Natural J coupling (NJC) analysis of the electron lone pair effect on NMR couplings: 2. The anomeric effects on ¹J(C,H) couplings and its dependence on solvent

The electronic origin of the influence of the anomeric effect (negative hyperconjugative interaction, NHI) on the Fermi contact (FC) term of ¹J(C,H) couplings has been studied from a theoretical point of view at the DFT-B3LYP level. The HN=CH₂ molecule was chosen as the primary model compound, in which both FC ¹J(C,H) couplings were decomposed into bond contributions with the natural J coupling dissection approach (NJC). Differences between the ¹J(C,H)^FC couplings for C-H bonds in synperiplanar and antiperiplanar orientations with respect to the nitrogen non-bonding electron pair closely follow the experimental trend. They are made up chiefly of three NJC contributions: 'bond', 'direct lone pair' and the 'carbon-core orbitals'. The NHI influence on these terms was studied by applying the natural bond orbital (NBO) deletion procedure to the charge transfer interaction into the antiperiplanar (C-H) antibond (n(N)→(C-H)*) prior to the NJC dissection calculation. The dielectric solvation effect on both the total FC terms and the respective NJC contributions was estimated by carrying out the calculations using the polarization continuum model. Inhibition of the anomeric effect is evident when the solvent polarity is increased. NHI saturates rapidly with increasing solvent dielectric. Specific solute-solvent interaction effects on ¹J(C,H) couplings were estimated by evaluating molecular complex models of the form CH₂=HN⋯S (S = H₂O and DMSO).