Acetyl group relaxation in four aromatic ketones

Dielectric absorption studies have been made of group relaxation of acetophenone, 1,4-diacetylbenzene,4-acetylbiphenyl, and 2-acetylfluorene in a polystyrene matrix, and two rigid molecules have also been examined. For acetyl group relaxation in acetophenone, ΔHE = 29.6 ± 0.6 kJ mol−1and ΔSE = 26 ± 3 JK−1mol−1(95% confidence intervals). The energy barrier for acetyl group relaxation is of the same order as that obtained from nmr data. Our results for acetophenone differ appreciably from the values reported for the gaseous phase by the far-infrared approach and from those by the dielectric absorption of aromatic ketones in benzene solution. The enthalpies of activation for acetyl group relaxation in 1,4-diacetylbenzene, 4-acetylbiphenyl, and 2-acetylfluorene are 29, 30, and 32 kJ mol−1, respectively, which values are identical within their experimental errors. These three substances have also been examined as pure solids, yielding enthalpies of activation for acetyl group relaxation of a similar magnitude.