Abstract
The natural J-coupling (NJC) method is applied to analyze the Fermi contact contribution of the NMR spin–spin coupling constant decomposing this contribution in terms of natural localized molecular orbitals. We investigated the influence of the basis set on the NJC analysis for the formyl group coupling constant (1JCHf) of benzaldehyde derivatives. NJC and other NBO analyses, like steric and natural Coulombic energy, were chosen to explain the influence of electron-donating and electron-withdrawing groups on 1JCHf for some substituted benzaldehydes (Me, OH, OMe, F, Cl, Br, I, and NO2). For the ortho derivatives, electronegative substituents near the C–Hf bond increase the 1JCHf coupling. This effect could be related to an increase in formyl carbon s character and changes in the carbon and hydrogen natural charges. This indicates that the substituents in ortho have a proximity effect on 1JCHf coupling mainly of electrostatic origin instead of the expected hyperconjugative interactions.
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