NMR STUDIES OF AN AMINE AND AMIDES IN A NEMATIC PHASE

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The technique of NMR spectroscopy in liquid crystal solvents 1.2 will be applied to a systematic study of the stereochemistry at the nitrogen atom in substituted amides and an amine. Such model compounds (amides), containing a peptide unit, are of fundamental biochemical and spectroscopic importance for studying the structure and conformation of peptides and proteins. The determination of molecular structure by the anisotropic NMR method is based on the fact that the intramolecular dipole-dipole couplings, dominating such spectra, are inversely related to the cube of the internuclear distances of the interacting nuclei. The amide system studied in the past, with the notable exception of N-methylformamide, have been studied primarily by anisotropic 1H NMR spectroscopy. However, a study of the proton spectra in conjunction with the 13C and/or 15N NMR spectra of isotopically enriched simple amides can result in a precise determination of molecular geometrical parameters, including an estimation of dihedral angles of distortion around the nitrogen atom with minimum assumptions of bond data. This is desirable as some of the bond data are of questionable reliability. The increased number of heteronuclear dipolar couplings in isotopically enriched (13C and/or 15N) systems permits the extraction of additional pieces of precise structural information, and this has been demonstrated for 15N- (13C)methyl(13C)formamide.7 Although an analysis of the anisotropic NMR spectra containing heteronuclei usually provides only the sum of spin-spin (JAX) and dipolar (DAX) coupling constants, namely (JAX+2DAX), it is feasible to apply a recently developed technique 8 of observing the anisotropic spectra by employing a mixture of two nematic phases of opposite 8 of observing the anisotropic spectra by employing a mixture of two nematic phases of opposite diamagnetic anisotropy. Such an observation at the "co-existence point" facilitates an experimental disentanglement of JAX from DAX parameters. In this proposal, a systematic study of isotopically enriched (15N)aniline and substituted amides such as (15N)acetamide, N>N>- dimethylform(13C)-amide and (15N)dimethylacetamide is presented. The long term objectives and the specific goals of this research are: (1) to determine the details of molecular structure, including the out-of-plane angles which the N-C'H3 and N-C"H3 bonds make with the symmetry plane in dimethylformamide and dimethylacetamide; (2) to examine trends and correlations in the extent of non-planar distortions as a function of the nature and the size of groups attached to the nitrogen; (3) to test the various models for describing the dynamics of intramolecular rotations of the methyl groups in substituted amides; and (4) to determine the liquid state quadrupole coupling constants of the various deuterons in these deuterated amides.