Molecular Insights into the Photochemistry of cis-Pinonic Acid and Its Complexes with O2 and H2O

cis-Pinonic acid (cPA) is one of the major photooxidation products of α-pinene and contributes to the formation of biogenic secondary organic aerosol (BSOA). Although the direct photolysis of cPA in aqueous solution has already been described, showing mainly Norrish II-type reaction, the molecular process, in particular the role of water and oxygen, has not yet been fully understood. This experimental work aims to study cis-pinonic acid and how its complexation with water and/or oxygen influences its ultraviolet (UV) photochemistry. One method for studying these complexes at the molecular level in their ground state is the cryogenic matrix technique coupled with Fourier transform infrared (FTIR) spectroscopy, which can be compared with density functional theory (DFT) theoretical calculations of the potential energy surface (PES) minima and their sets of vibrational frequencies. We studied cPA and its complexes in four cryogenic matrices: nitrogen, argon, neon, and xenon, to evaluate the potential matrix influence. The effects of broad band UV irradiation using a mercury vapor lamp under solar conditions (wavelength greater than 290 nm) can be contrasted with unfiltered irradiations. Although the observed effects are generally similar for all matrices, we clearly identified syn-anti isomerization of the carboxyl group in the nitrogen matrix even under solar conditions irradiation. When irradiated without a filter, carbon dioxide, carbon monoxide, and water are formed, showing the exclusive Norrish I-type reaction in the matrix. Neither water nor oxygen alters the photoinduced fragmentation. Even when it forms a complex with cPA, water appears to be a spectator to this fragmentation. However, the presence of oxygen considerably increases the amount of carbon dioxide and carbon monoxide produced, which confirms a triplet state transition during the electronic relaxation of cPA.