Mechanistic insight into the kinetic fragmentation of Norpinonic Acid in the gas phase: An experimental and DFT study

Abstract. Norpinonic acid has been known as an important α-pinene athmospheric SOA (Secondary Organic Aerosol) component. It is formed in the reaction of α-pinene, β-pinene or verbenone with atmospheric oxidizing reagents, such as ozone (O3) and hydroxy radicals. In the presented studies, tandem mass spectrometry techniques were used to determine the exact norpinonic acid fragmentation pathway in the gas phase. The precursor anion – deprotonated norpinonic acid (m/z 169) generated in an electrospray (ESI) source were subjected into the collision cell of the mass spectrometer and fragmented using CIE (Energy-Resolved Collision Induced Dissociation) technique. Based on the analysis of the breakdown curves, the experimental energy values required to initiate the gas – phase degradation processes were determined. Quantum chemical calculations of the reaction models for observed fragmentation processes were also constructed, including calculation of all transition states presented in the reaction mechanism. Comparison between the experimental and the theoretical threshold energies calculated at ωB97XD/6-311+G(2d,p) level of theory has shown a very good correlation. Two basic pathways of the fragmentation of the parent anion [M-H]⁻ (m/z 169) were observed. A first, lead to the decarboxylation product (m/z 125) and second to the loss of neutral molecule (C₄H₆O) together with the formation of anion m/z 99. Loss of C₃H₆ or C₂H₄O molecules and formation of the anion m/z 41, together with anion m/z 55, were found for fragment anion m/z 99. Further breaks down of anion m/z 125 give a rise of 69, 57 and 55 m/z ions. To confirm structures formed during ER-CID experiments, the gas-phase proton transfer reactions were examined of all Norpinonic acid anionic fragments with a series of neutral reagents, characterized by different Proton Affinity (PA) values. It was found that only m/z 55 and m/z 69 anionic fragmentation products have higher PA values and accept the proton from all neutral reagents. Based on PA differences analysis, the most possible chemical structures were proposed for the observed fragment anions.