Evidence of Gas-Phase Attachment of Molecular Oxygen to Deprotonated Hydroquinone During Ion-Mobility Mass Spectrometry

Abstract

Gas-phase addition of dioxygen to certain ions is a well-known phenomenon in mass spectrometry. For this reaction to occur, the presence of a distonic radical site on the precursor ion is thought to be a prerequisite. Herein, we report that oxygen adduct formation can take place also with deprotonated hydroquinone, which in fact is an even-electron species without a radical site. When the product-ion spectrum of the m/z 109 ion, generated by electrospray ionization from a solution of hydroquinone in acetonitrile, was recorded under ion-mobility conditions, a new peak was observed at m/z 141. However, an analogous peak was not visible in the spectrum acquired under nonmobility conditions (i.e., without any gas introduced to the mobility cell). Presumably, traces of oxygen present in the collision gas instigate an ion–molecule reaction to produce an adduct of m/z 141, which upon activation results in CO and H2O loss to form a product ion of m/z 95. Isotope-labeling studies confirmed that one of the hydrogen atoms from the hydroxy group and another from the aromatic ring contribute to the water loss instigated from the m/z 141 adduct. Furthermore, computational methods indicated the three-dimensional structure of the ground-state deprotonated hydroquinone to be distinctly different from those of its 1,2- and 1,3-isomers. Calculations predicted that all atoms in the two m/z 109 ions generated from catechol and resorcinol lie on one plane. In contrast, the structure of the m/z 109 ion from hydroquinone was significantly different. Computations predicted that the hydrogen atom on the intact hydroxyl group of deprotonated hydroquinone protrudes out of plane from rest of the atoms. Consequently, the exposed OH group can interact with an incoming dioxygen molecule. Computations conducted at the CAM-B3LYP/6-311++g­(2d,2p) level of theory detected a minimum energy crossing point (MECP) at −4.3 kJ mol–1 below the separated O2 + deprotonated hydroquinone triplet threshold. In contrast, similar calculations conducted for catechol and resorcinol yielded MECPs of +116.9 and +69.1 kJ mol–1, respectively, above the associated triplet thresholds. These results indicated that the curve crossing required to form singlet products upon reaction with triplet O2 is favorable in the case of hydroquinone and unfavorable in the cases of catechol and resorcinol. In practical terms, the selective oxygen addition appears to be a diagnostically useful reaction to differentiate hydroquinone from its ring isomers.

Does deprotonated benzoic acid lose carbon monoxide in collision‐induced dissociation?

“…Recently, the unusual CO 2 addition product ions were illustrated in the fragmentation of the [M − H] − ions of some pesticides (tolfenpyrad, tebufenpyrad and dicamba) under conventional tandem mass spectrometric conditions 8,9 . The formation of O 2 adduct in fragmentation processes of gaseous ions has been repeatedly reported due to the higher reactivity of dioxygen 10–14 . For example, Dülcks et al and Chai et al demonstrated that the unusual “neutral loss” of 14 u from some protonated nitro‐substituted benzodiazepines and 3‐(phenylthio)chromones was attributed to an attachment of molecular oxygen to the [M + H − NO 2 ] +• distonic fragment ion 12,13 .…”

“…The O 2 adduct formation has been deemed to occur mainly with distonic radical ions in most previous ion‐molecule reaction experiments 14–16 . Attygalle et al recently provided definitive evidence that the ion−molecule adduct formation with molecular oxygen could take place even from anions devoid of a radical site (deprotonated hydroquinone) in ion‐mobility mass spectrometry 11 . The above phenomena are not rare but often ignored because many MS users are not aware that trace amount of gaseous molecules in commercial mass spectrometers can participate in MS/MS process.…”

“…8,9 The formation of O 2 adduct in fragmentation processes of gaseous ions has been repeatedly reported due to the higher reactivity of dioxygen. [10][11][12][13][14] For example, Dülcks et al and Chai et al demonstrated that the unusual "neutral loss" of 14 u from some protonated nitro-substituted benzodiazepines and 3-(phenylthio)chromones was attributed to an attachment of molecular oxygen to the [M + H À NO 2 ] +• distonic fragment ion. 12,13 The O 2 adduct formation has been deemed to occur mainly with distonic radical ions in most previous ion-molecule reaction experiments.…”

“…[14][15][16] Attygalle et al recently provided definitive evidence that the ionÀmolecule adduct formation with molecular oxygen could take place even from anions devoid of a radical site (deprotonated hydroquinone) in ion-mobility mass spectrometry. 11 The above phenomena are not rare but often ignored because many MS users are not aware that trace amount of gaseous molecules in commercial mass spectrometers can participate in MS/MS process. Their product ions could be misleading during MS/MS spectra annotation if not recognized as adduct ions.…”