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

Earlier studies with 2-bromocyclohexanone demonstrated a measurable long-range coupling constant (4 JH2,H6 ) for the equatorial conformer, although 4 JH2,H4 and 4 JH4,H6 were not observed; as a consequence, it is inferred that the carbonyl group plays an important role particularly due to hyperconjugative interactions σC2H2 →π*C═O and σC6H6 →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 σC2H2 →π*C1═Y and σC6H6 →π*C1═Y (Y═O and S) hyperconjugative interactions for the equatorial conformer indeed contribute for the 4 JH2,H6 transmission mechanism. Surprisingly, the 4 JH2,H6 value is higher for the carbonyl compounds, although the interactions σC2H2 →π*C═Y and σC6H6 →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 JH2,H6 values. Moreover, both πC═S →σ*C─X and πC═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 JH2,H6 for the thiocarbonyl compounds appeared to be more sensitive than for the cyclohexanones.

Conformational analysis, stereoelectronic interactions and NMR properties of 2-fluorobicyclo[2.2.1]heptan-7-ols.

Four diastereoisomers of 2-fluorobicyclo[2.2.1]heptan-7-ols were computationally investigated by using quantum-chemical calculations, and their relative energies were analyzed on the basis of stereoelectronic interactions, particularly the presence or otherwise of the F∙∙∙HO intramolecular hydrogen bond in the syn-exo isomer. It was found through NBO and AIM analyses that such an interaction contributes to structural stabilization and that the (1h)J(F,H(O)) coupling constant in the syn-exo isomer is modulated by the n(F)→σ*(OH) interaction, i.e., the quantum nature of the F∙∙∙HO hydrogen bond.