Nuclear relaxation in coupled spin systems dissolved in a nematic phase—the AX and A2 spin systems

Abstract

Relaxation in coupled AX(¹³CHCl₃) and A₂(¹²CH₂Cl₂) spin systems has been studied in a nematic solvent. The process is modeled as intramolecular dipole–dipole interactions plus other mechanisms that are treated collectively as external random magnetic fields. The AX system provides evidence that extreme narrowing arguments are valid for small molecules dissolved in liquid crystals. Also, values of spectral densities show that relaxation by the intramolecular dipole–dipole mechanism and by the combined external random field mechanisms are of equal importance for proton longitudinal relaxation whereas the former mechanism dominates the carbon‐13 relaxation. Important details of the A₂ system are discussed for the first time with the two relaxation observables that are obtained when superposition of the transitions is eliminated by the anisotropic solvent. Extreme narrowing is assumed, and the relative importance of the various mechanisms is discussed. The random field type mechanisms, which include intermolecular dipole–dipole interactions with the solvent, are shown to be dominant. So the intramolecular model commonly used to account for the frequency dependence of proton T₁ in nematic liquid crystals does not obtain for small molecules dissolved in a nematic phase.