Conformational Space and Photochemistry of r-Terpinene

R-Terpinene is a natural product that is isolated from a variety of plant sources and is used in the pharmaceutical and perfume industries. In the atmosphere, under the influence of sunlight, R-terpinene undergoes a series of photochemical transformations and contributes to the formation of the secondary organic aerosols. In the present work, R-terpinene has been isolated in low-temperature xenon and argon matrices, and its structure and photochemistry were characterized with the aid of FTIR spectroscopy and DFT calculations. The theory predicts three conformers resulting from the rotation of the exocyclic CH(CH3)2 framework, that is, Trans (T) and Gauche (G+ and G-) forms. The two Gauche conformers were estimated to be higher in energy, by ca. 1.75 kJ mol-1, than the most stable Trans form. The signatures of all three conformers were found to be present in the experimental low-temperature matrix spectra with the T form dominating in diluted matrices. The conformational ratio was found to shift in favor of the G+/Gforms upon annealing of the matrices as well as in the neat R-terpinene liquid. UV-C (λ > 235 nm) irradiation of matrix-isolated R-terpinene led to its isomerization into an open-ring species, which is produced in the Z configuration and in the conformations that require the smallest structural rearrangements of both the reagent and matrix.