UV-spectroscopy, electronic structure and ozonolytic reactivity of sesquiterpenes: a theoretical study

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Sesquiterpenes, one of the most important classes of biogenic volatile organic compounds, are potentially significant precursors to secondary organic aerosols (SOAs) in nature. The electronic structure of sesquiterpenes and their reactivity in the ozonolysis reaction were investigated by density functional theory. Results from the CIS calculations combined with an analysis of transition intensities show that the first peaks in the ultraviolet (UV) spectra for saturated and unsaturated isomers are σ-σ and π-π transitions, respectively. The UV absorption wavelength and absorbency are dictated by the electronic structures of these compounds. An increase in the number of double bonds and formation of a conjugated system expand the range of absorption in the UV region. An isomer with an endocyclic C = C bond presents weaker UV transition intensity than its corresponding exocyclic isomer. Results from conceptual DFT chemical reactivity indices of isomers suggest that no quantitative linear relationships between the structural changes and their reactivity, such as different degrees of unsaturated C = C double bonds, or the number of substituents attached to the C = C bond were discovered. In the ozonolysis reaction of sesquiterpenes, isomers with larger steric hindrance of substituents or endocyclic C = C bond possess higher chemical reactivity compared to isomers with smaller steric hindrandce or with an exocyclic C = C bond. These results are imperative to a better understanding of SOAs production mechanisms in the troposphere.