Molecular motion and methyl group rotation barriers from 13C nmr relaxation times. Podacarpic acid derivatives

The 13 C nmr relaxation time data for a number of compounds related to podocarpic acid are presented. The T 1 values of these molecules were satisfactorily reproduced by application of Woessner's equations for an asymmetric top assuming that the preferred principal axis of the rotational diffusion tensor was either the preferred principal inertial axis or an axis containing the centre of mass and the heterosubstituent at C-4. The values for hindrance to rotation for the various methyl groups are given and their variations discussed.