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A crucial chain-branching step in autoignition is the decomposition of ketohydroperoxides (KHP) to form an oxy radical and OH. Other pathways compete with chain-branching, such as "Korcek" dissociation of γ-KHP to a carbonyl and an acid. Here we characterize the formation of a γ-KHP and its decomposition to formic acid+acetone products from observations of n-butane oxidation in two complementary experiments. In jet-stirred reactor measurements, KHP is observed above 590 K. The KHP concentration decreases with increasing temperature, whereas formic acid and acetone products increase. Observation of characteristic isotopologs acetone-d

Products formed in the reaction of C 2 H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C 6 H 6 . The photoionization spectrum of the C 6 H 6 product indicates that fulvene is formed with a branching fraction of (57 ± 30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.

The nitrogen molecular dication (N22+) has been proposed as a minor but significant component of the ionosphere of Saturn's moon Titan with an abundance comparable to that of several key monocations. It has also been suggested that the reactions of N22+ with H2 can provide a source of N2H2+ in Titan's atmosphere. This paper reports the results from experiments, using a position-sensitive coincidence technique, which reveal the chemical reactions forming pairs of monocations following collisions of the N22+ dication with H2(D2) at a centre-of-mass collision energy of 0.9(1.8) eV. These experiments show, in addition to single electron-transfer processes, a bond-forming pathway forming NH+ + H+ + N and allow an estimate to be made of the reaction cross section and the rate coefficient for this reaction. The correlations between the product velocities revealed by the coincidence experiments show that NH+ is formed via N atom loss from a primary encounter complex [N2H2]2+ to form NH22+, with this triatomic daughter dication then fragmenting to yield NH+ + H+. A computational investigation of stationary points on the lowest energy singlet and triplet [N2H2]2+ potential energy surfaces confirms the mechanistic deductions from the experiments and indicates that the formation of NH+ occurs solely, and efficiently, from the reaction of the c3Σ+u excited electronic state of N22+.

Thermal reactions of the CF32+ dication with argon lead to the formation of an ArCF22+ dication, a new type of metastable species with an argon−carbon bond. None of the other rare gases undergo a similar reaction with CF32+. For the lighter rare gases (He and Ne), no reactions with CF32+ other than those due to electronically excited reactant ions are observed, whereas for the heavier rare gases (Kr and Xe), the prevailing reactive pathways involve single-electron transfer. At elevated collision energies, single-electron transfer predominates for collisions with all rare gases (He−Xe).

The reaction between Ar 2+ and C 2 H 2 has been studied, at centre-of-mass collision energies ranging from 3 to 7 eV, using a position-sensitive coincidence technique to detect the monocation pairs, which are formed. Sixteen different reaction channels generating pairs of monocations have been observed, these channels arise from double-electron-transfer, single-electron-transfer and chemical reactions forming ArC + . Examination of the scattering diagrams and energetic information extracted from the coincidence data indicate that double-electron-transfer is a direct process, which does not involve a collision complex, and the derived energetics point towards a concerted, not stepwise, mechanism for the two-electron-transfer. As is commonly observed, single-electron-transfer from C 2 H 2 to Ar 2+ takes place via a direct mechanism, again not involving complexation. Most of the C 2 H 2 + products that are formed in the single-electron-transfer reactions possess significant (12–15 eV) internal energy and fragment rapidly within the electric field of the partner Ar + ion. The chemical reactions appear to proceed via a direct mechanism involving the initial formation of ArCH + , which subsequently fragments to form ArC + .

The products formed in the reaction between the methylidene radical (CH) and acrolein (CH2═CHCHO) are probed at 4 Torr and 298 K employing tunable vacuum-ultraviolet synchrotron light and multiplexed photoionization mass-spectrometry. The data suggest a principal exit channel of H loss from the adduct to yield C4H4O, accounting for (78 ± 10)% of the products. Examination of the photoionization spectra measured upon reaction of both CH and CD with acrolein reveals that the isomeric composition of the C4H4O product is (60 ± 12)% 1,3-butadienal and (17 ± 10)% furan. The remaining 23% of the possible C4H4O products cannot be accurately distinguished without more reliable photoionization spectra of the possible product isomers but most likely involves oxygenated butyne species. In addition, C2H2O and C3H4 are detected, which account for (14 ± 10)% and (8 +10, -8)% of the products, respectively. The C2H2O photoionization spectrum matches that of ketene and the C3H4 signal is composed of (24 ± 14)% allene and (76 ± 22)% propyne, with an upper limit of 8% placed on the cyclopropene contribution. The reactive potential energy surface is also investigated computationally, and specific rate coefficients are calculated with RRKM theory. These calculations predict overall branching fractions for 1,3-butadienal and furan of 27% and 12%, respectively, in agreement with the experimental results. In contrast, the calculations predict a prominent CO + 2-methylvinyl product channel that is at most a minor channel according to the experimental results. Studies with the CD radical strongly suggest that the title reaction proceeds predominantly via cycloaddition of the radical onto the C═O bond of acrolein, with cycloaddition to the C═C bond being the second most probable reactive mechanism.

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.

Using doubly ionized acetylene as a superelectrophilic reagent, the new rare-gas compounds HCCAr2+ and HCCKr2+ have been prepared for the first time in hyperthermal collisions of mass-selected C2H2(2+) with neutral rare gases (Rg). However, electron transfer from the rare gas to the acetylene dication as well as proton transfer from C2H2(2+) to the rare gas efficiently compete with formation of HCCRg2+. The computational investigations show that the formation of HCCRg2+ from acetylene dication is endothermic with Rg = He, Ne, Ar and Kr and only weakly exothermic with Xe. These energetic factors, as well as the pronounced competition with the other reactive channels help to explain why HCCRg2+ is only observed with Rg = Ar and Kr.

This work measures the absolute photoionization cross-section of the vinyl radical (σ vinyl(E)) between 8.1 and 11.0 eV. Two different methods were used to obtain absolute cross-section measurements: 193 nm photodissociation of methyl vinyl ketone (MVK) and 248 nm photodissociation of vinyl iodide (VI). The values of the photoionization cross-section for the vinyl radical using MVK, σ vinyl(10.224 eV) = (6.1 ± 1.4) Mb and σ vinyl(10.424 eV) = (8.3 ± 1.9) Mb, and using VI, σ vinyl(10.013 eV) = (4.7 ± 1.1) Mb, σ vinyl(10.513 eV) = (9.0 ± 2.1) Mb, and σ vinyl(10.813 eV) = (12.1 ± 2.9) Mb, define a photoionization cross-section that is ∼1.7 times smaller than a previous determination of this value.

The single electron transfer reactions between (13)CO(2)(2+) and (12)CO(2) and between (18)O(2)(2+) and (16)O(2) have been studied, using a position-sensitive coincidence technique, to test recently proposed explanations for the preferential dissociation of the (13)CO(2)(+) ion (the capture monocation) formed following electron transfer to (13)CO(2)(2+). In our studies of the carbon dioxide collision system, in agreement with previous work, the capture monocation shows a greater propensity to dissociate than the monocation formed from the neutral, (12)CO(2)(+) (the ejection monocation). The coincidence data clearly show that the dissociation pathways of the (13)CO(2)(+) and (12)CO(2)(+) ions are different and are consistent with the ejection monocation dissociating via population of the C(2)Sigma state, whilst the capture ion is predominantly directly formed in dissociative quartet states. This state assignment is in accord with an expected preference for one-electron transitions in the electron transfer process. A propensity for one-electron transitions also rationalizes our observation that following dissociative single electron transfer between (18)O(2)(2+) and (16)O(2) the ejection monocation ((16)O(2)(+)) preferentially dissociates; the opposite situation to that observed for carbon dioxide. The coincidence results for this reaction indicate the (16)O(2)(+) dissociation results from population of the B((2)Sigma) state. The less favoured dissociation of the capture monocation clearly involves population of a different electronic state(s) to those populated in the ejection ion. Indeed, the experimental data are consistent with the dissociation of the capture monocation via predissociated levels of the b((4)Sigma) state. Since the population of the B((2)Sigma) state from the neutral O(2) molecule involves a one-electron transition, and the population of the valence dissociative states of O(2)(+) from the dication are multi-electron processes, the preferential dissociation of the ejection monocation in this collision system can be rationalized by the same principles used to explain the electron transfer reactivity of CO(2)(2+) with CO(2).

The single-electron transfer reactions between helium atoms and the ground (3P) and first (1D) excited states of Ar2+(3p−2) have been investigated at a state-resolved level employing a position-sensitive coincidence mass spectrometer. Using this apparatus, the complete angular distributions of the Ar+(2P) and He+(2S) product ions arising from this electron transfer reaction have been determined at centre-of-mass collision energies ranging from 0.4 to 1.2 eV. The Ar+ product ions formed by the reaction of the 3P state of Ar2+ are predominantly forward scattered at collision energies between 1.2 and 0.6 eV, the distribution broadening with decreasing collision energy, and are scattered isotropically at the lowest collision energy investigated (0.4 eV). Product Ar+ ions arising from the reaction of He with the 1D state of Ar2+ have angular distributions which vary strongly with the collision energy over the range studied. No reactivity of the second (1S, 3p−2) excited state of Ar2+ is observed, allowing an upper limit of 0.02 to be placed on the relative reaction cross-section for this state with respect to that of the 1D state of Ar2+.