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

Abstract

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.