methyl reorientation in symmetrical alkenes and alkynes and in 1-alkenes and 1-alkynes

Abstract

The spin-lattice relaxation behaviour of some solid symmetrical alkenes and alkynes and in 1-alkenes and 1-alkynes down to about 100 K is described. The magnetization recovery curve is almost exponential, although methyl reorientation is the dominant relaxation mechanism. Spin diffusion probably breaks down “cross correlations”. No distinct effect of the double or triple bond on the methyl rotation barrier could be observed owing to the increasing influence of molecular motions with decreasing chain lengths. Replacement of some terminal methyl groups by a HCCH₂ or CCH group reduces the methyl group packing, which manifests itself in a lower methyl rotation barrier. The shape of the methyl rotation barrier is dependent on intermolecular interactions.