A theory is presented for fully anisotropic overall molecular tumbling with restricted group internal rotation. This formalism is used with 13C NMR relaxation data and results from molecular mechanics to compute the rotational diffusion rates and methyl group internal rotation rates for sodium taurocholate and ginsenoside‐Re in organic solvents. Although existing theories for steroid derivatives describe methyl internal rotation in terms of free isotropic motion, methyl rotation in ginsenoside‐Re is found to be both sluggish and spatially restricted. The energetic barrier to methyl internal rotation increases significantly with the bulkiness of flexible side‐chains in a series of steroid natural products.
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Wang et al. (1995) studied this question.