NMR study of molecular motion in some molecules containing t-butyl groups

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Continuous wave and pulse NMR measurements were carried out on phenol derivatives containing t-butyl groups [tri-t-butylphenol (TTBP), methylene-bis(2,6-di-t-butylphenol) (MBBP), and the galvinoxyl radical] to investigate the molecular motions of methyl and t-butyl groups. From second moment measurements we distinguish three temperature regions characteristic of different forms of molecular motions. Between the rigid-lattice limit (plateau region III) and plateau region I at high temperature where both methyl and t-butyl motions are present, there appeared shoulders in the second moment curve. The shoulders in the second moments and relaxation minimum at low temperature are satisfactorily explained by the motional model which includes nonequivalent methyl motions; two methyls in each t-butyl group at the ortho positions and all methyls in the t-butyl group at the para positions are responsible for these phenomena, while one methyl motion in each t-butyl group at the ortho positions and the entire motion of the t-butyl group are considerably restricted at low temperature. The relaxation mechanism at high temperature is discussed in terms of the nonequivalent methyl motions, assuming that one methyl group rotates slowly at a rate almost equal to that of the t-butyl group motion. The short relaxation time of the galvinoxyl radical is explained in terms of the electron–nuclear interactions and almost equal-weight contributions from the dipolar and scalar (hyperfine) interactions were found to account fully for the experimentally measured values.