Conformational analysis of cis-2-halocyclohexanols; Solvent effects by NMR and theoretical calculations

摘要

Conformational problems often involve very small energy differences, even low as 0.5 kcal mol~(-1). This accuracy can be achieved by theoretical methods in the gas phase with the appropriate accounting of electron correlation. The solution behavior, on the other hand, comprises a much greater challenge. In this study, we conduct and analysis for cis-2-fluoro-, cis-2-chloro-, and cis-2-bromocyclohexanol using low temperature NMR experiments and theoretical calculations (DFT, perturbation theory, and classical molecular dynamics simulations). In the experimental part, the conformers' populations were measured at 193 K in CD_2Cl_2, acetone-d_6, and methanol-d_4 solutions; the preferred conformer has the hydroxyl group in the equatorial and the halogen in the axial position (ea), and its population stays at about 60-70%, no matter the solvent or the halogen. Theoretical calculations, on the other hand, put the ae conformer at a lower energy in the gas phase (MP2/6-311++G(3df,2p)). Moreover, the theoretical calculations predict a markedly increase in the conformational energy on going from fluorine to bromine, which is not observed experimentally. The solvation models IEF-PCM and C-PCM were tested with two different approaches for defining the atomic radii used to build the molecular cavity, from which it was found that only with explicit consideration of hydrogens can the conformational preference be properly described. Molecular dynamic simulations in combination with ab initio calculations showed that the ea conformer is slightly favored by hydrogen bonding.