A study, using dielectric relaxation,¹³C N.M.R. relaxation and far infra-red techniques, of internal motion in some quasi-symmetrical dicarbonyl compounds

Dielectric relaxation measurements, 13C N.M.R. relaxation measurements and far infra-red spectral data are reported for pure diacetyl, pure fumaryl chloride and for dilute solutions of hexenedione and p-diacetylbenzene in non-polar solvents. The results are interpreted in terms of relaxation both by overall molecular motion and by internal rotation. The model of ‘ chemical relaxation ’ developed earlier is shown to be very useful and kinetic parameters are derived. The far infra-red spectra are discussed, with the aid of an adapted Hamiltonian for the internal rotation, and are shown to be consistent with an assignment of the strong bands in the 60–90 cm-1 regions to acetyl group rotation and those in the 140 cm-1 region to out-of-plane motion.