Probing long-range spin-spin coupling constants in 2-halo-substituted cyclohexanones and cyclohexanethiones: The role...

Earlier studies with 2-bromocyclohexanone demonstrated a measurable long-range coupling constant (⁴ J_H2,H6 ) for the equatorial conformer, although ⁴ J_H2,H4 and ⁴ J_H4,H6 were not observed; as a consequence, it is inferred that the carbonyl group plays an important role particularly due to hyperconjugative interactions σ_C2H2 →π*_C═O and σ_C6H6 →π*_C═O. In the present study, NBO analysis and coupling constant calculations were performed to cyclohexanone and cyclohexanethione alpha substituted with F, Cl, and Br, aiming to evaluate the halogen effect and acceptor character of the π* orbital on the long-range coupling pathway. The σ_C2H2 →π*_C1═Y and σ_C6H6 →π*_C1═Y (Y═O and S) hyperconjugative interactions for the equatorial conformer indeed contribute for the ⁴ J_H2,H6 transmission mechanism_. Surprisingly, the ⁴ J_H2,H6 value is higher for the carbonyl compounds, although the interactions σ_C2H2 →π*_C═Y and σ_C6H6 →π*_C═Y are more efficient for the thiocarbonyl compounds. Accordingly, the Fermi contact (FC) contribution for the thiocarbonyl compounds decays deeper than in ketones, thus reducing more the ⁴ J_H2,H6 values. Moreover, both π_C═S →σ*_C─X and π_C═S →σ*_C─H interactions seem to be stronger in thiocarbonyl than in carbonylic compounds. The implicit solvent effect (DMSO and water) on the coupling constant values was negligible when compared with the gas phase. On the other hand, an explicit solvent effect was found and ⁴ J_H2,H6 for the thiocarbonyl compounds appeared to be more sensitive than for the cyclohexanones.