A study of methyl group dynamics and barrier heights in a homologous series of unbranched ketones

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Abstract
The temperature dependence of the nuclear (proton) spin-lattice relaxation time, T 1, has been measured in the range 10–300 K for the following series of unbranched ketones; 2-butanone to 2-nonanone, 3-pentanone to 3-octanone and 4-heptanone. This data has been analysed to provide estimates for the magnitudes of the three-fold potential barriers to reorientation of all methyl groups in these materials. The corresponding methyl tunnel splittings have also been predicted. Measurements of six tunnel splittings in four of the samples encompassing an energy range of four orders of magnitude confirm these predictions to be accurate and provide refined values for the barrier heights. The tunnelling spectroscopy was performed using the techniques of high-resolution inelastic neutron scattering, field-cycling level-crossing N.M.R. spectroscopy and double sideband irradiation N.M.R. spectroscopy. The observed trends in barrier height within the series of materials have been rationalized in terms of the known molecular structure and inter- and intra-molecular contributions have been separately identified and accounted for. The four measured barrier heights in 2-pentanone have been employed to model the temperature dependence of T 1 using Clough et al.'s single parameter theory for methyl dynamics. The agreement with experiment is very good.