Extended Characterization of the Kinetics of α-Pinene Oxide Hydrolysis at Varying pH via Raman Spectroscopy

Secondary organic aerosols (SOA) derived from biogenic volatile organic compounds (BVOCs) affect climate and human health; however, there is uncertainty associated with the extent of their impact, in part due to the complexity of their physicochemical composition and varied heterogeneous chemistry. α-Pinene oxide is an oxidation product of α-pinene, the second most abundant BVOC, and it has been found to undergo rapid hydrolysis, forming campholenic aldehyde, trans-carveol, and trans-soberol, which can then react further. Although a mechanism for α-pinene oxide hydrolysis has been proposed and the kinetics have been studied previously, the rate constants for the initial step in this reaction have not been determined because the reaction occurred too quickly to detect any α-pinene oxide. Herein, Raman spectroscopy was used to probe α-pinene oxide hydrolysis in bulk aqueous solutions, expanding on previous kinetic analyses by making measurements on a shorter time scale (<5 min reaction time) and testing a wider range of acidity conditions. Campholenic aldehyde and pinol were the primary products, with pinol being produced only under highly acidic conditions. The pseudo-first-order rate constants for the initial consumption of α-pinene oxide were calculated to be on the order of 10-2-10-3 s-1. Additionally, full characterization of the experimental and density functional theory (DFT)-predicted Raman spectra and vibrational modes for α-pinene oxide and subsequent products is provided, highlighting signature vibrational modes that can be used for the identification of these chemical species. This work further develops the understanding of α-pinene oxide chemistry relevant to α-pinene-derived SOA.