The sign of the 'through-space' spin–spin coupling between methyl protons and the fluorine nucleus in 2,6-dimethylbenzoyl fluoride

Abstract

The spin–spin coupling constant over five formal bonds between 19F and methyl protons, [Formula: see text], in 2,6-dimethylbenzoyl fluoride is −3.1 Hz. Observation of a nonzero [Formula: see text] indicates an out-of-plane conformation for the carbonyl fluoride group and implies substantial nonbonded repulsions between the methyl and carbonyl fluoride groups. It is argued that [Formula: see text] is as small as −7 Hz when the C—F bond lies cis to a methyl group and that its magnitude is a consequence of the so-called 'through-space' coupling mechanisms. On the basis of INDO–MO–FPT computations, it is suggested that such observed couplings are a composite of large contributions of either sign and, therefore, that observed through-space 1H,19F couplings may be of either sign if conformational averaging occurs.