No abstract

The origin of the relative nuclear resonance shifts in monosubstituted benzenes has been investigated. In order to obtain more complete experimental information both C13 and H1 resonance shifts in a variety of aryl-X compounds were measured. The H1 resonances were measured on 5 mole % solutions in cyclohexane to minimize solvent effects; the carbon shifts were obtained from natural abundance C13 resonance measurements in the neat liquid. Unambiguous assignments of both H1 and C13 resonance spectra were made possible with the aid of deuterated compounds. The largest resonance shifts were observed for the carbon atom directly bonded to X. As in the corresponding CH3X compounds, these shifts arise primarily from the inductive and magnetic anisotropy effects of X. Magnetic anisotropy effects of X are also observable in both the C13 and H1 resonances at the ortho position. A very close correspondence between C13 and H1 resonances is observed at the para position, where the primary contribution to the relative shifts arises from resonance effects of X. This implies that the proton resonance responds to the π-electron density on the carbon to which it is bonded, and that under favorable conditions, both H1 and C13 resonance shifts might be employed to obtain information about the π-electron density distribution in aromatic systems. At the meta position the C13 resonance shifts are surprisingly small and uniform, indicating small or negligible inductive effects due to X, and there is no evident correlation with the meta-proton shifts. Both the C13 and H1 shifts at the para position show an approximate correlation with chemical reactivity parameters (Hammett σ constants) but no such correlation exists for the meta-carbon or meta-hydrogen shifts.

An investigation has been made of the major contributions which make up the relative chemical shifts in CH3X and CH3CH2X compounds. In order to obtain more detailed information, both the carbon and hydrogen chemical shifts were measured. The carbon shifts were obtained by measuring natural abundance C13 resonances in the pure liquids; the H1 resonances were measured on gaseous samples to avoid solvent effects. The results reveal surprisingly large contributions to both C13 and H1 shifts arising from magnetic anisotropy effects of the X substituent. In CH3X compounds, the contribution to the proton shifts is negative while that to the C13 shifts is positive. In CH3CH2X compounds, these effects contribute to the resonance shifts of carbon and hydrogen nuclei in both the methylene and methyl group. When such contributions are allowed for, an approximate correlation with the electronegativity of X can be obtained, indicating that inductive effects, together with anisotropy effects, account for the major part of the relative chemical shifts in these molecules. The quantitative determination of inductive parameters of substituents from chemical shift data is, however, somewhat limited. The presence of a large magnetic anisotropy within the molecule also affects the nuclear resonance shifts of neighboring molecules and gives rise to a ``solvent dilution shift''; for the C13 resonance of CH3I this amounts to 7.3 ppm.

Rksumk.-Les temps de relaxation nucleaire, T I Z et T I D , et les coefficients de diffusion Dl, et DL ont kt6 mesures en fonction de la temperature dans ethyl p-(p-Cthoxybenzylid6ne)-amino-cinnamate (EBAC) et dans TBBA. Les rksultats montrent que la diffusion contribue aux vitesses de relaxation dans les phases smectiques A, C et H de TBBA et les phases A et B de EBAC. La diffusion perpendiculairement au directeur est clairement du type difSusion liquide dans les phases A et C. Dans la phase B de EBAC, d'autre part, la diffusion se fait par sauts comme dans un pseudorbseau.

No abstract