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

Abstract

The 13C nmr relaxation time data for a number of compounds related to podocarpic acid are presented. The T1 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.

Carbon‐13 spin–lattice relaxation of tropane alkaloids. Anisotropic rotational motion of tropine and pseudotropine

“…in Ref. 25) the factor 1/2 in this equation is included into the c k terms, and the equation is given in the misleading form 1/TlDD = h2YH2yC2rph CkTk k = l…”

“…in Ref. 25) the factor 1/2 in this equation is included into the c k terms, and the equation is given in the misleading form 1/TlDD = h2YH2yC2rph CkTk k = l At the same time it is stated that the c k terms are the geometrical factors defined in Woessner's paper .l The factor 1/2, however, must be at the front of the expression if the c k terms are the same as those given by Woessner. Equation (1) can be used only if the orientation of the principal axes system of the diffusion tensor is known relative to the principal axes system of the dipole-dipole interaction" that is in a molecule-fixed frame. When the molecule is sufficiently symmetrical the principal axes system of the rotational diffusion and the moment of inertia tensors may coincide, as is normally the case with nonpolar molecules that have at least an ellipsoidal or D2,, symmetry.…”