Solid neopentane C(CH3)4as studied by nuclear magnetic resonance A detailed examination of methyl and molecular reorientation in the low temperature phase

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Abstract
The molecular motions in solid neopentane were investigated by the continuous-wave and the pulsed N.M.R. methods from 4·2 K to its melting point. A single broad minimum of the spin-lattice relaxation time T 1 at 100 K is caused by reorientation of the methyl groups and of the entire molecule. An attempt was made to interpret the T 1 data in the low-temperature phase by a detailed treatment, that is the triangular structure of methyl protons was taken into the calculation of intermethyl dipole interactions. The jumping rate (angular frequency) of methyl reorientation about its C 3 symmetry axis is ω 3 0=1·65×1012 exp (−8·0 kJ mol−1/RT) s−1 and the overall molecular reorientation about its C 2 or C 3 symmetry axis occurs in an isotropic (ω 2=ω 3=2·49×1O15 exp (−18·5 kJ mol−1/RT) s−1) or slightly anisotropic manner (ω 2≠ω 3). The potential barrier hindering the rotation of methyl groups is not a simple sinusoidal function and the tunnelling frequency of the methyl group is of the same magnitude as the local dipolar field.