Study of isotopic effects and determination of the quadrupole coupling constant from the nuclear magnetic resonance spectra of monodeuterobenzene in a nematic phase

Abstract

The proton and the deuteron magnetic resonance spectra of monodeuterobenzene have been studied in the nematic phase of a mixture of 50% 4-methoxybenzylidene-4-amino-α-methyl cinnamic acid-n-propyl ester and 50% anisole–azophenyl-n-capronate at room temperature.The proton spectrum is studied as a superposition of three 5-spin sub-spectra due to coupling with one deuteron (spin I = 1) and analyzed iteratively as a 7-spin system, treating the deuteron as two spin [Formula: see text] particles.The values of the dipole–dipole couplings show minor but significant deviations due to isotopic substitution, from those expected for the regular hexagonal geometry. Two parameters are necessary to define molecular orientation. The ratio of the two orientation parameters is reported.The ratios (rHD/rHH)ortho and (rCH − rCD)/(rHH)ortho are obtained. They indicate a reduction in the time average value of the C—D bond length compared to the C—H one.It is suggested that the orientation parameters from the proton spectrum of the molecule are used in order to determine the deuteron quadrupole coupling constant from deuteron magnetic resonance (d.m.r.) spectra of partially deuterated molecules dissolved in the nematic phase. The method is applied to the present case. Its limitations and the sources of errors are discussed.