Computational Comparison of Different Reagent Ions in the Chemical Ionization of Oxidized Multifunctional Compounds

Noora Hyttinen; Rasmus V Otkjær; Siddharth Iyer; Henrik G Kjaergaard; Matti P Rissanen; Paul O Wennberg; Theo Kurtén

doi:10.1021/acs.jpca.7b10015

Computational Comparison of Different Reagent Ions in the Chemical Ionization of Oxidized Multifunctional Compounds

J Phys Chem A. 2018 Jan 11;122(1):269-279. doi: 10.1021/acs.jpca.7b10015. Epub 2017 Dec 22.

Authors

Noora Hyttinen¹, Rasmus V Otkjær², Siddharth Iyer¹, Henrik G Kjaergaard², Matti P Rissanen³, Paul O Wennberg⁴, Theo Kurtén¹

Affiliations

¹ Department of Chemistry, University of Helsinki , P.O. Box 55, FI-00014 Helsinki, Finland.
² Department of Chemistry, DK-2100, Copenhagen Ø, University of Copenhagen , Copenhagen, Denmark.
³ Department of Physics, University of Helsinki , P.O. Box 64, FI-00014 Helsinki, Finland.
⁴ Division of Engineering and Applied Science and Division of Geological and Planetary Sciences, California Institute of Technology , 1200 East California Boulevard, Pasadena, California 91125, United States.

PMID: 29200296
DOI: 10.1021/acs.jpca.7b10015

Abstract

High pressure anion chemical ionization is commonly used for the detection of neutral molecules in the gas phase. The detection efficiency in these measurements depends on how strongly the reagent ion binds to the neutral target molecule. We have calculated the binding strength of nitrate (NO₃^-), acetate (CH₃C(O)O^-), lactate (CH₃CH(OH)C(O)O^-), trifluoroacetate (CF₃C(O)O^-), trifluoromethanolate (CF₃O^-), bromide (Br^-), and iodide (I^-) reagent ions to ten different products derived from the OH radical-initiated oxidation of butadiene. We found that the binding of these oxidation products to the reagent ions depends almost linearly on the number of oxygen atoms in the target molecule, with the precise chemical identity of the compound (e.g., the number and relative position of hydroxyl or hydroperoxy groups) playing a more minor role. For acetate, the formation free energy decreases on average by around 4 kcal/mol when the number of oxygen atoms in the sample molecule increases by one. For the other reagent ions the corresponding decrease is around 3 kcal/mol. For all of the molecules studied, acetate forms the most stable clusters and I^- the least stable. We also investigated the effect of humidity on the chemical ionization by calculating how strongly water molecules bind to both the reagent ions and the ion-molecule clusters. Water binds much more strongly to the reagent ion monomers compared to the reagent ion "dimers" (defined here as a cluster of the reagent anion with the corresponding neutral conjugate acid, e.g., HNO₃(NO₃^-)) or the ion-molecule clusters. This likely leads to a stronger humidity dependence when using reagent ions that are not able to form reagent ion dimers (such as CF₃C(O)O^-, CF₃O^-, Br^-, and I^-).