Bond-forming reactions of dications: Production of ArO+ and ArO2+ in the reaction of Ar2+ with O2

Bond-forming and electron-transfer reactivity between Ar2+ and N2

Collisions between Ar2+ and N2 have been studied using a coincidence technique at a centre-of-mass (CM) collision energy of 5.1 eV. Four reaction channels generating pairs of monocations are observed: Ar+ + N2+, Ar+ + N+, ArN+ + N+ and N+ + N+. The formation of Ar+ + N2+ is the most intense channel, displaying forward scattering but with a marked tail to higher scattering angles. This scattering, and other dynamics data, is indicative of direct electron transfer competing with a 'sticky' collision between the Ar2+ and N2 reactants. Here Ar+ is generated in its ground (2P) state and N2+ is primarily in the low vibrational levels of the C2Σu+ state. A minor channel involving the initial population of higher energy N2+ states, lying above the dissociation asymptote to N+ + N, which fluoresce to stable states of N2+ is also identified. The formation of Ar+ + N+ by dissociative single electron transfer again reveals the involvement of two different pathways for the initial electron transfer (direct or complexation). This reaction pathway predominantly involves excited states of Ar2+ (1D and 1S) populating N2+* in its dissociative C2Σu+, 22Πg and D2Πg states. Formation of ArN+ + N+ proceeds via a direct mechanism. The ArN+ is formed, with significant vibrational excitation, in its ground (X3Σ-) state. Formation of N+ + N+ is also observed as a consequence of double electron transfer forming N22+. The exoergicity of the subsequent N22+ dissociation reveals the population of the A1Πu and D3Πg dication states.

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.

Review: gas-phase ion chemistry of the noble gases: recent advances and future perspectives

This review article surveys recent experimental and theoretical advances in the gas-phase ion chemistry of the noble gases. Covered issues include the interaction of the noble gases with metal and non-metal cations, the conceivable existence of covalent noble-gas anions, the occurrence of ion-molecule reactions involving singly-charged xenon cations, and the occurrence of bond-forming reactions involving doubly-charged cations. Research themes are also highlighted, that are expected to attract further interest in the future.

Oxygen-containing diatomic dications in the gas phase

A variety of oxygen-containing diatomic dications XO(2+) can be produced in the gas phase by prolonged high-current (16)O(-) ion surface bombardment (oxygen ion beam sputtering) of a wide range of sample materials. These gas-phase species were detected by mass spectrometry at half-integer m/z values for ion flight times of the order of ~10-5 s. Examples provided here include ion mass spectra of AsO(2+), GaO(2+), SbO(2+), AgO(2+), CrO(2+) and BeO(2+). A detailed theoretical study of the diatomic dication system BeO(2+) is also presented.

Bond-forming reactions of dications with molecules: a computational and experimental study of the mechanisms for the formation of HCF2+ from CF3(2+) and H2

The QCISD and QCISD(T) quantum chemical methods have been used to characterize the energetics of various possible mechanisms for the formation of HCF2+ from the bond-forming reaction of CF3(2+) with H2. The stationary points on four different pathways leading to the product combinations HCF2+ + H+ + F and HCF2+ + HF+ have been calculated. All four pathways begin with the formation of a collision complex [H2-CF3]2+, followed by an internal hydrogen atom migration to give HC(FH)F2(2+). In two of the mechanisms, immediate charge separation of HC(FH)F2(2+) via loss of either HF+ or a proton, followed by loss of an F atom, yields the experimentally observed bond-forming product HCF2+. For the other two mechanisms, internal hydrogen rearrangement of HC(FH)F2(2+) to give C(FH)2F(2+), followed by charge separation, yields the product CF2H+. This product can then overcome a 2.04 eV barrier to rearrange to the HCF2+ isomer, which is 1.80 eV more stable. All four calculated mechanisms are in agreement with the isotope effects and collision energy dependencies of the product ion cross sections that have been previously observed experimentally following collisions between CF3(2+) and H2/D2. We find that in this open-shell system, CCSD(T) and QCISD(T) T1-diagnostic values of up to 0.04 are acceptable. A series of angularly resolved crossed-beam scattering experiments on collisions of CF3(2+) with D2 have also been performed. These experiments show two distinct channels leading to the formation of DCF2+. One channel appears to correspond to the pathway leading to the ground state 1DCF2+ + D+ + F product asymptote and the other to the 3DCF2+ + D+ + F product asymptote, which is 5.76 eV higher in energy. The experimental kinetic energy releases for these channels, 7.55 and 1.55 eV respectively, have been determined from the velocities of the DCF2+ product ion and are in agreement with the reaction mechanisms calculated quantum chemically. We suggest that both of these observed experimental channels are governed by the reaction mechanism we calculate in which charge separation occurs first by loss of a proton, without further hydrogen atom rearrangement, followed by loss of an F atom to give the final products 1DCF2+ + D+ + F or 3DCF2+ + D+ + F.

On a Possible Growth Mechanism for Polycyclic Aromatic Hydrocarbon Dications: C7H62++C2H2

The mechanism of the bond-forming reaction between C(7)H(6) (2+) and C(2)H(2) to yield C(9) entities has been investigated by density functional theory calculations with close comparison with experimental data. It is shown that the reaction produces the C(9)H(6) (2+) and C(9)H(7) (2+) di-cations with geometries most probably derived from the indene skeleton. In comparison, the formation of linear structures of di-cations is much more energy-demanding and therefore appears improbable.

Bimolecular reactions of molecular dications: reactivity paradigms and bond-forming processes

The bimolecular reactivity of molecular dications in the gas phase is reviewed from an experimental point of view. Recent research has demonstrated that in addition to the ubiquitous occurrence of electron transfer in the reactions of gaseous dications with neutral molecules, bond-forming reactions play a much larger role than anticipated before. Thus, quite a number of hydrogen-containing dications show proton transfer to neutral reagents as an abundant or even as the major pathway, and also the nature of the neutral reagent itself is decisive for the amount of proton transfer which takes place. Further, several hydrocarbon dications C(m)H(n)(2+) of medium size (m = 6-14, n = 6-10) undergo bond-forming reactions with unsaturated hydrocarbons such as acetylene or benzene, thereby offering new routes for the formation of larger aromatic compounds under extreme conditions such as interstellar environments. Likewise, recent results on the bimolecular reactivity of multiply charged metal ions have revealed the occurrence of a number of new bond-forming reactions which open promising prospects for further research.

Electron transfer and bond-forming reactions following collisions of Cl2+ and HCl2+ with CO

Collisions between Cl(2+) and CO have been investigated using time-of-flight mass spectrometry over a collision energy range between 2.2 eV and 7.1 eV in the centre-of-mass frame. The formation of Cl(+), CO(+) and C(+) in electron transfer reactions has been detected and an unusual bond-forming reaction which generates CCl(2+) has also been observed. The reactive cross-sections, in arbitrary units, for the electron transfer reactions have been evaluated. To extract these cross sections we employ a new method of analysing mass spectral intensities for crossed-beam experiments, an algorithm which allows inter-comparison of the fluxes of all the ionic products from the electron transfer reactions. The observed electron transfer reactivity has been rationalized by calculations based on Landau-Zener theory. To account for the observation of CCl(2+), we have calculated the relevant energetics showing that the lowest lying doublet state of this dication is bound and is energetically accessible at our collision energies. These energetic arguments indicate that electron transfer in the exit channel between the separating CCl(2+) and O atom probably forms C(+) ions via the dissociation of CCl(+). Additionally, collisions between HCl(2+) and CO have been studied at collision energies from 2.2 to 7.0 eV in the centre-of-mass frame. In this collision system, proton transfer to form HCO(+) is observed to compete efficiently with dissociative and non-dissociative electron transfer.

Bond-Forming Reactions of Molecular Dications as a New Route to Polyaromatic Hydrocarbons

Mass-selected CmHn2+ dications of medium size (m = 6-14, n = 6-10) undergo bond-forming reactions in the presence of acetylene with Cm+2Hn2+ + H2 as the major product channel. These unprecedented reactions of organic dications offer a feasible route for the formation of polycyclic aromatic hydrocarbons under extreme conditions, such as in interstellar clouds.

Probing the intrinsic features and environmental stabilization of multiply charged anions

Multiply charged anions (MCAs) represent exotic, highly energetic species in the gas-phase due to their propensity to undergo unimolecular decay via electron loss or ionic fragmentation. There is considerable fundamental interest in these systems since they display novel potential energy surfaces that are characterized by Coulomb barriers. Over recent years, considerable progress has been made in understanding the factors that affect the stability, decay pathways and reactivity of gas-phase MCAs, mainly as a result of the application of electrospray ionization as a generic technique for transferring solution-phase MCAs into the gas-phase for detailed characterization. We review contemporary work in this field, focusing on the factors that control the intrinsic stability of MCAs, both as isolated gas-phase ions, and on their complexation with solvent molecules and counter-ions. While studies of MCAs are primarily of fundamental interest, several classes of important biological ions are commonly observed as MCAs in the gas-phase (e.g. oligonucleotides, sugars). Recent results for biologically relevant ions are emphasised, since a fundamental understanding of the properties of gas-phase MCAs will be highly valuable for developing further analytical methods to study these important systems.

The formation of NO+ from the reaction of N22+ with O2

We have studied the potentially ionospherically significant reaction between N(2)2+ with O2 using position-sensitive coincidence spectroscopy. We observe both nondissociative and dissociative electron transfer reactions as well as two channels involving the formation of NO+. The NO+ product is formed together with either N+ and O in one bond-forming channel or O+ and N in the other bond-forming channel. Using the scattering diagrams derived from the coincidence data, it seems clear that both bond-forming reactions proceed via a collision complex [N2O2]2+. This collision complex then decays by loss of a neutral atom to form a daughter dication (NO2(2+) or N2O2+), which then decays by charge separation to yield the observed products.

Experimental studies of the dynamics of the bond-forming reactions of CF22+ with H2O using position-sensitive coincidence spectroscopy

The dynamics of the product channels forming OCF(+)+H(+)+HF and HCF(2) (+)+H(+)+O following the collisions of CF(2) (2+) with H(2)O have been investigated with a new position-sensitive coincidence experiment at a center-of-mass collision energy of 5.6 eV. The results show the formation of OCF(+) occurs via the formation of a doubly charged collision complex [H(2)O-CF(2)](2+) which subsequently undergoes a charge separating dissociation to form H(+) and HOCF(2) (+). The HOCF(2) (+) monocation subsequently fragments to form HF+OCF(+). The lifetimes of the collision complex and the HOCF(2) (+) ion are at least of the order of their rotational period. The kinetic energy release in this reaction indicates that it involves the ground state of CF(2) (2+) and forms the ground electronic states of OCF(+) and HF. The mechanism for forming HCF(2) (+) involves the direct and rapid abstraction of a hydride ion from H(2)O by CF(2) (2+). The resulting OH(+) ion subsequently fragments to H(+)+O, on a time scale at least comparable with its rotational period.

The Bond-Forming Reactions of Atomic Dications with Neutral Molecules: Formation of ArNH+ and ArN+ from Collisions of Ar2+ with NH3

An experimental and computational study has been performed to investigate the bond-forming reactivity between Ar(2+) and NH(3). Experimentally, we detect two previously unobserved bond-forming reactions between Ar(2+) and NH(3) forming ArN(+) and ArNH(+). This is the first experimental observation of a triatomic product ion (ArNH(+)) following a chemical reaction of a rare gas dication with a neutral. The intensity of ArNH(+) was found to decrease with increasing collision energy, with a corresponding increase in the intensity of ArN(+), indicating that ArN(+) is formed by the dissociation of ArNH(+). Key features on the potential energy surface for the reaction were calculated quantum chemically using CASSCF and MRCI methods. The calculated reaction mechanism, which takes place on a singlet surface, involves the initial formation of an Ar-N bond to give Ar-NH(3)(2+). This complexation is followed by proton loss via a transition state, and then loss of the two remaining hydrogen atoms in two subsequent activationless steps to give the products (3)ArN(+) + H(+) + 2H. This calculated pathway supports the sequential formation of ArN(+) from ArNH(+), as suggested by the experimental data. The calculations also indicate that no bond-forming pathway exists on the ground triplet surface for this system.