Carbon-13 Spin–Lattice Relaxation Times for Selected Steroids

Abstract

The spin–lattice relaxation time (T1) data for a number of androstane and cholestane derivatives is presented. The NT1 value of carbon-3 in several of the compounds is found to be shorter than the average NT1 value for the other ring carbons which is interpreted in terms of anisotropic motion with the long steroid axis being the preferred axis of rotation. As this effect is only observed for carbon-3, an effective molecular correlation time can be calculated from the average NT1 value for all other CH and CH2 carbons. The correlation time (τi) for internal rotation of a methyl group is shown to depend on the number of 1,3 diaxial methyl group – hydrogen interactions. Thus τi(C-19) is shorter than τi(C-18) for androstane, τi(C-19) is shorter for trans-cholestanol than for either cis-cholestanol or cholesterol. The effects of substituents and conformational changes on τi as well as the effect of solvent on T1 are discussed.