Conformational Flexibility in Terpenes: Vibrational Circular Dichroism (VCD), Infrared and Raman Study of <i>S</i>-(−)-Perillaldehyde

Abstract

S-(-)-Perillaldehyde (4-isopropenylcyclohex-1-ene-1-carbaldehyde) is a secondary metabolite and an atmospheric pollutant obtained from the oxidation of other terpenes, as limonene and alpha-pinene, among others. For the first time, a study of the molecular structure and the infrared and Raman spectra of this chemical is 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), being three of these equatorial rotamers the most stable (ca. 93%). A complete and reliable assignment of the IR and Raman spectra of the title compound is made, taking into account these three last rotamers, and the results are used to carry out a detailed interpretation, for the first time to our knowledge, of the VCD spectrum of this compound in the 1500-900 cm (-1) region.