Solute–solute interaction in liquid crystal solvents: a nuclear magnetic resonance study of pyridine–iodine molecular complex in the nematic phase

Abstract

¹ H N.m.r. spectra of [¹⁴N]- and [¹⁵N]-pyridine partially oriented in nematic phase IV in the presence of iodine show dipolar couplings (D_ij) increasing linearly as a function of iodine concentration. The spectral changes induced by iodine can be rationalized, to a first approximation, by assuming that the pyridine molecule is in equilibrium, in the oriented phase, with the pyridine–iodine charge transfer complex so that a considerable variation in the mean orientation of the pyridine moiety occurs. Attempts to reproduce the measured dipolar couplings of the exchanging complex on the basis of an average geometry and orientation afforded only dramatic distortion effects on the apparent geometry. Looking, moreover, at the geometry of [¹⁵N]pyridine in phase IV and in other liquid crystals geometrical distortions were also found, probably due to solute–solvent interactions, except for the ZLI 1167 phase, where an r^α structure in close agreement with gas-phase microwave measurements was observed. The results suggest that, for pyridine in the nematic phase in the presence of iodine, at least a three-site mechanism is operating, one of the sites being the complexed pyridine.