Effect of Electronegativity and Magnetic Anisotropy of Substituents on C13 and H1 Chemical Shifts in CH3X and CH3CH2X Compounds
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.