Modeling the Charging of Highly Oxidized Cyclohexene Ozonolysis Products Using Nitrate-Based Chemical Ionization

Several extremely low volatility organic compounds (ELVOCs) formed in the ozonolysis of endocyclic alkenes have recently been detected in laboratory and field studies. These experiments have been carried out with chemical ionization atmospheric pressure interface time-of-flight mass spectrometers (CI-APi-TOF) with nitrate ions as reagent ions. The nitrate ion binds to the detected species through hydrogen bonds, but it also binds very strongly to one or two neutral nitric acid molecules. This makes the measurement highly selective when there is an excess amount of neutral nitric acid in the instrument. In this work, we used quantum-chemical methods to calculate the binding energies between a nitrate ion and several highly oxidized ozonolysis products of cyclohexene. These were then compared with the binding energies of nitrate ion-nitric acid clusters. Systematic configurational sampling of the molecules and clusters was carried out at the B3LYP/6-31+G* and ωB97xD/aug-cc-pVTZ levels, and the final single-point energies were calculated with DLPNO-CCSD(T)/def2-QZVPP. The binding energies were used in a kinetic simulation of the measurement system to determine the relative ratios of the detected signals. Our results indicate that at least two hydrogen bond donor functional groups (in this case, hydroperoxide, OOH) are needed for an ELVOC molecule to be detected in a nitrate ion CI-APi-TOF. Also, a double bond in the carbon backbone makes the nitrate cluster formation less favorable.