Association and Proton Transfer Reactions With a Series of Organic Molecules

In this study, we present reactions of $$ with a series of analytes (A): acetone (C₃H₆O), methyl vinyl ketone (C₄H₆O), methyl ethyl ketone (C₄H₈O), and eight monoterpene isomers (C₁₀H₁₆) using a Selective Reagent Ionization Time-of-Flight Mass Spectrometer (SRI-ToF-MS). We studied the ion-molecule reactions at collision energies of 55 and 80 meV. The ketones, having a substantially lower proton affinity than NH₃, produce only cluster ions $$ (A) in detectable amounts at 55 meV. At 80 meV, no cluster ions were detected meaning that these adduct ions are formed by strongly temperature dependent association reactions. Bond energies of cluster ions and proton affinities for most monoterpenes are not known and were estimated by high level quantum chemical calculations. The calculations reveal monoterpene proton affinities, which range from slightly smaller to substantially higher than the proton affinity of NH₃. Proton affinities and cluster bond energies allow to group the monoterpenes as a function of the enthalpy for the dissociation reaction $$ . We find that this enthalpy can be used to predict the $$ (A) cluster ion yield. The present study explains product ion formation involving $$ ion chemistry. This is of importance for chemical ionization mass spectrometry (CIMS) utilizing $$ as well as $$ (H₂O) as reagent ions to quantitatively detect atmospherically important organic compounds in real-time.