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<p>Norpinonic acid has been known as an important <span class="inline-formula"><i>α</i></span>-pinene atmospheric secondary organic aerosol (SOA) component. It is formed in the reaction of <span class="inline-formula"><i>α</i></span>-pinene, <span class="inline-formula"><i>β</i></span>-pinene or verbenone with atmospheric oxidizing reagents. In the presented study, tandem mass spectrometry techniques were used to determine the exact norpinonic acid fragmentation pathway in the gas phase. The precursor anion – deprotonated norpinonic acid (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="55e7511b997f0b1e1b80fcef0834494e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-9309-2024-ie00001.svg" width="23pt" height="14pt" src="acp-24-9309-2024-ie00001.png"/></svg:svg></span></span> 169), generated in an electrospray (ESI) source – was introduced into the collision cell of the mass spectrometer and fragmented using the energy-resolved collision-induced dissociation (ER-CID) technique. The experimental energy values of degradation processes were determined via analysis of the breakdown curves. Quantum chemical calculations of the reaction models were also constructed, including calculation of all transition states. Comparison between the experimental and the theoretical threshold energies calculated at a <span class="inline-formula"><i>ω</i></span>B97XD/6-311<span class="inline-formula">+</span>G(2d,p) theoretical level has shown a good correlation. Two basic pathways of the fragmentation of the parent anion <span class="inline-formula">[M-H]⁻</span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="d93d3cda2a0fb8602765a5ab31f8bec1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-9309-2024-ie00002.svg" width="23pt" height="14pt" src="acp-24-9309-2024-ie00002.png"/></svg:svg></span></span> 169) were observed. Firstly this leads to the decarboxylation product (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c87305775cdf966f8a4d16ada7bea42b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-9309-2024-ie00003.svg" width="23pt" height="14pt" src="acp-24-9309-2024-ie00003.png"/></svg:svg></span></span> 125) and secondly to the loss of a neutral molecule (<span class="inline-formula">C₄H₆</span>O), together with the formation of the anion <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="38084c98e5949371829830faf37384c9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-9309-2024-ie00004.svg" width="23pt" height="14pt" src="acp-24-9309-2024-ie00004.png"/></svg:svg></span></span> 99. On the other hand, the breakdown of the anion <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c5ac927fad9592af10e0b6c28e1c46de"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-9309-2024-ie00005.svg" width="23pt" height="14pt" src="acp-24-9309-2024-ie00005.png"/></svg:svg></span></span> 125 gives rise to the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>m</mi><mo>/</mo><mi>z</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="279239c08ce5ed5dd0fbeb16cbabd960"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-9309-2024-ie00006.svg" width="23pt" height="14pt" src="acp-24-9309-2024-ie00006.png"/></svg:svg></span></span> 69, 57 and 55 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 proton affinity (PA) values. Based on PA difference analysis, the most possible chemical structures were proposed for the observed fragment anions.</p>