Spin–spin coupling between proximate protons on neighbouring aromatic rings and between hydroxyl protons in 2,2′-dihydroxy-4-methoxybenzophenone. Coupling through the hydrogen bond

Abstract

The doubly hydrogen bonded conformation of 2,2′-dihydroxy-4-methoxybenzophenone ensures substantial nonbonded interactions between C—H bonds on neighbouring aromatic rings and gives rise to spin–spin coupling between the protons in those bonds. Because of the relative orientation of the C—H bonds containing the coupled protons, the coupling represents a direct mechanism which probably does not depend on the orbitals of the carbon atom. The observed coupling between the hydroxyl protons, formally over eight bonds, is small and indicative of an inappreciable covalent character of the hydrogen bond, in agreement with recent conclusions that a weak hydrogen bond is best represented by electrostatic and non-bonded forces only.