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

Exploring three-body fragmentation of acetylene trication

The three-body breakup of [C₂H₂]³⁺ formed in collision with Xe⁹⁺ moving at 0.5 atomic units of velocity is studied by using recoil ion momentum spectroscopy. Three-body breakup channels leading to (H⁺, C⁺, CH⁺) and (H⁺, H⁺, C₂ ⁺) fragments are observed in the experiment and their kinetic energy release is measured. The breakup into (H⁺, C⁺, CH⁺) occurs via concerted and sequential modes, whereas the breakup into (H⁺, H⁺, C₂ ⁺) shows only the concerted mode. By collecting events coming exclusively from the sequential breakup leading to (H⁺, C⁺, CH⁺), we have determined the kinetic energy release for the unimolecular fragmentation of the molecular intermediate, [C₂H]²⁺. By using ab initio calculations, the potential energy surface for the lowest electronic state of [C₂H]²⁺ is generated, which shows the existence of a metastable state with two possible dissociation pathways. A discussion on the agreement between our experimental results and these ab initio calculations is presented.

Regioselective Bond-Forming and Hydrolysis Reactions of Doubly Charged Vanadium Oxide Anions in the Gas Phase

The gas-phase reactivity of vanadium-containing dianions, NaV3O92− and its hydrated form H2NaV3O102−, were probed towards sulphur dioxide at room temperature by ion-molecule reaction (IMR) experiments in the collision cell of an ion trap mass spectrometer. The sequential addition of two SO2 molecules to the NaV3O92− dianion leads to the breakage of the stable V3O9 backbone, resulting in a charge separation process with the formation of new V-O and S-O bonds. On the contrary, the H2NaV3O102− hydroxide species reacts with SO2, promoting regioselective hydrolysis and bond-forming processes, the latter similar to that observed for the NaV3O92− reactant anion. Kinetic analysis shows that these reactions are fast and efficient with rate constants of the 10−9 (±30) cm3 s−1 molecule−1 order of magnitude.

Unusual hydroxyl migration in the fragmentation of β-alanine dication in the gas phase

Experimental and theoretical investigations show that hydroxyl migration leads to unexpected fragmentation dynamics of β-alanine dication in the gas phase.

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.

First and second ionization energies of 1,3,5-trimethylbenzene and 2,4,6-trimethylpyridine

The first and second ionization energies of trimethyl substituted analogs of benzene and pyridine are determined by means of mass spectrometry in conjunction with synchrotron radiation. The first ionization energy of 1,3,5-trimethylbenzene amounts to (8.38 ± 0.05) eV and the second ionization energy to (22.8 ± 0.1) eV. The first ionization energy of 2,4,6-trimethylpyridine is determined as (8.65 ± 0.05) eV and the second ionization energy as (23.0 ± 0.1) eV. The ionization energies are compared with those of unsubstituted benzene and pyridine and the effects of the methyl groups are evaluated by means of isodesmic reactions. As expected, it is found that the electron-donating effect of the methyl groups stabilizes neutral pyridine and doubly charged pyridine more than neutral benzene and doubly charged benzene, respectively. Surprisingly, the opposite effect is found for the radical cations, which is ascribed to the unfavorable degenerate electronic structure of benzene radical-cation, which disappears upon the methyl substitution.

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.

On the C-C Coupling of the Naphthylium Ion with Methane

Unlike other medium-sized hydrocarbon cations CmHn+ (m = 7-11, n = 6-12), the naphthylium ion C10H7+ undergoes a thermal reaction with methane to form the C-C coupled product C11H9+ concomitant with dehydrogenation. This reaction, which might be of relevance in the context of the growth of hydrocarbon species under extreme conditions, is suggested to lead to a benzylium-type cation in analogy to the C-C coupling of phenyl cations with methane.

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.

Bond-formation versuselectron transfer: C–C-coupling reactions of hydrocarbon dications with benzene

The bimolecular reactions of several hydrocarbon dications C(m)H(n)(2+) (m = 6-10, n = 4-9) with neutral benzene are investigated by tandem mass spectrometry using a multipole instrument. Not surprisingly, the major reaction of C(m)H(n)(2+) with benzene corresponds to electron transfer from the neutral arene to the dication resulting in the pair of monocationic products C(m)H(n)(+) + C(6)H(6)(+). In addition, also dissociative electron transfer takes place, whereas proton transfer from the C(m)H(n)(2+) dication to neutral benzene is almost negligible. Interestingly, the excess energy liberated upon electron transfer from the neutral arene to the C(m)H(n)(2+) dication is not equally partitioned in the monocationic products in that the cations arising from the dicationic precursor have a higher internal energy content than the monocations formed from the neutral reaction partner. In addition to the reactions leading to monocationic product ions, bond-forming reactions with maintenance of the two-fold charge are observed, which lead to a condensation of the C(m)H(n)(2+) dications with neutral benzene under formation of intermediate C(m+6)H(n+6)(2+) species and then undergo subsequent losses of molecular hydrogen or neutral acetylene. This reaction complements a recently proposed dicationic route for the formation of polycyclic aromatic hydrocarbons under extreme conditions such as they exist in interstellar environments.

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.