Computational and Experimental Investigation of the Detection of HO2 Radical and the Products of Its Reaction with Cyclohexene Ozonolysis Derived RO2 Radicals by an Iodide-Based Chemical Ionization Mass Spectrometer

Siddharth Iyer; Xucheng He; Noora Hyttinen; Theo Kurtén; Matti P Rissanen

doi:10.1021/acs.jpca.7b01588

Computational and Experimental Investigation of the Detection of HO₂ Radical and the Products of Its Reaction with Cyclohexene Ozonolysis Derived RO₂ Radicals by an Iodide-Based Chemical Ionization Mass Spectrometer

J Phys Chem A. 2017 Sep 14;121(36):6778-6789. doi: 10.1021/acs.jpca.7b01588. Epub 2017 Aug 29.

Authors

Siddharth Iyer¹, Xucheng He¹, Noora Hyttinen¹, Theo Kurtén¹, Matti P Rissanen¹

Affiliation

¹ Department of Chemistry, and ‡Department of Physics, University of Helsinki , Helsinki, 00100, Finland.

PMID: 28796517
DOI: 10.1021/acs.jpca.7b01588

Abstract

The HO₂ radical is an important atmospheric molecule that can potentially influence the termination of autoxidation processes of volatile organic compounds (VOCs) that lead to the formation of highly oxygenated multifunctional compounds (HOMs). In this work, we demonstrate the direct detection of the HO₂ radical using an iodide-based chemical ionization mass spectrometer (iodide-CIMS). Expanding on the previously established correlation between molecule-iodide binding enthalpy and iodide-CIMS instrument sensitivity, the experimental detection of the HO₂ radical was preceded by the quantum chemical calculation of the HO₂*I^- cluster (PBE/aug-cc-pVTZ-PP level), which showed a reasonably strong binding enthalpy of 21.60 kcal/mol. Cyclohexene ozonolysis intermediates and closed-shell products were next detected by the iodide-CIMS. The ozone-initiated cyclohexene oxidation mechanism was perturbed by the introduction of the HO₂ radical, leading to the formation of closed-shell hydroperoxides. The experimental investigation once again followed the initial computational molecule-iodide binding enthalpy calculations. The quantum chemical calculations were performed at the PBE/aug-cc-pVTZ-PP level for radicals and DLPNO-CCSD(T)/def2-QZVPP//PBE/aug-cc-pVTZ-PP level for the closed-shell products. A comparison between the iodide-CIMS and nitrate-CIMS spectra with identical measurement steps revealed that the iodide-CIMS was able to detect the low-oxidized (O/C ratio 0.5 and 0.66) cyclohexene ozonolysis monomer products more efficiently than nitrate-CIMS. Higher-oxidized monomers (O/C ratio 1 to 1.5) were detected equally well by both methods. An investigation of dimers showed that both iodide- and nitrate-CIMS were able to detect the dimer compositions possibly formed from reactions between the peroxy radical monomers considered in this study. Additionally, iodide-CIMS detected organic ions that were formed by a previously suggested mechanism of dehydroxylation of peroxy acids (and deoxygenation of acyl peroxy radicals) by H₂O*I^- clusters. These mechanisms were computationally verified.