A theoretical study on UV-spectroscopy, electronic structure and reactivity properties of sesquiterpenes

Abstract. Sesquiterpenes, a class of biogenic volatile organic compounds, are important precursors to secondary organic aerosols (SOAs) in nature. Using density functional theory (DFT), conceptual DFT, time-dependent (TD) DFT, configuration interaction with single excitation (CIS), and Zerner's intermediate neglect of differential overlap (ZINDO) methods, the electronic structures, spectroscopy, and reactivity of sesquiterpenes were systematically investigated. Results from the CIS calculations show the best consistency in the excited energies and allow for assigning and predicting newly found sesquiterpenes. The results suggest that the first peaks in the ultraviolet-visible (UV-vis) absorption spectra for saturated and unsaturated isomers are σ–σ* and π–π* transitions, respectively. It can be deduced from the transit intensities of the isomers that an isomer with an endocyclic C = C bond presents weaker UV transition intensity than its corresponding exocyclic isomer. The electronic structures of these compounds were also analyzed by comparing published UV-spectroscopy with advanced theoretical calculations. α-Zingiberene and longicyclene are the most and least reactive in electron-transfer reactions, respectively. No quantitative linear relationships were discovered between the changes in transit energies, DFT chemical reactivity indices of isomers, different degrees of unsaturated C = C double bonds, or the number of substituents attached to the C = C bond. The larger steric hindrance of substituents or exocyclic C = C bond is related directly to higher chemical reactivity possessed by the isomer compared to a corresponding isomer with smaller steric hindrandce or with an endo C = C bond. These results are imperative to a better understanding of SOA production mechanisms in the troposphere.