A positive 6J(H,CHO) in some meta derivatives of benzaldehyde. A simple model
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Abstract
6 J(H,CHO), the long-range coupling constant between the aldehydic and para protons in benzaldehyde, has not been detected, possibly because the σ–π interaction giving rise to a negative coupling is intrinsically rather small and because the internal barrier to rotation about the [Formula: see text] bond is large. However, 6 J(H,CHO) in some meta substituted derivatives is actually positive and as large as 0.09 Hz in 3,5-difluorobenzaldehyde; the barrier to internal rotation in this molecule is some 4 kJ/mol lower than in the parent molecule. The magnitude of 6 J(H,CHO) in these derivatives correlates well with σ R.ST values, a recent set of substituent constants derived from 13 C nuclear magnetic resonance chemical shifts of the β carbon in styrene derivatives. It is hypothesized that the substituents with negative σ R.ST values stabilize an ionic valence bond structure that has a positive 6 J(H,CHO). A brief discussion of 4 J(H,CHO) in some of these molecules is also presented. Keywords: NMR, spin coupling NMR, benzaldehyde.
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