Upper Limits to Electric-Field-Induced Nuclear Magnetic Dipole—Dipole Couplings in Polar Liquids

Abstract

The alignment of polar molecules in liquids by an external electric field is investigated by pulsed nuclear magnetic resonance methods. Sensitive rotating-frame techniques are described for the detection of the expected induced nuclear magnetic dipole—dipole coupling. In no case could an induced coupling be detected. The upper experimental limits for the induced couplings disagree with the predictions of the Lorentz localfield model. For nitrobenzene and p-nitrotoluene, disagreement is also obtained with the Onsager local-field model by a reduction of almost an order of magnitude in the expected size of the effect. The negative results for these two compounds are indicative of strong local antiparallel ordering of the molecular electric dipoles. The induced coupling in p-nitrotoluene, reported by Buckingham and McLauchlan using standard NMR methods, is found to be absent in this investigation, which employs a measuring technique more sensitive by at least one order of magnitude.