Intramolecular isotope effects in the reactions of CF32+ and CO22+ with HD

Competition of electron transfer, dissociation, and bond-forming processes in the reaction of the CO(2)(2+) dication with neutral CO(2)

The bimolecular reactivity of the CO(2)(2+) dication with neutral CO(2) is investigated using triple quadrupole and ion-ion coincidence mass spectrometry. Crucial for product analysis is the use of appropriate isotope labelling in the quadrupole experiments in order to distinguish the different reactive pathways. The main reaction corresponds to single-electron transfer from the neutral reagent to the dication, i.e. CO(2)(2+) + CO(2) --> 2CO(2)(+); this process is exothermic by almost 10 eV, if ground state monocations are formed. Interestingly, the results indicate that the CO(2)(+) ion formed when the dication accepts an electron dissociates far more readily than the CO(2)(+) ion formed from the neutral CO(2) molecule. This differentiation of the two CO(2)(+) products is rationalized by showing that the population of the key dissociative states of the CO(2)(+) monocation will be favoured from the CO(2)(2+) dication rather than from neutral CO(2). In addition, two bond-forming reactions are observed as minor channels, one of which leads to CO(+) and O(2)(+) as ionic products and the other affords a long-lived C(2)O(3)(2+) dication.

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.

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.

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.