Conformational preference of a chiral terpene: vibrational circular dichroism (VCD), infrared and Raman study of S-(-)-limonene oxide.

S-(-)-Limonene oxide (4-isopropenyl-1-methyl-7-oxabicyclo[4.1.0]heptane) is a secondary metabolite and an atmospheric pollutant product of oxidation of other terpenes, such as limonene and alpha-pinene, among others. For the first time, a theoretical study of the molecular structure and a theoretical and experimental analysis of the infrared and Raman spectra of this chemical species are presented. Theoretical calculations reveal the existence of two conformers depending on the position of the isopropenyl group (axial and equatorial) and twelve rotamers (six equatorials and six axials), with the six equatorial ones as the most stable (around 97%) on the basis of Boltzmann's distribution law. In the current work, from a reliable assignment of the IR and Raman spectra and with the help from the study of the VCD spectrum of the title compound, five rotamers have been detected experimentally in the liquid phase. The present work reveals that IR, Raman and VCD are helpful complementary techniques to characterize flexible systems, as terpenes, which present several conformers.