PNNL

Abstract

Cis-pinic acid is one of the most important oxidation products of a-pinene – a key monoterpene compound in biogenic emission processes affecting the atmosphere. Molecular level understanding of interactions involved in the cluster formations around cis-pinic acid is an important and necessary prerequisite toward ascertaining its role in the aerosol formation processes. In this work, we studied the structures and energetics of the solvated clusters of cis-pinate (cis-PA2–), the doubly deprotonated dicarboxylate of cis-pinic acid, with H2O, CH3OH, and CH3CN by negative ion photoelectron spectroscopy and ab initio theoretical calculations. We found that cis-PA2– prefers being solvated alternately on the two ?CO2– groups with increase of solvent coverage, a well-known solvation pattern that has been observed in microhydrated linear dicarboxylate dianion (DCn2–) clusters. Experiments and calculations further reveal an intriguing feature for the existence of the asymmetric type isomers for cis-PA2–(H2O)2 and cis-PA2–(CH3OH)2, in which both solvent molecules interact with only one of the ?CO2– groups – a phenomena that has not been observed in DCn2– water clusters and exhibits the subtle effect of the rigid four-membered carbon ring brought in on the cis-PA2– solvation. The dominant interactions between cis-PA2– and solvent molecules are forming bidentate O–···H–O H-bonds for H2O, O–···H–O and O–···H–C H-bonds for CH3OH, and tridentate O–···H–C H-bonds for CH3CN. The formation of inter-solvent H-bonds between H2O and CH3CN is found to be favorable in mixed solvent clusters, distinctly different from that between H2O and CH3OH. These findings have important implications for understanding the mechanism of cluster growth and nucleation of atmospheric organic aerosols and the nature of structure-function relationship of proteins containing carboxylate groups under various solvent environments.