Structures and energetics of hydrated deprotonated cis-pinonic acid anion clusters and their atmospheric relevance.

Pinonic acid, a C10-monocarboxylic acid with a hydrophilic -CO2H group and a hydrophobic hydrocarbon backbone, is a key intermediate oxidation product of α-pinene - an important monoterpene compound in biogenic emission processes that influences the atmosphere. Molecular interaction between cis-pinonic acid and water is essential for understanding its role in the formation and growth of pinene-derived secondary organic aerosols. In this work, we studied the structures, energetics, and optical properties of hydrated clusters of the cis-pinonate anion (cPA-), the deprotonated form of cis-pinonic acid, by negative ion photoelectron spectroscopy and ab initio theoretical calculations. Our results show that cPA- can adopt two different structural configurations - open and folded. In the absence of waters, the open configuration has the lowest energy and provides the best agreement with the experiment. The added waters, which mainly interact with the negatively charged -CO2- group, gradually stabilize the folded configuration and lower its energy difference relative to the most stable open-configured structure. Thermochemical and equilibrium hydrate distribution analyses suggest that the mono- and di-hydrates are likely to exist in humid atmospheric environments with high populations. The detailed molecular description of cPA- hydrated clusters unraveled in this study provides a valuable reference for understanding the initial nucleation process and aerosol formation involving organics containing both hydrophilic and hydrophobic groups, as well as for analyzing the optical properties of those organic aerosols.