Molecular reorientation in three orientationally disordered molecular crystals by incoherent neutron scattering

Abstract

Incoherent neutron scattering measurements have been made on three substances in both their ordered and orientationally disordered crystalline phases. The compounds studied were pivalic acid, 1,2-dichlorethane and succinonitrile.

In all three cases no energy broadening of the quasi-elastic spectrum was observed in the low temperature phase, indicating that rotational correlation times are >∼10^–10 s.

For pivalic acid and dichlorethane the results are consistent with a uniaxial rotational motion, most probably by jump steps of π/3 or less, but little information is obtainable about the details of the rotational potential function. The motion may be described by a rotational diffusion constant D_r which in both cases is near 10¹¹ rad s^–1 at the m.p. and for dichloroethane an activation energy of 5 ± 4 kJ mol^–1 is found. For pivalic acid the methyl group torsional frequency has been identified and simple analysis in terms of a 3-fold cosine potential implies a jump frequency for methyl group reorientation of [graphic omitted] 10¹⁰ s^–1.

In the case of succinonitrile it has been established that both trans–gauche isomerisation and four-fold jump reorientation of the trans-isomers about the N–N molecular axis [the crystal (h00) direction] are observed in the neutron experiment. Values are obtained for appropriate correlation times and for energy barriers for the two kinds of motion and a comparison made with the results of other kinds of experiment.