Characterisation of the structure, deuterium quadrupolar tensors, and orientational order of acenaphthene, a rigid, prochiral molecule, from the NMR spectra of samples dissolved in nematic and chiral nematic liquid crystalline solvents

Abstract

Molecules like acenaphthene which have a point group symmetry of C_2v behave as though their symmetry is C₂ when dissolved in chiral nematic liquid crystalline solvents. To quantify this effect a sample of perdeuterated acenaphthene dissolved in the chiral nematic solvent formed by dissolving poly-(γ-benzyl-L-glutamate), PBLG, in CHCl₃ has been studied by deuterium NMR spectroscopy. The quadrupolar splittings obtained were used to determine the orientational order parameters of acenaphthene-d₁₀ when dissolved in PBLG/CHCl₃. To do this it was necessary to also record and analyse the proton and deuterium spectra given by a sample containing both acenaphthene and acenaphthene-d₁₀ in a non-chiral liquid crystalline solvent. The proton spectrum is very complex, and was analysed only after first recording and analysing the simpler ¹H-{²H} spectrum given by a sample of acenaphthene-d₆ . This procedure finally yielded a set of dipolar couplings for the fully protonated molecule, which after correction for vibrational motion, were used to determine both the relative positions of the protons and the orientational order of the molecules. This information was then used to derive the quadrupolar coupling constants from the measured quadrupolar splittings. The lowering of the symmetry of the orientational distribution function is quantified by the angle α by which the principal axes of the molecular orientational order matrix of acenaphthene dissolved in PBLG/CHCl₃ are rotated out of the plane defined by the aromatic ring. The values of α are in the range 1.5 ± 0.1° to 1.7 ± 0.1° for the temperature range 295–330 K.