Nuclear magnetic resonance studies in liquid crystal solvents

Abstract

A brief account of the historical development, theoretical treatment, and experimental aspects of nuclear magnetic resonance spectroscopy in liquid crystal solvents is given. Results of anisotropic NMR spectral investigations of thiazole and monodeuterated thiazoles, 2,5-dibromothiophene, N,N-dimethylformamide, and N,N-dimethylformamide-d7 are presented. The NMR spectrum of thiazole oriented in a nematic phase has been studied. Since the PMR spectrum of oriented systems are usually dominated by the dipolar couplings, a system of three non-equivalent protons without symmetry provides only two equations for the three unknowns (three dipolar couplings). The use of deuterium substitution to augment the number of available equations in these underdetermined systems has been explored. It is shown that the synthesis of 2d-thiazole and 5d-thiazole followed by a determination of their anisotropic spectra can aid in the spectral analysis of thiazole. A combination of the three dipolar couplings from the results of this study with the microwave structural data of thiazole enabled a determination of the elements of the order matrix. The direction of the largest component of the principal order tensor for thiazole is shown to be shifted from the dipole moment direction by only 8°. It is proposed that dipole moment might be important in influencing the orientation of some small molecules. The anisotropic proton and carbon-l3 NMR spectra of 2,5-dibromothiophene have been studied. A theoretical analysis of this problem shows that the carbon-l3 spectrum depends only on the sums of certain dipolar couplings. A computer program has been written to aid in the extraction of the individual dipolar couplings from their sums. Assuming the remainder of the molecular geometry and combining the results of the dipolar couplings from the proton and the carbon-l3 spectra, the interproton distance in 2,5-dibromothiophene has been o computed as 2.64 ± 0.02 A. The anisotropic proton magnetic resonance spectrum of N,N-dimethylformamide (DMF) has been analyzed. From the derived dipolar coupling constants and using the best available structure of DMF it is shown that the NMR results are consistent only with rapidly inverting nonplanar conformers. Although the extent of the nonplanar distortions as measured by the two out-of-plane angles depend on the model assumed to describe the internal rotation of the methyl groups, the results of this spectral study could not be fitted to a reasonable planar structure of DMF. The quadrupole splittings in the deuteron magnetic resonance spectrum of DMF-d7 have also been examined with a view to measure the quadrupole coupling constants. The elements of the order tensor necessary for the calculation of the quadrupole coupling constants were obtained by an examination and analysis of the proton spectrum of a mixture of DMF and DMF-d7 under the same conditions of temperature and concentration. Assuming that the field gradient has axial symmetry, the quadrupole coupling constants (in kilohertz) along the bond axis were determined to be 162 ± 5 for the methyl deuterons trans to the amide deuteron, 161 ± 5 for those cis to the amide deuteron (with the assumption of 108.5° for the DCD bond angle within the CD 3 groups). The quadrupole coupling constant of the amide deuteron is sensitive to the assumed model of the methyl rotation and it lies in the range of 154 to 181 KHz.