Richard A. J. O'HAir

Based on gas-phase models, a Cu ^II -formate-functionalised UiO-67-bipy framework was synthesised and shown to generates a catalytically active species upon treatment with l -ascorbic acid, enabling H ₂ and CO ₂ production from formic acid.

Rationale: Carbon–carbon double bonds are ubiquitous structural motifs in natural products and pose a key challenge in locating their presence in analytes via MS‐based methods. Here, we use the alkaloid reserpine as a model system to examine whether OAD reactions can locate quaternary double bonds embedded in fused multicyclic structures. Methods: OAD reactions were carried out on the Shimadzu LCMS‐9050 ESI mass spectrometer equipped with an OAD RADICAL SOURCE I. Protonated reserpine, N‐ methylated and N‐ benzylated reserpine derivatives and dehydroreserpine were generated via electrospray ionisation, mass‐selected by the first quadrupole and exposed to a plasma of hydrogen atoms, hydroxyl radicals and O atoms in the collision cell. The OAD products were then identified using the TOF mass analyser. DFT calculations were at the PW6B95‐D3/def2‐QZVPP/MN15‐D3/def2‐TZVP level of theory. Results: OAD reactions of protonated reserpine yielded an O atom adduct [M + H + O] ⁺ ( m/z 625) and a fragment ion at m/z 450, which are not observed via CID. DFT calculations reveal that triplet oxygen addition to the indole ring double bond is exothermic and that ring cleavage is a stepwise process. New reaction pathways were found in the OAD reactions of the N‐ methylated and N‐ benzylated reserpine derivatives and dehydroreserpine, including fragments from loss of the methyl/benzyl group. Conclusions: Reserpine shows an OAD‐specific fragment ion at m/z 450. DFT calculations suggest that its formation occurs via a stepwise exothermic mechanism involving triplet oxygen addition to the double bond of the indole ring. OAD‐MS of N‐ methylated and N‐ benzylated reserpine was overshadowed by fragmentation reactions involving loss of the N‐ R group.

Arylacetylides such as phenylacetylide (PhCC⁻, 1) are important nucleophiles used in synthetic chemistry yet they have rarely been studied as bare carbanions. In this work, the phenylacetylide anion was formed via electrospray ionization (ESI) or multistage mass spectrometry (MSⁿ) experiments and subsequently examined in the gas phase by negative ion photoelectron spectroscopy (NIPES) and ion–molecule reactions (IMR), alongside theoretical calculations to probe its fundamental structure and reactivity. Photoelectron spectra of PhCC⁻ (1) revealed vertical (VDE) and adiabatic detachment energies (ADE), both being 3.220 eV. The latter value is also the electron affinity (EA) of the phenylethynyl radical (PhCC˙) from which a bond dissociation energy (BDE) of 131.3 ± 2.5 kcal mol⁻¹ of phenylacetylene (PhCCH) was derived using a gas-phase thermochemical cycle. Detachment of PhCC⁻ at 266 nm likely involves photoexcitation to an excited electronic state of the anion from which resonant electron autodetachment occurs, supported by Franck–Condon factor (FCF) simulations. The phenylacetylide anion reacts with methyl iodide (CH₃I) and allyl iodide (C₃H₅I) via S_N2 nucleophilic displacement with measured rate coefficients of 4.29 and 3.66 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, respectively. The mechanisms associated with these displacement reactions are explained and understood in terms of the reaction kinetics, natural bond orbital (NBO) theory, and density functional theory (DFT) calculations.

The optical and photo-decomposition properties of the [Mo ₆ I ₁₄ ] ²⁻ cluster complex and the photo-reactivity towards O ₂ and CO ₂ of its photo-products are examined by ion trap mass spectrometry experiments and quantum chemistry simulations.

Copper(II) aryl species are proposed intermediates in copper catalysed C‐X coupling reactions. Here, the formation and reactions of the cationic copper complexes [(phen)Cu(Ar)]⁺ (phen = 1,10‐phenanthroline; Ar = Ph, p‐MeC₆H₄, p‐MeOC₆H₄, p‐MeCOC₆H₄) are explored in the gas‐phase using electrospray ionisation (ESI) multistage mass spectrometry (MSⁿ) experiments in a linear ion trap mass spectrometer together with Density Functional Theory (DFT) calculations. A survey of three routes to the preparation of [(phen)Cu(Ph)]⁺ showed that transmetalation of [(phen)₃Cu][BPh₄]⁺ and desulfination of [(phen)Cu(O₂SPh)]⁺ both produced the desired complex [(phen)Cu(Ph)]⁺, but decarboxylation of [(phen)Cu(O₂CPh)]⁺ gave a higher yield with fewer competing side reactions, making it the preferred route to the other aryl complexes [(phen)Cu(Ar)]⁺. The aryl complexes [(phen)Cu(Ar)]⁺ undergo protonolysis reactions with formic acid to yield a formate complex [(phen)Cu(O₂CH)]⁺; react with N,N′‐diisopropylcarbodiimide to form an adduct, likely by insertion to give [(phen)Cu((NiPr)₂CAr)]⁺; undergo C–S bond‐forming reactions with dimethyl disulfide; and react with allyl iodide by Ar–I bond‐forming reactions in preference to Ar–allyl bond cross‐coupling reactions. The potential reaction mechanisms associated with the different reaction pathways were explored by DFT calculations.

The potential for ion-molecule reactions to identify alkene functional groups in substrates with increasing oxygenation is explored by using linear ion-trap mass spectrometry. Electrophilic attack by the iodiranium ion, c-C₂H₄I⁺ (m/z 155), proceeds at the collision rate with terminal alkenes RCH═CH₂ (R = n-Pr), allyl ethers (R = MeOCH₂), and nonconjugated methyl esters (R = MeO₂CCH₂), favoring transfer of the iodenium cation to the point of unsaturation by π-ligand exchange. Density functional theory (DFT) calculations at M06-2X-D3/def2-TZVP revealed a single energy well for π-ligand exchange, allowing for a barrierless and hence kinetically favorable reaction pathway. In contrast, ring-opening of the iodiranium ion with subsequent elimination of HI proceeds via a relatively high barrier and is only a minor product channel despite being thermodynamically preferable. Conjugation of the carbon-carbon double bond to an ester group in methyl acrylate had a significant impact on the reaction partitioning with exclusive attack by the carbonyl oxygen to give an allylic stabilized oxonium ion. However, substitution at the point of unsaturation with an inductively donating methyl group enabled the formation of a π-ligand exchange product, which underwent secondary reactions with the neutral. Comparison of the ion-molecule reactions of methyl crotonate, methyl methacrylate, and methyl 3-butenoate with c-C₂H₄I⁺ show this shift in reaction partitioning between ring-opening by the ester carbonyl oxygen and π-ligand exchange is related to the nucleophilicity of the carbon-carbon double bond. The branching ratios of these product channels reveal significant differences between these three structural isomers encompassing conjugated and nonconjugated unsaturated esters.

A previous gas-phase study has uncovered formal catalytic cycles for the dehydrogenation of model liquid organic hydrogen carriers (LOHCs) pyrrolidine, N-methylpyrrolidine, and piperidine by the coordinatively unsaturated half-sandwich cyclopentadienyl iron cation, [CpFe]+. That work is extended here to the well-known condensed-phase [CpFe(CO)2]+ cation, which was generated via electrospray ionization for gas-phase reactions with model LOHCs in a linear ion trap mass spectrometer, in which the helium bath gas was seeded with 0.1% carbon monoxide. The initial ion-molecule reaction (IMR) was exothermic enough to expel one CO molecule from the complex to form [CpFe(CO)L]+ (L = pyrrolidine, N-methylpyrrolidine, or piperidine). Collision-induced dissociation (CID) of these cations revealed two fragmentation pathways: (i) removal of another CO molecule leading to the species [CpFeL]+ that was studied previously; (ii) dehydrogenation of the ligand L (except for L= N-methylpyrrolidine). Two new formal catalytic cycles (for dehydrogenation of pyrrolidine and piperidine) were found that operate via a combination of IMR and CID experiments and which rely on the presence of CO for re-ligation of iron complexes. Density functional theory calculations were performed to compute the structures of all species observed as well as the reaction energetics.

Rationale: Polyfluoroalkyl substances (PFAS) like perfluorooctanoic acid have persistent environmental and physiological effects. This study investigates the degradation of C _n F _2n+1 CO ₂ ⁻ ( n = 1–7) with neutral radical fragmentation under oxygen attachment dissociation (OAD). Unique fragments absent from collision‐induced dissociation (CID) are observed. Further, potential mechanisms are uncovered by density functional theory (DFT) calculations. Methods: From a standard mixture of PFAS, straight‐chain perfluorinated carboxylic acids with carbon chain lengths of one to eight were separated via liquid chromatography and transferred to the gas phase via negative‐mode electrospray ionisation. Each C _n F _2n+1 CO ₂ ⁻ of interest was mass selected and fragmented via both CID and OAD in a quadrupole time‐of‐flight mass spectrometer. DFT optimisations of structures were performed at M06/6‐31+g(d), and single point energy calculations were performed at M06‐2X/aug‐cc‐pVTZ for C ₃ F ₇ CO ₂ ⁻ . Results: Decarboxylation was observed from both CID and OAD, but fluorine abstraction and hydroxyl addition only occurred with OAD. The DFT calculations suggest that C ₃ F ₆ ^–• ( m / z 150) is most likely formed from by H• attack onto a β‐ C‐F bond, then loss of HF, finally decarboxylation. Further, C ₃ F ₅ O ⁻ ( m / z 147) likely arises from C ₃ F ₆ ^–• recombining with OH• to produce energised C ₃ F ₆ OH ⁻ ions, followed by α‐ or β‐ elimination of HF to give enolate and/or epoxide‐type products. Conclusions: OAD of C ₃ F ₇ CO ₂ ⁻ yields unique product ions C ₃ F ₆ ^–• ( m / z 150) and C ₃ F ₅ O ⁻ ( m / z 147) absent from collision‐induced dissociation. DFT calculations suggest an intricate pathway of H• attack onto a β C‐F bond, then loss of HF, decarboxylation, recombination with OH•, and finally α‐ or β‐ elimination of HF to give the products.