title: Competitive Deprotonation and Superoxide [O₂⁻•)] Radical-Anion Adduct Formation Reactions of Carboxamides under Negative-Ion Atmospheric-Pressure Helium-Plasma Ionization (HePI) Conditions.
abstract: Carboxamides bearing an N-H functionality are known to undergo deprotonation under negative-ion-generating mass spectrometric conditions. Herein, we report that N-H bearing carboxamides with acidities lower than that of the hydroperoxyl radical (HO-O(•)) preferentially form superoxide radical-anion (O2(-•)) adducts, rather than deprotonate, when they are exposed to the glow discharge of a helium-plasma ionization source. For example, the spectra of N-alkylacetamides show peaks for superoxide radical-anion (O2(-•)) adducts. Conversely, more acidic amides, such as N-alkyltrifluoroacetamides, preferentially undergo deprotonation under similar experimental conditions. Upon collisional activation, the O2(-•) adducts of N-alkylacetamides either lose the neutral amide or the hydroperoxyl radical (HO-O(•)) to generate the superoxide radical-anion (m/z 32) or the deprotonated amide [m/z (M - H)(-)], respectively. For somewhat acidic carboxamides, the association between the two entities is weak. Thus, upon mildest collisional activation, the adduct dissociates to eject the superoxide anion. Superoxide-adduct formation results are useful for structure determination purposes because carboxamides devoid of a N-H functionality undergo neither deprotonation nor adduct formation under HePI conditions.
authors.affiliation: Center for Mass Spectrometry, Department of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ, 07030, USA.
authors.forename: Isra
authors.initials: I
authors.lastname: Hassan
authors.affiliation: Center for Mass Spectrometry, Department of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ, 07030, USA.
authors.forename: Spencer
authors.initials: S
authors.lastname: Pinto
authors.affiliation: Center for Mass Spectrometry, Department of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ, 07030, USA.
authors.forename: Carl
authors.initials: C
authors.lastname: Weisbecker
authors.affiliation: Center for Mass Spectrometry, Department of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ, 07030, USA. aattygal@stevens.edu.
authors.forename: Athula B
authors.initials: AB
authors.lastname: Attygalle
keywords: Adducts; Carboxamides; Helium-plasma ionization (HePI); Superoxide radical anion