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

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–).