The effect of substituents on proton coupling constants: Simple theoretical correlations

The nature and types of lone-pair effects on nuclear spin coupling constants are reviewed in the context of a localized bond description of molecular electronic structure. Emphasis is placed on the importance of residual delocalization involving the otherwise lone-pair orbital, in terms of which the effect of an X lone-pair, when compared with an isoelectronic Y-H or X-H+ group or with an X-R group, and its orientational dependence can be interpreted. One-, two-and three-bond coupling constants are considered and the importance of lone-pair effects for configurational and conformational information is stressed and illustrated. They also serve for a better understanding of substituent inductive effects on coupling constants.A large collection of illustrative examples are presented, with particular attention paid to couplings involving H, C, N, F and P nuclei, organized in a systematic manner into nine categories. The signs of the lone-pair effects on the reduced coupling constants are found to be independent of the actual nuclei under study in the same category. This is taken as an indication that the electron lone-pairs mainly affect the Fermi contact contribution to the coupling and, accordingly, an interpretation is given in terms of simple sum-over-states models. In addition, symmetry-based relationship are established involving the sign of lone-pair effects in coupling constants between nuclei which are a different number of bonds apart.KEY WORDS NMR Multinuclear spin coupling Lone-pair effects Substituent effects Stereochemistry MO Sum-over-states models Electron delocalization CONTENTS

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