Relaxation in an <i>A</i>2 coupled spin system dissolved in a perdeuterated liquid crystal: The intermolecular dipolar random field cross relaxation term

Abstract

A perdeuterated nematic liquid crystal N- (p-ethoxybenzylidene) -p-n-butylaniline (EBBA-d23, 98% deuterium) was synthesized to study relaxation of small molecules dissolved in anisotropic solvents. Proton spin–lattice relaxation in the coupled A2 spin system of methylene chloride (CH2Cl2) was studied using both perhydro and perdeutero EBBA as the solvent. The relaxation was modeled as intramolecular dipole–dipole interactions plus other mechanisms that were treated collectively as fluctuating external random magnetic fields. Comparison of relaxation in EBBA and EBBA-d23 allowed the first experimental measurement of the correlation coefficient between external random fields at the two protons due to dipolar interactions of the methylene protons with the protons or deuterons of the solvent (78% correlated). The selective nuclear Overhauser effect was also studied.