No abstract

C NMR chemical shifts are reported for several alkyl or aryl amines, acetamides, diacetamides and ammonium and trimethylammonium salts. Substituent effects, determined with respect to chemical shifts of amino derivatives, are rationalized in terms of y effects, steric interactions and, for benzene derivatives, in terms of competition between inductive (a,) and resonance (uR) effects. These perturbations can be used to assign chemical shifts of complex structures and to study certain chemical processes. 13

Thirty-four benzoxazoles, oxazole, and 10 o-aminophenols have been studied by 13C nmr spectroscopy. All the signals have been attributed from substituent effects. The structure of products obtained by nitration of benzoxazole have been determined by 13C nmr. The shifts induced by substitution at the 2 position have been discussed as a function of an empirical model using the structural parameters F, R, and Q*. Azido–tetrazole equilibria (N3 in position 2) and prototropic tautomerism (NH2, OH, and SH in position 2) have been also discussed. The chemical shifts and coupling constants of oxazole and unsubstituted benzoxazole have been compared.

We have recorded the "C n.m.r. spectra of thiones and thioethers in the 1,3-diazole and 1J-diazine series with various alkyl substituents at the nitrogen atoms. Some analogous oxygen containing heterocycles were also examined. We have shown that in the thiocarbonylated compounds the thiol thione equilibrium is displaced towards the thione form, but that "C n.m.r. gives only qualitative results. In the sulphur containing derivatives the isopropyl group is in a fixed conformational position because of the steric hindrance of the sulphur atom. Substitution by a rerbbutyl group leads to unexpected y values. We ascribe this phenomenon to ring deformation or to variations in the valence angles of the substituted nitrogen atoms.Nous avons enregistrt les spectres de RMN du carbone-13 de thiones et de thioethers, en strie diazolique et diazinique-1,3, diversement substituts sur les atomes d'azote par des groupements alkyles, ainsi que certaines des structures oxygtntes homologues. Toutes les raies de rhsonance ont Btt attributes. Dans le cas des dtrivts thiocarbonylts, nous montrons que l'tquilibre thiol thione est dtplact vers la forme thione, mais que la RMN du carbone-13 ne peut conduire qu'h une tvaluation qualitative de la position de cet tquilibre.L'analyse des effets de substituant montre que, contrairement aux composts oxygknb, Ie groupement isopropyle adopte dans les dtrivts soufrts une conformation privilkgite par suite de l'encombrement sttrique dO B l'atome de soufre. La substitution par un groupement tertiobutyle conduit B des valeurs des effets y inattendus. Nous attribuons ce phgnom5ne B des dgfonnations de cycle ou B des variations des angles valentiels de l'azote porteur du substituant, provoqutes par des interactions sttriques entre le substituant et le cycle correspondant. Nous relions ces modifications B certaines modalitts de la rtactivitt, et B des risultats de spectroscopie photoClectronique.