Conformation and reorientation of acetophenone in solution. A proton and deuterium magnetic resonance study in a nematic solvent

Abstract

The deuterium spectrum of 2,3,4,5,6-pentadeuteroacetophenone dissolved in the nematogen Merck Phase V has been recorded and the quadrupolar splittings used to determine elements of the ordering matrix. Two kinds of multiple resonance experiment are used to obtain D_CH3, the dipolar coupling between protons in the methyl group. In one experiment both halves of a quadrupolarsplit doublet in the deuterium spectrum are irradiated whilst observing the proton spectrum, and experiments on a sample of CH₃CD₂Br dissolved in Phase V are used to show the improved efficiency of this triple resonance experiment at removing proton–deuterium spin–spin coupling. In the second experiment, which is also demonstrated on the CH₃CD₂Br sample as well as on the acetophenone, one half of a quadrupolar-split doublet in the deuterium spectrum is irradiated whilst observing the proton-spectrum. In this case the proton spectrum is shown to yield D_CH3 and also the relative sign of a quadrupole and dipolar coupling. The information obtained from these experiments is used to show that acetophenone exists predominantly in the two forms with the CCOC and phenyl ring carbons coplanar. It is also shown that the molecules reorients in the liquid crystal on each internal rotation about the phenyl—C(O) bond.