Selected ice nanoparticle accelerator hypervelocity impact mass spectrometer (SELINA-HIMS): features and impacts of charged particles.

The selected ice nanoparticle accelerator, SELINA, was used to prepare beams of single ice particles with positive or negative charge. Positively charged particles were prepared from deionized water and 0.05-0.2 molar solutions of sodium chloride in water, and negatively charged ice particles were generated from water without salt. Depending on the electrospray source configuration, the measured particles vary from 50 to 1000 nm in diameter. The kinetic energy per charge for all particles was set to 200 eV by the collisional equilibration in quadrupoles, which resulted in primary velocities up to 600 m/s for the lowest m/z particles. The electrospray ionization and thus particle formation from SELINA become less efficient with increasing salt concentration, resulting in a lower detected particle frequency and size. Good instrument operation is achievable for concentrations below 0.2 M. After we have verified and characterized positively and negatively charged ice particles, we have combined SELINA with a target and time-of-flight spectrometer for a 'proof-of-principle' post acceleration of 120 nm particles towards hypervelocity (v ~ 3000 m/s) and detection of fragments from the particle impact (SELINA-HIMS). General conditions are discussed for the acceleration of particles between 50 and 1000 nm to velocities well above 3000 m/s with SELINA-HIMS instrument. This article is part of the theme issue 'Dust in the Solar System and beyond'.

Reactivity of the Ethenium Cation (C2H5+) with Ethyne (C2H2): A Combined Experimental and Theoretical Study.

The gas-phase reaction between the ethyl cation (C2H5+) and ethyne (C2H2) 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 C2H5Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C2H5+, 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 [C4H7]+ 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) C3H3++CH4, with BR = 0.76±0.05 and (b) C4H5++H2, with BR = 0.22±0.02. A third channel giving C2H3+ in association with C2H4 is shown to emerge at both high internal excitation of C2H5+ 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, CH3CH2CHCH+, from which barrierless and exothermic pathways exist for the production of both cyclic c-C3H3+ and linear H2CCCH+ isomers of the main product channel. For the minor C4H5+ product, two isomers are energetically accessible: the three-member cyclic isomer c-C3H2(CH3)+ and the higher energy linear structure CH2CHCCH2+, 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).

Charged Ice Particle Beams with Selected Narrow Mass and Kinetic Energy Distributions.

Small ice particles play an important role in atmospheric and extraterrestrial chemistry. Circumplanetary ice particles that are encountered by space probes at hypervelocities play a critical role in the determination of surface and subsurface properties of their source bodies. Here we present an apparatus for the generation of low-intensity beams of single mass-selected charged ice particles under vacuum. They are produced via electrospray ionization of water at atmospheric pressure and undergo evaporative cooling when transferred to vacuum through an atmospheric vacuum interface. m/z selection is achieved through two subsequent quadrupole mass filters operated in the variable-frequency mode within a range of m/z values between 8 × 104 and 3 × 107. Velocity and charge of the selected particles are measured using a nondestructive single-pass image charge detector. From the known electrostatic acceleration potentials and settings of the quadrupoles the particle masses could be obtained and be accurately controlled. It has been shown that the droplets are frozen within the transit time of the apparatus such that ice particles are present after the quadrupole stages and finally detected. The demonstrated correspondence between particle mass and specific quadrupole potentials in this device allows preparation of beams of single particles with a repetition rate between 0.1 and 1 Hz with various diameter distributions from 50 to 1000 nm at 30-250 eV of kinetic energy per charge. This corresponds to velocities and particle masses quickly available between 600 m/s (80 nm) and 50 m/s (900 nm) and particle charge numbers (positive) between 103 and 104[e], depending upon size.

OLYMPIA-LILBID: A New Laboratory Setup to Calibrate Spaceborne Hypervelocity Ice Grain Detectors Using High-Resolution Mass Spectrometry.

The coupling of an Orbitrap-based mass analyzer to the laser-induced liquid beam ion desorption (LILBID) technique has been investigated, with the aim to reproduce the mass spectra recorded by Cassini's Cosmic Dust Analyzer (CDA) in the vicinity of Saturn's icy moon Enceladus. LILBID setups are usually coupled with time-of-flight (TOF) mass analyzers, with a limited mass resolution (∼800 m/Δm). Thanks to the Orbitrap technology, we developed a unique analytical setup that is able to simulate hypervelocity ice grains' impact in the laboratory (at speeds in the range of 15-18 km/s) with an unprecedented high mass resolution of up to 150 000 m/Δm (at m/z 19 for a 500 ms signal duration). The results will be implemented in the LILBID database and will be useful for the calibration and future data interpretation of the Europa Clipper's SUrface Dust Analyzer (SUDA), which will characterize the habitability of Jupiter's icy moon Europa.

State-Selected Reactivity of Carbon Dioxide Cations ( CO 2 + ) With Methane.

The reactivity of CO 2 + with CD4 has been experimentally investigated for its relevance in the chemistry of plasmas used for the conversion of CO2 in carbon-neutral fuels. Non-equilibrium plasmas are currently explored for their capability to activate very stable molecules (such as methane and carbon dioxide) and initiate a series of reactions involving highly reactive species (e.g., radicals and ions) eventually leading to the desired products. Energy, in the form of kinetic or internal excitation of reagents, influences chemical reactions. However, putting the same amount of energy in a different form may affect the reactivity differently. In this paper, we investigate the reaction of CO 2 + with methane by changing either the kinetic energy of CO 2 + or its vibrational excitation. The experiments were performed by a guided ion beam apparatus coupled to synchrotron radiation in the VUV energy range to produce vibrationally excited ions. We find that the reactivity depends on the reagent collision energy, but not so much on the vibrational excitation of CO 2 + . Concerning the product branching ratios ( CD 4 + / CD 3 + /DOCO+) there is substantial disagreement among the values reported in the literature. We find that the dominant channel is the production of CD 4 + , followed by DOCO+ and CD 3 + , as a minor endothermic channel.

The reaction of C5N- with acetylene as a possible intermediate step to produce large anions in Titan's ionosphere.

A theoretical and experimental investigation of the reaction C5N- + C2H2 has been carried out. This reaction is of astrophysical interest since the growth mechanism of large anions that have been detected in Titan's upper atmosphere by the Cassini plasma spectrometer are still largely unknown. The experimental studies have been performed using a tandem quadrupole mass spectrometer which allows identification of the different reaction channels and assessment of their reaction thresholds. Results of these investigations were compared with the predictions of ab initio calculations, which identified possible pathways leading to the observed products and their thermodynamical properties. These computations yielded that the majority of these products are only accessible via energy barriers situated more than 1 eV above the reactant energies. In many cases, the thresholds predicted by the ab initio calculations are in good agreement with the experimentally observed ones. For example, the chain elongation reaction leading to C7N-, although being slightly exoergic, possesses an energy barrier of 1.91 eV. Therefore, the title reaction can be regarded to be somewhat unlikely to be responsible for the formation of large anions in cold environments such as interstellar medium or planetary ionospheres.

Effects of collision energy and vibrational excitation of CH3+ cations on its reactivity with hydrocarbons: But-2-yne CH3CCCH3 as reagent partner.

The methyl carbocation is ubiquitous in gaseous environments, such as planetary ionospheres, cometary comae, and the interstellar medium, as well as combustion systems and plasma setups for technological applications. Here we report on a joint experimental and theoretical study on the mechanism of the reaction CH3+ + CH3CCCH3 (but-2-yne, also known as dimethylacetylene), by combining guided ion beam mass spectrometry experiments with ab initio calculations of the potential energy hypersurface. Such a reaction is relevant in understanding the chemical evolution of Saturn's largest satellite, Titan. Two complementary setups have been used: in one case, methyl cations are generated via electron ionization, while in the other case, direct vacuum ultraviolet photoionization with synchrotron radiation of methyl radicals is used to study internal energy effects on the reactivity. Absolute reactive cross sections have been measured as a function of collision energy, and product branching ratios have been derived. The two most abundant products result from electron and hydride transfer, occurring via direct and barrierless mechanisms, while other channels are initiated by the electrophilic addition of the methyl cation to the triple bond of but-2-yne. Among the minor channels, special relevance is placed on the formation of C5H7+, stemming from H2 loss from the addition complex. This is the only observed condensation product with the formation of new C-C bonds, and it might represent a viable pathway for the synthesis of complex organic species in astronomical environments and laboratory plasmas.

Selected ion flow tube study of the reactions of H3 O+ and NO+ with a series of primary alcohols in the presence of water vapour in support of selected ion flow tube mass spectrometry.

RATIONALE: Alcohols are often present in foods and other biological media, including exhaled breath, urine and cell culture headspace. For their analysis by selected ion flow tube mass spectrometry (SIFT-MS), the ion chemistry initiated by the reactions of the reagent ions H3 O+ and NO+ with alcohol molecules in the presence of water molecules needs to be understood and quantitatively described. METHODS: The reactions of H3 O+ and NO+ ions have been studied with the primary alcohols, methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol and 1-hexanol, under the conditions used for SIFT-MS analyses (1 Torr He; 0.1 Torr air sample; 300 K) and over a range of sample gas humidity from 1% to 5.5%. RESULTS: The H3 O+ reactions led to the formation of protonated alcohol molecules MH+ and their hydrates MH+ (H2 O)1,2,3 and (MH+ -H2 O) fragment ions. The NO+ reactions were observed to proceed mainly via hydride ion transfer, resulting in the formation of [M-H]+ product ions. Formation of the NO+ M adduct ions was also observed due to ligand switching between the NO+ (H2 O)1,2 hydrated reagent ions and M, and via direct NO+ /M association in the case of ethanol. The variation in the percentages of the hydrated product ions with the air sample humidity is reported. CONCLUSIONS: This detailed study has provided the kinetics data, including the secondary hydrated ion product distributions, for the reactions of a number of volatile primary alcohols with the SIFT-MS reagent ions H3 O+ and NO+ , which allows their analyses by SIFT-MS in humid air and also helps in the interpretation of proton transfer reaction (PTR)-MS data. Copyright © 2016 John Wiley & Sons, Ltd.

Is the Reaction of C3N(-) with C2H2 a Possible Process for Chain Elongation in Titan's Ionosphere?

The reaction of C3N(-) with acetylene was studied using three different experimental setups, a triple quadrupole mass spectrometer (Trento), a tandem quadrupole mass spectrometer (Prague), and the "CERISES" guided ion beam apparatus at Orsay. The process is of astrophysical interest because it can function as a chain elongation mechanism to produce larger anions that have been detected in Titan's ionosphere by the Cassini Plasma Spectrometer. Three major products of primary processes, C2H(-), CN(-), and C5N(-), have been identified, whereby the production of the cyanide anion is probably partly due to collisional induced dissociation. The formations of all these products show considerable reaction thresholds and also display comparatively small cross sections. Also, no strong signals of anionic products for collision energies lower than 1 eV have been observed. Ab initio calculations have been performed to identify possible pathways leading to the observed products of the title reaction and to elucidate the thermodynamics of these processes. Although the productions of CN(-) and C5N(-) are exoergic, all reaction pathways have considerable barriers. Overall, the results of these computations are in agreement with the observed reaction thresholds. Due to the existence of considerable reaction energy barriers and the small observed cross sections, the title reaction is not very likely to play a major role in the buildup of large anions in cold environments like the interstellar medium or planetary and satellite ionospheres.

A Pilot Study of Ion - Molecule Reactions at Temperatures Relevant to the Atmosphere of Titan.

Reliable theoretical models of the chemical kinetics of the ionosphere of Saturn's moon, Titan, is highly dependent on the precision of the rates of the reactions of ambient ions with hydrocarbon molecules at relevant temperatures. A Variable Temperature Selected Ions Flow Tube technique, which has been developed primarily to study these reactions at temperatures within the range of 200-330 K, is briefly described. The flow tube temperature regulation system and the thermalisation of ions are also discussed. Preliminary studies of two reactions have been carried out to check the reliability and efficacy of kinetics measurements: (i) Rate constants of the reaction of CH3+ ions with molecular oxygen were measured at different temperatures, which indicate values in agreement with previous ion cyclotron resonance measurements ostensibly made at 300 K. (ii) Formation of CH3+ ions in the reaction of N2+ ions with CH4 molecules were studied at temperatures within the range 240-310 K which showed a small but statistically significant decrease of the ratio of product CH3+ ions to reactant N2+ ions with reaction temperature.

Selective Generation of the Radical Cation Isomers [CH3CN](•+) and [CH2CNH](•+) via VUV Photoionization of Different Neutral Precursors and Their Reactivity with C2H4.

Experimental and theoretical studies have been carried out to demonstrate the selective generation of two different C2H3N(+) isomers, namely, the acetonitrile [CH3CN](•+) and the ketenimine [CH2CNH](•+) radical cations. Photoionization and dissociative photoionization experiments from different neutral precursors (acetonitrile and butanenitrile) have been performed using vacuum ultraviolet (VUV) synchrotron radiation in the 10-15 eV energy range, delivered by the DESIRS beamline at the SOLEIL storage ring. For butanenitrile (CH3CH2CH2CN) an experimental ionization threshold of 11.29 ± 0.05 eV is obtained, whereas the appearance energy for the formation of [CH2CNH](•+) fragments is 11.52 ± 0.05 eV. Experimental findings are fully supported by theoretical calculations at the G4 level of theory (ZPVE corrected energies at 0 K), giving a value of 11.33 eV for the adiabatic ionization energy of butanenitrile and an exothermicity of 0.49 for fragmentation into [CH2CNH](•+) plus C2H4, hampered by an energy barrier of 0.29 eV. The energy difference between [CH3CN](•+) and [CH2CNH](•+) is 2.28 eV (with the latter being the lowest energy isomer), and the isomerization barrier is 0.84 eV. Reactive monitoring experiments of the [CH3CN](•+) and [CH2CNH](•+) isomers with C2H4 have been performed using the CERISES guided ion beam tandem mass spectrometer and exploiting the selectivity of ethylene that gives exothermic charge exchange and proton transfer reactions with [CH3CN](•+) but not with [CH2CNH](•+) isomers. In addition, minor reactive channels are observed leading to the formation of new C-C bonds upon reaction of [CH3CN](•+) with C2H4, and their astrochemical implications are briefly discussed.

Reactions of State-Selected Atomic Oxygen Ions O(+)((4)S, (2)D, (2)P) with Methane.

An experimental study has been carried out on the reactions of state selected O(+)((4)S, (2)D, (2)P) ions with methane with the aims of characterizing the effects of both the parent ion internal energy and collision energy on the reaction dynamics and determining the fate of oxygen species in complex media, in particular the Titan ionosphere. Absolute cross sections and product velocity distributions have been determined for the reactions of (16)O(+) or (18)O(+) ions with CH4 or CD4 from thermal to 5 eV collision energies by using the guided ion beam (GIB) technique. Dissociative photoionization of O2 with vacuum ultraviolet (VUV) synchrotron radiation delivered by the DESIRS beamline at the SOLEIL storage ring and the threshold photoion photoelectron coincidence (TPEPICO) technique are used for the preparation of purely state-selected O(+)((4)S, (2)D, (2)P) ions. A complete inversion of the product branching ratio between CH4(+) and CH3(+) ions in favor of the latter is observed for excitation of O(+) ions from the (4)S ground state to either the (2)D or the (2)P metastable state. CH4(+) and CH3(+) ions, which are by far the major products for the reaction of ground state and excited states, are strongly backward scattered in the center of mass frame relative to O(+) parent ions. For the reaction of O(+)((4)S), CH3(+) production also rises with increasing collision energy but with much less efficiency than with O(+) excitation. We found that a mechanism of dissociative charge transfer, mediated by an initial charge transfer step, can account very well for all the observations, indicating that CH3(+) production is associated with the formation of H and O atoms (CH3(+) + H + O) rather than with OH formation by an hydride transfer process (CH3(+) + OH). Therefore, as the CH4(+) production by charge transfer is also associated with O atoms, the fate of oxygen species in these reactions is essentially the O production, except for the reaction of O(+)((4)S), which also produces appreciable amounts of H2O(+) ions but only at very low collision energy. The production of O atoms and the nature of the states in which they are formed are discussed for the reactions of O(+) ions with CH4 and N2.

The unimolecular chemistry of protonated and deprotonated 2,2-dinitroethene-1,1-diamine (FOX-7) studied by tandem mass spectrometry and computational chemistry.

2,2-Dinitroethene-1,1-diamine (FOX-7) was studied by means of electrospray ionization (ESI) and chemical ionization (CI) mass spectrometry in both positive and negative ion mode. Detailed mechanisms of unimolecular fragmentations of protonated and deprotonated FOX-7 were investigated using high- and low- energy collision-induced dissociation (CID) mass spectrometry, neutral fragment reionization mass spectrometry and quantum chemistry calculations. In deprotonated FOX-7, elimination of the carbodiimide molecule was identified as the energetically most favored fragmentation channel, closely resembling the base hydrolysis of FOX-7. The dinitromethanide ion is formed during this fragmentation as revealed by comparison with CID mass spectra of an isobaric ion prepared by the ESI of authentic sodium dinitromethanide. The proton affinity of FOX-7 was estimated as 855 kJ mo(-1) by high-accuracy quantum chemistry calculations. This value corresponds to protonation at the C-2 position, though the oxygen-protonated tautomer was found to be nearly isoenergetic in the gas phase. In acetonitrile, the nitro group-protonated FOX-7 was found to be significantly less stable then its C-2 tautomer. These theoretical findings are clearly reflected in differences in fragmentations of ESI- and CI-generated [M+H(]+) ions. Interestingly, the consecutive losses of OH∙ and NO2∙ radicals instead of a whole HNO3 molecule were found to account for the most abundant fragment ion in the positive ESI CID mass spectra. In the CI-generated [M+H](+) and [M+D](+) ions, substantial internal energy effects upon the CID were observed.

Reactions of doubly ionized benzene with nitrogen and water: a nitrogen-mediated entry into superacid chemistry.

Even in the highly diluted gas phase, rather than electron transfer the benzene dication C(6)H(6)(2+) undergoes association with dinitrogen to form a transient C(6)H(6)N(2)(2+) dication which is best described as a ring-protonated phenyl diazonium ion. Isotopic labeling studies, photoionization experiments using synchrotron radiation, and quantum chemical computations fully support the formation of protonated diazonium, which is in turn a prototype species of superacidic chemistry in solution. Additionally, reactions of C(6)H(6)(2+) with background water involve the transient formation of diprotonated phenol and, among other things, afford a long-lived C(6)H(6)OH(2)(2+) dication, which is attributed to the hydration product of Hogeveen's elusive pyramidal structure of C(6)H(6)(2+), as the global minimum of doubly ionized benzene. Nitrogen is essential for the formation of the C(6)H(6)OH(2)(2+) dication in that it mediates the formation of the water adduct, while the bimolecular encounter of the C(6)H(6)(2+) dication with water only leads to (dissociative) electron transfer.

The oxidation of natural flavonoid quercetin.

This study explains the controversies in the literature concerning the number of electrons involved in the oxidation of quercetin. This stems from inappropriate handling samples, which require strict anaerobic conditions. The redox potential of quercetin strongly depends on the pH and on the presence of dissociation forms in solution.

On the stability of the bioactive flavonoids quercetin and luteolin under oxygen-free conditions.

The natural flavonoid compounds quercetin (3,3',4',5,7-pentahydroxyflavone) and luteolin (3',4',5,7-tetrahydroxyflavone) are important bioactive compounds with antioxidative, anti-allergic, and anti-inflammatory properties. However, both are unstable when exposed to atmospheric oxygen, which causes degradation and complicates their analytical determinations. The oxidative change of these flavonoids was observed and followed by UV-visible spectrophotometry, both in aqueous and ethanolic solutions. The distribution of the degradation products in aqueous media was monitored by LC-MS and LC-DAD analysis. The amounts of oxidative reaction products increase with the exposure time. The oxidative degradation reduces the pharmacological efficiency of these antioxidants and renders analytical determination inaccurate. The oxidative changes in flavonoid test solutions can explain the inconsistent dissociation constants reported in the literature. Dissociation constants of quercetin and luteolin were determined both by alkalimetric titration and by UV-visible spectrophotometry under deaerated conditions. The values pK(1) = 5.87 ± 0.14 and pK(2) = 8.48 ± 0.09 for quercetin, and pK(1) = 5.99 ± 0.32 and pK(2) = 8.40 ± 0.42 for luteolin were found.

Double ionization of cycloheptatriene and the reactions of the resulting C7H(n)2+ dications (n = 6, 8) with xenon.

The formation and fragmentation of the molecular dication C(7)H(8)(2+) from cycloheptatriene (CHT) and the bimolecular reactivities of C(7)H(8)(2+) and C(7)H(6)(2+) are studied using multipole-based tandem mass spectrometers with either electron ionization or photoionization using synchrotron radiation. From the photoionization studies, an apparent double-ionization energy of CHT of (22.67 ± 0.05) eV is derived, and the appearance energy of the most abundant fragment ion C(7)H(6)(2+), formed via H(2) elimination, is determined as (23.62 ± 0.07) eV. Analysis of both the experimental data as well as results of theoretical calculations strongly indicate, however, that an adiabatic transition to the dication state is not possible upon photoionization of neutral CHT and the experimental value is just considered as an upper bound. Instead, an analysis via two different Born-Haber cycles suggests (2)IE(CHT) = (21.6 ± 0.2) eV. Further, the bimolecular reactivities of the C(7)H(n)(2+) dications (n = 6, 8), generated via double ionization of CHT as a precursor, with xenon as well as nitrogen lead, inter alia, to the formation of the organo-xenon dication C(7)H(6)Xe(2+) and the corresponding nitrogen adduct C(7)H(6)N(2)(2+).

Selected ion flow tube study of ion-molecule reactions of N(+)(3P) and Kr+ with C3 hydrocarbons propane, propene, and propyne.

Reactions of (14)N(+)((3)P), (15)N(+)((3)P), and Kr(+) with propane, propene, and propyne were studied using the selected ion flow tube, SIFT, technique. Thermal rate constants in all N(+)/C(3) systems were k = (2 ± 0.4) × 10(-9) cm(3) molecule(-1) s(-1), close to the collisional rate constants. With propane and propene, only hydrocarbon ions were found among the products of reactions with N(+); in propyne about 15% of the products were N-containing ions (C(3)H(2)N(+), C(2)H(4)N(+), C(2)H(3)N(+), C(2)H(2)N(+)), and the rest were hydrocarbon ions. A comparison with product ions from electron transfer between Kr(+) (of recombination energy similar to that for N(+)((3)P)) and the C(3) hydrocarbons and further analysis of the results led to an estimation of an approximate ratio of electron transfer vs hydride-ion transfer reactions leading to the hydrocarbon product ions: in propane the ratio was 2:1, in propene 3:1, and in propyne 5:1. A fraction of product ions resulted from reactions leading to the excited neutral product N*.

Formation of organoxenon dications in the reactions of xenon with dications derived from toluene.

The bimolecular reactivity of xenon with C(7)H(n)(2+) dications (n=6-8), generated by double ionization of toluene using both electrons and synchrotron radiation, is studied by means of a triple-quadrupole mass spectrometer. Under these experimental conditions, the formation of the organoxenon dications C(7)H(6)Xe(2+) and C(7)H(7)Xe(2+) is observed to occur by termolecular collisional stabilization. Detailed experimental and theoretical studies show that the formation of C(7)H(6)Xe(2+)+H(2) from doubly ionized toluene (C(7)H(8)(2+)) and xenon occurs as a slightly endothermic, direct substitution of dihydrogen by the rare gas with an expansion to a seven-membered ring structure as the crucial step. For the most stable isomer of C(7)H(6)Xe(2+), an adduct between the cycloheptatrienyldiene dication and xenon, the computed binding energy of 1.36 eV reaches the strength of (weak) covalent bonds. Accordingly, electrophiles derived from carbenes might be particularly promising candidates in the search for new rare-gas compounds.

Comparative study of mono- and dinuclear complexes of late 3d-metal chlorides with N,N-dimethylformamide in the gas phase.

Mono- and binuclear complexes of N,N-dimethylformamide (DMF) with chlorides of the divalent, late 3d metals M = Co, Ni, Cu, and Zn are investigated by means of electrospray ionization (ESI). Specifically, ESI leads to monocations of the type [(DMF)(n)MCl](+) and [(DMF)(n)M(2)Cl(3)](+), of which the species with n = 2 and 3 were selected for in-depth studies. The latter include collision-induced dissociation experiments, gas-phase infrared spectroscopy, and calculations using density functional theory. The mononuclear complexes [(DMF)(n)MCl](+) almost exclusively lose neutral DMF upon collisional activation with the notable exception of the copper complex, for which also a reduction from Cu(II) to Cu(I) concomitant with the release of atomic chlorine is observed. For the dinuclear clusters, there exists a competition between loss of a DMF ligand and cluster degradation via loss of neutral MCl(2) with decreasing cluster stability from cobalt to zinc. For the specific case of [(DMF)(n)ZnCl](+) and [(DMF)(n)Zn(2)Cl(3)](+), ion-mobility mass spectrometry indicates the existence of two isomeric cluster ions in the case of [(DMF)(2)Zn(2)Cl(3)](+) which corroborates parallel theoretical predictions.