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Interest in the observation and characterization of organic isomers in astronomical environments has grown rapidly with an increase in the sensitivity of detection techniques. Accurate modeling and interpretation of these environments require experimental isomer-specific reactivity and spectroscopic measurements. Given the abundance of formaldehyde (H 2 CO) in various astrophysical objects, the properties and reactivities of its cation isomers H 2 CO •+ and HCOH •+ are of significant interest. However, for the hydroxymethylene radical cation HCOH •+ (and its isotopologue DCOH •+ ), detailed reactivity studies have been limited by the lack of suitable experimental methods to generate this isomer with high purity. Here, potential approaches to the isomer-selective generation of HCOH •+ and DCOH •+ are characterized through differential reactivity measurements. While the dissociative photoionization of cyclopropanol (c-CH 2 CH 2 CHOH) is determined to be unsuitable, the dissociation of methanol- d 3 (CD 3 OH) allows for the formation of DCOH •+ with a fractional abundance of >99% at photon energies below 14.8 eV. These results will allow future spectroscopic and reactivity measurements of HCOH •+ /DCOH •+ to be conducted, laying the groundwork for future detection and incorporation into models of the interstellar medium.

The gas phase reactivity of the radical cation isomers H 2 CNH˙ + (methanimine) and HCNH 2 ˙ + (aminomethylene) with propene (CH 3 CHCH 2 ) has been investigated by measuring absolute reactive cross sections and product branching ratios, under single collision conditions, as a function of collision energy (in the range ∼0.07-11.80 eV) using guided ion beam mass spectrometry coupled with VUV photoionization for selective isomer generation. Experimental results have been merged with theoretical calculations to elucidate reaction pathways and structures of products. The H 2 CNH˙ + isomer is over a factor two more reactive than HCNH 2 ˙ + . A major channel from both isomers is production of protonated methanimine CH 2 NH 2 + via hydrogen-atom transfer reaction but, while H 2 CNH˙ + additionally gives charge and proton transfer products, the HCNH 2 ˙ + isomer leads instead to protonated vinylimine CH 2 CHCHNH 2 + , produced alongside CH 3 ˙ radicals. The reactions have astrochemical implications in the build up of chemical complexity in both the interstellar medium and the hydrocarbon-rich atmospheres of planets and satellites.

We report a study on the reactive collision of S+(4S) with H2, HD, and D2 combining guided ion beam experiments and quantum-mechanical calculations. It is found that the reactive cross sections reflect the existence of two different mechanisms, one being spin-forbidden. Using different models, we demonstrate that the spin-forbidden pathway follows a complex mechanism involving three electronic states instead of two as previously thought. The good agreement between theory and experiment validates the methodology employed and allows us to fully understand the reaction mechanism. This study also provides new fundamental insights into the intersystem crossing process., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/).)

Structural change of ions induced by collision with a neutral has been studied in a guided ion beam tandem mass spectrometer, using Time-Of-Flight measurements and SIMION simulation. The exothermic catalytic isomerization of HOC + to HCO + is used to explore the new methodology. Isomerization is catalyzed via a proton transport mechanism through the interplay of a neutral molecule, the catalyst. Four different potential catalysts, Ne, D 2 , CH 4, and C 18 O, were studied at different collision energies. SIMION simulation of the ion path and collision in the instrument leads to the highlight of a specific signature related to the catalytic isomerization in the time-of-flight spectra. This signature is used to identify the experimental conditions where isomerization takes place. Only C 18 O, at low collision energies, gives a clear signature of catalytic isomerization, and a quantitative estimate of the catalyzed isomerization cross-section and rate constant is derived. This new methodology is sensitive to clear presence of catalyzed isomerization and can be used in instruments designed for cross-section measurements, provided low collision energy is used and ion bunching is available., (© 2024 The Author(s). Journal of Mass Spectrometry published by John Wiley & Sons Ltd.)

The gas-phase reaction between the ethyl cation (C 2 H 5 ) and ethyne (C + ) and ethyne (C 2 ) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C 2 ) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C 2 Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C 5 Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C 2 , which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C 5 + , which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C 4 H 7 C + PES and elucidate the underlying reaction mechanisms. Two main product channels have been identified at a centre-of-mass collision energy of ∼0.1 eV: (a) C 3 H 3 C + +CH 4 , with BR = 0.76±0.05 and (b) , with BR = 0.22±0.02. A third channel giving C 4 H 5 in association with C + +H 2 is shown to emerge at both high internal excitation of C 2 H 3 + in association with C 2 H 4 , from which barrierless and exothermic pathways exist for the production of both cyclic c-C 2 H 5 + and high collision energies. From CS measurements, energy-dependent total rate constants in the range 4.3×10-11-5.2×10-10 cm3·molecule-1·s-1 have been obtained. Theoretical calculations indicate that both channels stem from a common covalently bound intermediate, CH 3 isomers of the main product channel. For the minor C 2 CHCH + product, two isomers are energetically accessible: the three-member cyclic isomer c-C 3 H 3 + and linear H 2 CCCH + isomers of the main product channel. For the minor C 4 H 5 + product, two isomers are energetically accessible: the three-member cyclic isomer c-C 3 H 2 (CH 3 ) + and the higher energy linear structure CH 2 CHCCH 2 + , but their formation requires multiple isomerization steps and passages via transition states lying only 0.11 eV below the reagents' energy, thus explaining the smaller BR. Results have implications for the modeling of hydrocarbon chemistry in the interstellar medium and the atmospheres of planets and satellites as well as in laboratory plasmas (e.g., plasma-enhanced chemical vapor deposition of carbon nanotubes and diamond-like carbon films).