A conformational study of (1S, 2R, 5S)-(+)-menthol using vibrational circular dichroism spectroscopy

We report the experimental and predicted absorption and vibrational circular dichroism (VCD) spectra of(1S, 2R, 5S)-(+)-menthol. The harmonic force fields and atomic polar tensors (APTs) were obtained for 10 conformers using density functional theory (DFT) with the Becke3LYP functional and the 6-31G** basis set. The atomic axial tensors (AATs) were obtained with restricted Hartree-Fock (RHF) theory and a 6-31G basis set. VCD intensities were predicted from vibronic coupling theory (VCT). The predicted absorption and VCD spectra based on an equilibrium mixture of conformers corresponded well with the experimental spectra, indicating a reasonable geometry and frequency analysis for each of the conformers. The largest contributions towards the equilibrium population derive from conformers in which the cyclohexane ring occupies a chair conformation, the isopropyl group is directed away fi-om the hydroxyl group, and only the hydroxyl group assumes different conformations.