NMR study of induced dielectric alignment in pure and binary liquids

The nmr spectra of polar liquids may be affected by application of a strong external electric field. Dipolar molecules are slightly aligned by this field and the nuclear quadrupole couplings and the direct spin—spin dipolar interactions may no longer be-averaged to zero. The proton spectrum of p-nitrotoluene, modified by dipolar induced couplings, is given as an example. Quadrupolar electric field effects have been measured in the '4N spectra of a series of pure nitriles and nitrocompounds. From these spectra the molecular alignment, induced by the electric field, has been obtained. Alignments of both polar and nonpolar molecules have been detected in the nmr spectra of nitrobenzene—deuterobenzene mixtures, if the polar molecules are aligned by the electric field, the orientation is transferred to the nonpolar particles. Theoretical expressions for the alignments in pure and binary liquids are discussed. Short range interactions between the molecules, giving rise to angular correlations, turn out to be important. This is particularly apparent in the case of the alignment of nonpolar molecules in mixtures. INTRODUCTION When an external electric field is applied to a polar liquid the molecular dipoles will tend to align themselves along this field. In competition with this effect the thermal motion disturbs the ordering but on the average the molecules will preserve a preferential orientation. As a result the nmr spectrum of the liquid may be modified because molecular tensor properties; which have no influence on the spectrum of an isotropic liquid, become manifest. Well known examples are the direct dipole—dipole interactions between nuclear spins and the quadrupole interaction between a nucleus and the inhomogeneous electric field originating from the surrounding electrons and nuclei. The possibility of observing the effect of molecular alignment, induced by an electric field, in nuclear magnetic resonance phenomena was mentioned as early as 1954 by Hahn1. Since then several attempts2 have been made to detect dipolar-induced electric field effects in nmr spectra. Most Present address: Department of Physical Chemistry, University of Nymegen, Nymegen, The Netherlands.