Mass spectrometric and computational studies on the reaction of aromatic peroxyl radicals with phenylacetylene using the distonic radical ion approach

Molecular Oxygen (O2) Artifacts in Tandem Mass Spectra

Peak annotation plays an important role in mass spectral evaluation of the NIST 2023 tandem mass spectral library. While most fragment ions are formed by neutral losses, there are peaks that represent adduct ions from these fragments. Previously, we have reported two main types of addition reactions in the collision cell, namely addition of H2O and N2. Here we report a different reaction in the collision cell, with addition of O2 leading to a small peak that could only be assigned to a peroxyl radical ion. For example, some protonated iodoaromatics lose an iodine atom to form a radical cation [M+H-I]+•, which reacts with O2 to generate a peroxyl radical ion [M+H-I+O2]+•. Higher concentrations of O2 result in higher peroxyl radical peaks, which become dominant while the precursor ions are consumed, as examined by five compounds under different concentrations of O2. The correlation of the peroxyl radical peak intensities to the concentration of O2 provides a tool to estimate trace amounts of O2 within the instrument. In the NIST 2023 tandem mass spectral library, the peaks for [M+H-X+O2]+· are most abundant in numbers and in intensity for X = NO2 or I, are much less abundant for X = Br, and are rare for X = Cl. Other leaving groups in this library are SO3H, SO2NH2, CSNH2, CO2C6F5, SO2CH3, and COCH3. The O2 addition reaction is also observed with negative ions in this library. While adducts of H2O and N2 often constitute major peaks, the peaks of the peroxyl radicals under standard conditions are mostly very small and may be mistaken for noise, but their correct annotation improves the quality of the spectra and is important when comparing spectra from different instruments or conditions.

Reactions of a distonic peroxyl radical anion influenced by SOMO–HOMO conversion: an example of anion-directed channel switching

Deprotonation of a remote site in a peroxyl radical energetically buries the singly occupied molecular orbital, suppressing radical-driven oxidation and promoting reactions involving the anion site.

Regioselectivity of aryl radical attack onto isocyanates and isothiocyanates

The combination of multistage mass spectrometry experiments employing the distonic radical approach together with DFT calculations are used to examine addition of the N-methyl-pyridinium-4-yl radical cation (γ-NMP) to iso(thio)cyanates in the gas-phase. The type of products formed depend on the nature of the iso(thio)cyanate: (1) hydrogen atom abstraction occurs for alkyl isocyanates; (2) aryl isocyanates undergo radical-ipso substitution; (3) radical attack occurs at the C[double bond, length as m-dash]C bond of allyl isocyanate; (4) radical attack occurs at the C[double bond, length as m-dash]S bond of isothiocyanates to generate S adducts of γ-NMP and isonitriles. DFT calculations provide insight into the reactivity differences of these heterocumulenes towards the electrophilic C-centered γ-distonic radical cations. Translation of these gas phase results to the solution phase were hampered by dominating radical recombination reactions which appear to be favoured over the radical-iso(thio)cyanate reactions.

Direct Evidence for the Origin of Bis-Gold Intermediates: Probing Gold Catalysis with Mass Spectrometry

Gold-catalyzed alkyne hydration was studied by using in situ reacting mass spectrometry (MS) technology. By monitoring the reaction process in solution under different conditions (regular and very diluted catalyst concentrations, different pH values) and examining the reaction occurrence in the early reaction stage (1-2 ms after mixing) with MS, we collected a series of experimental evidence to support that the bis-gold complex is a potential key reaction intermediate. Furthermore, both experimental and computational studies confirmed that the σ,π-bis-gold complexes are not active intermediates toward nucleophilic addition. Instead, formation of geminally diaurated complex C is crucial for this catalytic process.

Environmental Polymer Degradation: Using the Distonic Radical Ion Approach to Study the Gas-Phase Reactions of Model Polyester Radicals

A novel precursor to the distonic O- and C-centered radical cations Oxo⁺O^• and Oxo⁺C^• was designed and synthesized, which represents model systems for radicals produced during polyester degradation. The precursor is equipped with a nitrate functional group, which serves as a masked site for these alkoxyl and carbon radicals that are unleashed through collision-induced dissociation (CID). Oxo⁺O^• and Oxo⁺C^• feature a cyclic carboxonium ion as permanent charge tag to enable monitoring their ion-molecule reactions on the millisecond to second time scale in the ion trap of the mass spectrometer. The reactions of Oxo⁺O^• and Oxo⁺C^• with cyclohexene, cyclohexadiene, ethyl acetate, 1,1-dimethoxyethane, and 1,2-dimethoxyethane, which exhibit structural features present in both intact and defective polyesters, were explored through product and kinetic studies to identify "hot spots" for radical-induced damage in polyesters. All reactions with Oxo⁺O^• were extremely fast and proceeded predominantly through HAT. Oxo⁺C^• was about two orders of magnitude less reactive and did not noticeably damage aliphatic ester moieties through hydrogen abstraction on the time scale of our experiments. Radical addition to alkene π systems was identified as an important pathway for C-radicals, which needs to be included in polymer degradation mechanisms.

The role of peroxyl radicals in polyester degradation – a mass spectrometric product and kinetic study using the distonic radical ion approach

Mass spectrometric techniques were employed to reveal detailed insight into the reaction of peroxyl radicals with structural motifs in polyesters.

Unimolecular reaction chemistry of a charge-tagged beta-hydroxyperoxyl radical

The study of unimolecular isomerization and decomposition of a charge-tagged β-hydroxyperoxyl radical anion ˙CH₂C(OH)(CH₃)CH₂C(O)O⁻ using mass spectrometry, quantum mechanical calculations and master equation kinetic simulations.