NOED, COSY and HETCOR experiments, in conjunction with molecular mechanics calculations, were used to assign the 'H and "C NMR spectra of the terpenoids a-ionone and methyl 0-methylpodwarpate and to investigate them stereocbemistry. These compounds were used as models to investigate the ability of molecular mechanics and associated calculations to predict 'H spin-lattice relaxation rates in molecules of this type.

LAWRENCE D. COLEBROOK and NING QING. Can. J. Chem. 70, 2 154 (1992).'H and I3C spin-lattice relaxation rates of the methyl groups of 15 steroids have been measured, and rotational barriers of the methyl groups have been calculated, using force field methods. Correlations between relaxation rates and computed rotational barriers have been investigated. 'H and "C relaxation rates are consistent, and correlate with rotational barriers in simpler steroids. Steroids that are highly substituted with polar groups, and that must be investigated in polar solvents, tend to show poor correlations, an observation that is attributed to limitations in the computational model employed, which does not account for the effects of specific solvation or molecular association. In dilute solution ( 5 0 . 1 M in a deuteriated solvent) the spin-lattice relaxation of most 'H and I3c nuclei is dominated by the intramolecular dipole-dipole mechanism. For a molecule tumbling isotropically, the dipole-dipole contribution, R,(dd), to the overall relaxation rate is given by eq.[ l ] in terms of the relaxation contribution between two nonequivalent nuclei, i, j:where y, and yj are the magnetogyric ratios of the i and j nuclei, rij is their internuclear separation, and s,.(i, j) is the motional correlation time of the vector between them. The net R, value for a nucleus is dependent on the overall tumbling rate of the molecule as a unit, plus the contribution from any additional motion of a substituent group that carries that particular nucleus. Because of the l/r6 term in eq.[ l ] , the influence of a relaxation pathway is attenuated very rapidly with increasing distance between nuclei, so that it is the near neighbours of a nucleus which dominate its relaxation. A methyl proton relaxes largely to its neighbouring protons on the same carbon atom. The carbon atom of a methyl group is relaxed almost exclusively by the protons it carries. Differences between the I H relaxation rates of methyl groups in the same molecule arise, therefore, largely from differences in the correlation times for local rotational motion of the methyl groups, while differences between the I3c relaxation rates of methyl groups in the same molecule arise almost exclusively from this origin (1). An increase in the rate of methyl group rotation reduces the efficiency of the dipole-dipole relaxation mechanism.Relaxation in small molecules, which experience fast tumbling and fast methyl group rotation, may be compli-'~u t h o r to whom correspondence may be addressed. '~evision received January 14, 1992. cated by contributions from the spin-rotation mechanism (2), which has an inverse dependence on the rotational rate of the methyl group (3). The effect of the spin-rotation contribution is to reduce the sensitivity of the overall relaxation rate, which is usually dominated by the dipole-dipole mechanism (I), to steric factors that influence the methyl group rotational rate.The sensitivity of methyl group relaxation in steroids to steric factors was first systematically investi...

A Study of Computed Rotational Barriers of Methyl Groups and 1H and 13C Spin-Lattice Relaxation Rates in Some Steroids.-In seven steroids, e.g. (I)-(III), the measured spin-lattice relaxation rates correlate well with the computed methyl groups rotational barriers. In eight highly substituted steroids, e.g. (IV)-( VI), the correlation may break down due to an inadequate computation model, which does not take into account molecular interactions and solvation effects. -(COLEBROOK, L. D.; QING, N.; Can.