Some remarks on the diamagnetic anisotropy of C-C and C-H bonds in saturated hydrocarbons

Abstract

Data on the diagmagnetic anisotropies of C-C and C-H bonds obtained by measurements of chemical shifts, crystal diamagnetic susceptibilities, and Cotton-Mouton constants are discussed. It is suggested that for both C-C and C-H bonds the absolute value of the transverse susceptibility is greater than the absolute value of the longitudinal susceptibility. A relation for the anisotropy of a straight chain paraffin in terms of the bond anisotropies is derived. It is concluded that the crystal susceptibilities and Cotton-Mouton constants can be explained if the C-H bond anisotropy is greater than about one-half of the C-C bond anisotropy. It is suggested that values for the bond anisotropies obtained from proton chemical shifts are too high, owing to other contributions to the proton shielding, and to the breakdown of the classical magnetic dipole equation. Values for the C-C and C-H bond anisotropies consistent with the available data are given.