Decomposition of Halogenated Molybdenum Sulfide Dianions [Mo3S7X6]2- (X = Cl, Br, I)

Molybdenum sulfides are considered a promising and inexpensive alternative to platinum as a catalyst for the hydrogen evolution reaction. In this study, we perform collision-induced dissociation experiments in the gas phase with the halogenated molybdenum sulfides [Mo₃S₇Cl₆]^2-, [Mo₃S₇Br₆]^2-, and [Mo₃S₇I₆]^2-. We show that the first fragmentation step for all three dianions is charge separation via loss of a halide ion. As a second step, further halogen loss competes with the dissociation of a disulfur molecule, whereas the former becomes energetically more favorable and the latter becomes less favorable from chlorine via bromine to iodine. We show that the leaving S₂ group is composed of sulfur atoms from two bridging groups. These decomposition pathways differ drastically from the pure [Mo₃S₁₃]^2- clusters. The obtained insight into preferred dissociation pathways of molybdenum sulfides illustrate possible reaction pathways during the activation of these substances in a catalytic environment.

Bimolecular reactions of S2+ with Ar, H2 and N2: reactivity and dynamics

The reactivity, energetics and dynamics of bimolecular reactions between S²⁺ and three neutral species (Ar, H₂ and N₂) have been studied using a position-sensitive coincidence methodology at centre-of-mass collision energies below 6 eV. This is the first study of bimolecular reactions involving S²⁺, a species detected in planetary ionospheres, the interstellar medium, and in anthropogenic manufacturing processes. The reactant dication beam employed consists predominantly of S²⁺ in the ground ³P state, but some excited states are also present. Most of the observed reactions involve the ground state of S²⁺, but the dissociative electron transfer reactions appear to exclusively involve excited states of this atomic dication. We observe exclusively single electron-transfer between S²⁺ and Ar, a process which exhibits strong forward scatting typical of the Landau-Zener style dynamics observed for other dicationic electron transfer reactions. Following collisions between S²⁺ + H₂, non-dissociative and dissociative single electron-transfer reactions were detected. The dynamics here show evidence for the formation of a long-lived collision complex, [SH₂]²⁺, in the dissociative single electron-transfer channel. The formation of SH⁺ was not observed. In contrast, the collisions of S²⁺ + N₂ result in the formation of SN⁺ + N⁺ in addition to the products of single electron-transfer reactions.

Ionisation of PF3: absolute partial electron ionisation cross sections and the formation and reactivity of dication states

Absolute partial electron ionisation cross sections, and precursor-specific partial electron ionisation cross sections, for the formation of cations from phosphorus trifluoride (PF₃) are reported over the electron energy range 50-200 eV. The absolute values are determined by the measurement of cross sections relative to the formation of PF₃⁺ using 2D ion-ion coincidence time-of-flight mass spectrometry and subsequent scaling using binary encounter-Bethe calculations of the total ionisation cross section. This new dataset significantly augments the partial ionisation cross sections for electron ionization of PF₃ found in literature, addressing previous discrepancies in the branching ratios of product ions, and provides the first values for the precursor-specific cross sections. Comparisons to calculated cross sections from the literature are encouraging, although there are discrepancies for individual ions. The coincidence experiments indicate that double and triple ionisation generate approximately 20% of the cationic ionisation products at 200 eV electron energy. One dissociative dication state, dissociating to PF₂⁺ + F⁺, is clearly identified as the lowest triplet state of PF₃²⁺ and five different dications (PF₃²⁺, PF₂²⁺, PF²⁺, P²⁺ and F²⁺) are detected in the mass spectra. The dication energetics revealed by the experiments are supported by a computational investigation of the dication's electronic structure. The cross sections reported will allow more accurate modelling of the role of the ionization of PF₃ in energetic environments. A first investigation of the bimolecular reactivity of metastable states of PF₃²⁺ is also reported. In collisions with Ar, O₂ and CO dissociative single electron transfer dominates the product ion yield, whereas collision-induced dissociation of the dication is important following collisions with Ne. Consideration of the energetics of these processes indicates that the reactant dication beam contains ions in both the ground singlet state and the first excited triplet state. The deduction regarding the longevity of the triplet state is supported by metastable signals in the coincidence spectra. Weak signals corresponding to the formation of ArF⁺ are detected following PF₃²⁺ collisions with Ar, and experimental and computational considerations indicate this new chemical bond is formed via a collision complex.

Bond-forming and electron-transfer reactivity between Ar2+ and O2

The reactivity, energetics and dynamics of the bimolecular reactions between Ar²⁺ and O₂ have been studied using a position sensitive coincidence methodology at a collision energy of 4.4 eV.

Structural Properties of Gas-Phase Molybdenum Oxide Clusters [Mo4O13]2-, [HMo4O13]-, and [CH3Mo4O13]- Studied by Collision-Induced Dissociation

Molybdenum oxide-based catalysts are widely used for the ammoxidation of toluene, methanation of CO, or hydrodeoxygenation. As a first step towards a gas-phase model system, we investigate here structural properties of mass-selected [Mo₄O₁₃]^2-, [HMo₄O₁₃]^-, and [CH₃Mo₄O₁₃]^- by a combination of collision-induced dissociation (CID) experiments and quantum chemical calculations. According to calculations, the common structural motif is an eight-membered ring composed of four MoO₂ units and four O atoms. The 13th O atom is located above the center of the ring and connects two to four Mo centers. For [Mo₄O₁₃]^2- and [HMo₄O₁₃]^-, dissociation requires opening or rearrangement of the ring structure, which is quite facile for the doubly charged [Mo₄O₁₃]^2-, but energetically more demanding for [HMo₄O₁₃]^-. In the latter case, the hydrogen atom is found to stay preferentially with the negatively charged fragments [HMo₂O₇]^- or [HMoO₄]^-. The doubly charged species [Mo₄O₁₃]^2- loses one MoO₃ unit at low energies while Coulomb explosion into the complementary fragments [Mo₂O₆]^- and [Mo₂O₇]^- dominates at elevated collision energies. [CH₃Mo₄O₁₃]^- affords rearrangements of the methyl group with low barriers, preferentially eliminating formaldehyde, while the ring structure remains intact. [CH₃Mo₄O₁₃]^- also reacts efficiently with water, leading to methanol or formaldehyde elimination.

Energy partitioning in single-electron transfer events between gaseous dications and their neutral counterparts

Electron-transfer reactions between hydrocarbon dications and neutral hydrocarbons lead to an unequal deposition of the excess energy from the reaction in the pair of monocations formed. The initial observation of this phenomenon was explained by the different states accessible upon single-electron capture by a dication compared to single-electron ejection from a neutral compound. Alternatively, however, isomeric structures of the dicationic species, pronounced Franck-Condon effects, as well as excess energy in the dicationic precursors could cause the asymmetric energy partitioning in such dication/neutral collisions. Here, the investigation of this phenomenon in an interdisciplinary cooperation is described, shedding light not only upon a possible solution of the problem at hand, but also providing an example for the synergistic benefits of international research networks applying complementary approaches.

Electronic state selectivity in dication-molecule single electron transfer reactions: NO(2+) + NO

The single-electron transfer reaction between NO(2+) and NO, which initially forms a pair of NO(+) ions, has been studied using a position-sensitive coincidence technique. The reactivity in this class of collision system, which involves the interaction of a dication with its neutral precursor, provides a sensitive test of recent ideas concerning electronic state selectivity in dicationic single-electron transfer reactions. In stark contrast to the recently observed single-electron transfer reactivity in the analogous CO(2)(2+)/CO(2) and O(2)(2+)/O(2) collision systems, electron transfer between NO(2+) and NO generates two product NO(+) ions which behave in an identical manner, whether the ions are formed from NO(2+) or NO. This observed behaviour is in excellent accord with the recently proposed rationalization of the state selectivity in dication-molecule SET reactions using simple propensity rules involving one-electron transitions.

Doubly-charged ions in the planetary ionospheres: a review

This paper presents a review of the current knowledge on the doubly-charged atomic and molecular positive ions in the planetary atmospheres of the Solar System. It is focused on the terrestrial planets which have a dense atmosphere of N(2) or CO(2), i.e. Venus, the Earth and Mars, but also includes Titan, the largest satellite of Saturn, which has a dense atmosphere composed mainly of N(2) and a few percent of methane. Given the composition of these neutral atmospheres, the following species are considered: C(++), N(++), O(++), CH(4)(++), CO(++), N(2)(++), NO(++), O(2)(++), Ar(++) and CO(2)(++). We first discuss the status of their detection in the atmospheres of planets. Then, we provide a comprehensive review of their complex and original photochemistry, production and loss processes. Synthesis tables are provided for those ions, while a discussion on individual species is also provided. Methods for detecting doubly-charged ions in planetary atmospheres are presented, namely with mass-spectrometry, remote sensing and fine plasma density measurements. A section covers some original applications, like the possible effect of the presence of doubly-charged ions on the escape of an atmosphere, which is a key topic of ongoing planetary exploration, related to the evolution of a planet. The results of models, displayed in a comparative way for Venus, Earth, Mars and Titan, are discussed, as they can predict the presence of doubly-charged ions and will certainly trigger new investigations. Finally we give our view concerning next steps, challenges and needs for future studies, hoping that new scientific results will be achieved in the coming years and feed the necessary interdisciplinary exchanges amongst different scientific communities.