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For pt.I see ibid., vol.8, p.2274 (1975). Replacement of all hydrogen atoms other than those of the 4-methyl group by deuterium atoms in crystals of 4-methyl 2,6-di-tertiary butyl phenol reduces the 4-methyl group tunnelling rotation frequency at 4.2K from 9.45 GHz to 8.47 GHz.
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Clough et al. (1976) studied this question.
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