Neutron scattering study of methyl group rotation in solid para-azoxyanisole (PAA)

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Abstract
Using incoherent neutron scattering, we show that the methyl groups in solid PAA undergo random rotational motion around their three-fold axis, and that rotations about other molecular axes on the same time scale can be excluded. A model for uniaxial rotational jumps between three equidistant sites on a circle of radius a = 1.032 Å is fitted to the quasielastic spectra. The mean jump time τ between two consecutive jumps is determined and found to vary from 2.93 x 10-11 to 0.95 x 10-11 s between 17 and 117 °C, yielding an activation energy of Ea = 2.54 ± 0.13 kcal/mole. The intense peak observed in the inelastic region at 31 meV energy transfer is assigned to the ν = 1 → ν = 0 torsional frequency of the methyl group in a three-fold potential. For a cosine potential one obtains a barrier V3 = 4.21 ± 0.37 kcal/mole. Attempts to relate Ea and V3 values lead to the conclusion that the potential shape differs appreciably from the cosine form. Our results are then compared with other experimental data. In particular, it seems that the barrier to rotation of the methyl groups arises mainly from intramolecular interactions