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To understand the origin of interstellar molecules we rely on astrochemical models, the gas-phase networks of which contain ≥7000 reactions. However, just a tiny fraction of them have parameters derived in laboratory experiments. Theoretical quantum mechanical (QM) calculations can also provide this information. Unfortunately, sometimes theoretical predictions and experimental values disagree, as is the case for the paradigmatic reaction CH3OH + OH → CH3O + H2O. Both laboratory experiments and QM calculations found an unexpected increase in the rate coefficients with decreasing temperature. However, experimental and theoretical estimates of the rate coefficients diverge by up to two orders of magnitude at the low temperatures of interest in interstellar chemistry. This work aims to test whether astronomical observations can help untangle this confusing situation. To this end, we first carried out new QM calculations to derive the rate coefficients of the major destruction reaction of the methoxy radical, CH3O + H, and then we compared astronomical observations from the IRAM/NOEMA Large Programme SOLIS with astrochemical model predictions. Our new rate coefficient for the CH3O + H reaction is 5–10 times larger than that in the astrochemical data base KIDA in the 10–100 K range. When including the new methoxy destruction rate coefficients, the comparison between observations and model predictions favours the rate coefficients of the CH3OH + OH reaction from QM calculations. We conclude that QM calculations are an important alternative to laboratory experiments when it comes to the harsh conditions of interstellar objects and that astronomical observations can be used to constraint the rate coefficients of relevant reactions. [ABSTRACT FROM AUTHOR]

In this paper we report a combined experimental and theoretical study on the dynamics of the insertion reaction C(1D)+D2 at 15.5 kJ mol-1 collision energy. Product angular and velocity distributions have been obtained in crossed beam experiments and quasiclassical trajectory (QCT) and rigorous statistical calculations have been performed on the recent and accurate ab initio potential energy surface of Bussery-Honvault, Honvault, and Launay at the energy of the experiment. The molecular-beam results have been simulated using the theoretical calculations. Good agreement between experiment and both QCT and statistical predictions is found. [ABSTRACT FROM AUTHOR]

The reaction OH+D2→HOD+D was studied in a crossed beams experiment at a collision energy of 6.3 kcal/mol. Center-of-mass translational energy and angular distributions were determined. The HOD product shows a very pronounced backward scattering. Thirty-two percent of the available energy is released as product translational energy. [ABSTRACT FROM AUTHOR]

Until recently, the chemistry of boronyl (BO), a diatomic radical isolectronic with the cyano (CN) species, has remained unknown. The boronyl group is characterized by a boron-oxygen multiple bond, and because of the inherent electron deficiency of the boron atom, boronyls (RBO) are highly reactive and typically only exist in their cyclotrimeric form (RBO)3. Due to their invaluable role as reactants, the isolation of the monomers in gas phase experiments has been extensively sought after by the organic synthesis and physical organic chemistry communities but never achieved. Besides the interests from a physical organic and synthetic point of view, boronyls also play a role as reaction intermediates in boron-assisted rocket propulsion systems.In this Account, we review recent experimental work in which gas phase organo boronyl monomers (RBO) are formed via bimolecular reactions of the boronyl radical (BO) with C2-C6 unsaturated hydrocarbons. The investigated hydrocarbons are widely exploited as fuels, and their reactions with boronyl radicals under single collision conditions lead to the formation of organo boronyls. Our studies also elucidate the mechanisms of their formation reactions thus furnishing a comprehension at the molecular level of this reaction class. The variety of the employed hydrocarbon substrates has allowed us to systematically classify the chemical behavior of the boronyl radicals. With the exception of the case of the dimethylacetylene reaction, the boron monoxide radical versus atomic hydrogen exchange mechanisms were always open leading to the formation of highly unsaturated organo boronyl monomers (RBO), which could be easily identified because they cannot trimerize under single collision conditions.Besides the hydrogen displacement pathway, methylacetylene, dimethylacetylene, and propylene, carrying one or two methyl groups, were also found to eliminate a methyl group. In all systems, the reactions were barrierless, indirect, and initiated by addition of the boron monoxide radical to the π electron density of the hydrocarbon molecule, with the radical center located at the boron atom of the BO radical, thus leading to doublet radical intermediates. These intermediates either decompose via hydrogen or methyl loss or isomerize prior to their decomposition via atomic hydrogen or migration of the BO moiety. A consistent trend suggests that all exit transition states are rather tight with those involved in the hydrogen atom loss depicting exit barriers of typically 25 to 35 kJ mol-1, whereas the methyl loss pathways are associated with tighter exit transition states located about 30-50 kJ mol-1 above the separated products. Further, the overall energetics suggest that those bimolecular reactions are exoergic by 40-90 kJ mol-1. These findings confirm that this reaction class leads to the formation of highly unsaturated organo boronyl molecules. [ABSTRACT FROM AUTHOR]

Following a recent investigation on the N(2D) + H2O reaction [Homayoon et al., J. Phys. Chem. Lett.5, 3508 (2014)], we report on an experimental and theoretical study of the isotopologue N(2D) + D2O reaction. Crossed molecular beam (CMB) experiments were conducted at a collision energy of 10.3 kcal mol–1. Quasiclassical trajectory calculations were performed on a recent potential energy surface to derive the centre-of-mass functions necessary to simulate the CMB laboratory distributions. Excellent agreement was found. The importance of the channel leading to HON/DON was confirmed. The inclusion of this channel, in addition to that leading to the isomer HNO/DNO, can affect the models considering the coupling between nitrogen and oxygen chemistry in the upper atmosphere of Titan. [ABSTRACT FROM AUTHOR]

We review the progress made in the understanding of the dynamics of the reactions of ground state oxygen atoms, O(3P), with unsaturated hydrocarbons (acetylene, ethylene, allene, propyne and propene) which are of great relevance, besides from a fundamental point of view, in combustion chemistry and of interest also in atmosphere- and astro-chemistry. Advances in this area have been made possible by an improved crossed molecular beams (CMBs) instrument with rotating mass spectrometric detection and time-of-flight analysis which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power. This apparatus offers the capability of identifying virtually all primary reaction channels, characterising the dynamics, and determining the branching ratios (BRs) for these polyatomic multichannel nonadiabatic reactions. The reactive scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevantab initiopotential energy surfaces (PESs). For the simpler prototypical systems, such as O(3P) + ethylene, detailed comparisons with synergic state-of-the-art quasiclassical trajectory surface-hopping calculations on full dimensionalab initiocoupled triplet and singlet PESs have recently been possible. For more complex systems, such as O(3P) + propene, comparisons with the results of synergic statistical calculations of BRs onab initiocoupled triplet and singlet PESs, have also been carried out very recently. The combined experimental/theoretical approach has allowed for a better understanding of the mechanism of these reactions. And overall has deepened considerably our understanding of chemical reactivity; in addition, these studies provide an important bridge between CMBs dynamics and thermal kinetics as well as valuable information for improving combustion and astrochemistry models. [ABSTRACT FROM PUBLISHER]

The CN + C 2 H 4 reaction has been investigated experimentally, in crossed molecular beam (CMB) experiments at the collision energy of 33.4 kJ/mol, and theoretically, by electronic structure calculations of the relevant potential energy surface and Rice–Ramsperger–Kassel–Marcus (RRKM) estimates of the product branching ratio. Differently from previous CMB experiments at lower collision energies, but similarly to a high energy study, we have some indication that a second reaction channel is open at this collision energy, the characteristics of which are consistent with the channel leading to CH 2 CHNC + H. The RRKM estimates using M06L electronic structure calculations qualitatively support the experimental observation of C 2 H 3 NC formation at this and at the higher collision energy of 42.7 kJ/mol of previous experiments. [ABSTRACT FROM AUTHOR]

Highlights: [•] The O+CH3CCH reaction is studied in crossed beam experiments with mass spectrometric detection. [•] Product distributions have been measured for the H- and CH3-displacement channels. [•] The yield of the CH3-displacement channel is thrice that of the H-displacement channel. [•] The yield is affected by the intersystem crossing to the singlet potential energy surface. [ABSTRACT FROM AUTHOR]

The reaction dynamics of the boronmonoxide radical (11BO; X2Σ+) with diacetylene(C4H2; X1Σg+) were investigated at a nominal collision energyof 17.5 kJ mol–1employing the crossed molecularbeam technique and supported by ab initioand statistical(RRKM) calculations. The reaction is governed by indirect (complexforming) scattering dynamics with the boron monoxide radical addingwith its boron atom to the carbon–carbon triple bond of thediacetylene molecule at one of the terminal carbon atoms without entrancebarrier. This leads to a doublet radical intermediate (C4H211BO), which undergoes unimolecular decompositionthrough hydrogen atom emission from the C1 carbon atom via a tightexit transition state located about 18 kJ mol–1abovethe separated products. This process forms the hitherto elusive boronyldiacetylenemolecule (HCCCC11BO; X1Σ+) in a bimolecular gas phase reaction under single collisionconditions. The overall reaction was determined to be exoergic by62 kJ mol–1. The reaction dynamics are comparedto the isoelectronic diacetylene (C4H2; X1Σg+)–cyanoradical (CN; X2Σ+) systemstudied previously in our group. The characteristics of boronyl-diacetyleneand the boronyldiacetylene molecule (HCCCC11BO; X1Σ+) as well as numerous intermediatesare reported for the first time. [ABSTRACT FROM AUTHOR]

The dynamics of the H displacement channels in the reactionN(2D) + C2H4have been investigatedbythe crossed molecular beam technique with mass spectrometric detectionand time-of-flight analysis at two different collision energies (17.2and 28.2 kJ/mol). The interpretation of the scattering results isassisted by new electronic structure calculations of stationary pointsand product energetics for the C2H4N groundstate doublet potential energy surface. RRKM statistical calculationshave been performed to derive the product branching ratio under theconditions of the present experiments and of the atmosphere of Titan.Similarities and differences with respect to a recent study performedin crossed beam experiments coupled to ionization via tunable VUVsynchrotron radiation are discussed (Lee, S.-H.; et al. Phys.Chem. Chem. Phys.2011, 13,8515–8525). Implications for the atmospheric chemistry of Titanare presented. [ABSTRACT FROM AUTHOR]

Abstract: The autoionization dynamics of water molecules induced by Ne∗(3P2,0) collisions has been studied. The energy dependence of the total ionization cross section is measured together with relative abundances of product ions in the 0.05–0.15eV range. The data are analyzed by using an optical potential model within a semiclassical approach. The real part of the potential is formulated applying a semiempirical method, while the imaginary part has been used in the fitting procedure of the data adjusting its pre-exponential factor. The good agreement between calculations and experiment confirms the attractive nature of the potential energy surface driving the Ne∗–H2O dynamics. [Copyright &y& Elsevier]

The 0(3P) + C2H4 reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet- and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Primary products from five competing channels (H + CH2CHO, H + CH3CO, H2 + CH2CO, CH3 + HCO, CH2+ CH20) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-lonization mass-spectrometric detection at a collision energy of 8.4 kcal/mol. As some of the observed products can only be formed via ISC from triplet to singlet PESs, from the product BRs the extent of ISC Is inferred. A new full-dimensional PES for the triplet state as well as spin-orbit coupling to the singlet PES are reported, and roughly half a million surface hopping trajectories are run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential crosssections. Both theory and experiment find almost equal contributions from the two PESs to the reaction, posing the question of how important is It to consider the ISC as one of the nonadiabatic effects for this and similar systems Involved in combustion chemistry. Detailed comparisons at the level of angular and translational energy distributions between theory and experiment are presented for the two primary channel products, CH3 + HCO and H + CH2CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions. [ABSTRACT FROM AUTHOR]

In our laboratory a recent series of experiments by means of the crossed molecular beam (CMB) scattering technique with mass-spectrometric detection and time-of-flight analysis has been instrumental in fostering progress in the understanding of the dynamics of both simple triatomic insertion reactions and complex polyatomic addition–elimination reactions exhibiting competing channels. In the first part of this review we survey the advances made in the comprehension of the dynamics of the insertion reactions involving excited carbon, nitrogen and oxygen atoms – C( 1 D), N( 2 D), O( 1 D) – with H 2 (D 2 ), as made possible by synergistic comparisons of experimental reactive differential cross-sections with the results of exact quantum, quasiclassical trajectory and statistical calculations on reliable ab initio potential energy surfaces. Related experimental and theoretical work from other laboratories is noted throughout. In the second part, we review the progress made in the understanding of the dynamics of polyatomic multichannel reactions, such as those of ground state oxygen and carbon atoms, O( 3 P) and C( 3 P), with the simplest alkyne, acetylene, and alkene, ethylene, as made possible by the gained capability of identifying virtually all primary reaction channels, characterising their dynamics, and determining their branching ratios. Such a capability is based on an improved crossed molecular beam instrument which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power, and variable beam crossing angle for a larger collision energy range and increased angular and velocity resolution. The scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevant ab initio potential energy surfaces. These detailed studies on polyatomic multichannel reactions provide an important bridge between crossed beam dynamics and thermal kinetics research. [ABSTRACT FROM AUTHOR]

The reaction involving atomic carbon in its first electronically excited state 1D and methane has been investigated in crossed molecular beam experiments at a collision energy of 25.3 kJ mol-1. Electronic structure calculations of the underlying potential energy surface (PES) and Rice-Ramsperger-Kassel-Marcus (RRKM) estimates of rates and branching ratios have been performed to assist the interpretation of the experimental results. The reaction proceeds via insertion of C(1D) into one of the C-H bonds of methane leading to the formation of the intermediate HCCH3 (methylcarbene or ethylidene), which either decomposes directly into the products C2H3 + H or C2H2 + H2 or isomerizes to the more stable ethylene, which in turn dissociates into C2H3 + H or H2CC + H2. The experimental results indicate that the H-displacement and H2-elimination channels are of equal importance and that for both channels the reaction mechanism is controlled by the presence of a bound intermediate, the lifetime of which is comparable to its rotational period. On the contrary, RRKM estimates predict a very short lifetime for the insertion intermediate and the dominance of the H-displacement channel. It is concluded that the reaction C(1D) + CH4 cannot be described statistically and a dynamical treatment is necessary to understand its mechanism. Possibly, nonadiabatic effects are responsible for the discrepancies, as triplet and singlet PES of methylcarbene cross each other and intersystem crossing is possible. Similarities with the photodissociation of ethylene and with the related reactions N(2D) + CH4, O(1D) + CH4 and S(1D) + CH4 are also commented on. [ABSTRACT FROM AUTHOR]

The reaction of O(3P) with C2H4, of importance in combustion and atmospheric chemistry, stands out as paradigm reaction involving not only the indicated triplet state potential energy surface (PES) but also an interleaved singlet PES that is coupled to the triplet surface. This reaction poses great challenges for theory and experiment, owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Crossed molecular beam (CMB) scattering experiments with soft electron ionization detection are used to disentangle the dynamics of this polyatomic multichannel reaction at a collision energy Ec of 8.4 kcal/mol. Five different primary products have been identified and characterized, which correspond to the five exothermic competing channels leading to H + CH2CHO, H + CH3CO, CH3 + HCO, CH2 + H2CO, and H2 + CH2CO. These experiments extend our previous CMB work at higher collision energy (Ec ∼ 13 kcal/mol) and when the results are combined with the literature branching ratios from kinetics experiments at room temperature (Ec ∼ 1 kcal/mol), permit to explore the variation of the branching ratios over a wide range of collision energies. In a synergistic fashion, full-dimensional, QCT surface hopping calculations of the O(3P) + C2H4 reaction using ab initio PESs for the singlet and triplet states and their coupling, are reported at collision energies corresponding to the CMB and the kinetics ones. Both theory and experiment find almost an equal contribution from the triplet and singlet surfaces to the reaction, as seen from the collision energy dependence of branching ratios of product channels and extent of intersystem crossing (ISC). Further detailed comparisons at the level of angular distributions and translational energy distributions are made between theory and experiment for the three primary radical channel products, H + CH2CHO, CH3 + HCO, and CH2 + H2CO. The very good agreement between theory and experiment indicates that QCT surface-hopping calculations, using reliable coupled multidimensional PESs, can yield accurate dynamical information for polyatomic multichannel reactions in which ISC plays an important role. [ABSTRACT FROM AUTHOR]

Experimental and theoretical differential cross sections for the reactions between Cl atoms and two isotopic variants of molecular hydrogen (H[sub 2] and D[sub 2]) are presented. The experimental results have been obtained by using the crossed molecular beam method with mass spectrometric detection. The theoretical results have been computed using both the quasiclassical trajectory and quantum mechanical (QM) methods. The potential energy surface employed for the calculations is the ab initio BW2 surface by Bian and Werner [J. Chem. Phys. 112, 220 (2000)]. The theoretical results have been directly compared to the experiments in the laboratory frame at a collision energy (E[sub c]) of 4.25 and 5.85 kcal/mol for the Cl+H[sub 2] reaction and of 4.9 and 6.3 kcal/mol for the Cl+D[sub 2] reaction. The agreement between QM results and experiment is quite satisfactory for the Cl+D[sub 2] reaction, especially for the low collision energy, while for Cl+H[sub 2] is less good, especially when considering data at the lower E[sub c]. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The CN+C2H4 reaction has been investigated in crossed molecular beam (CMB) experiments at a high collision energy of 42.7kJ/mol. Differently from previous CMB experiments at lower collision energies and a room temperature multiplexed time-resolved mass spectrometry experiment, we have some indication that a second reaction channel opens, the characteristics of which are consistent with the slightly endothermic channel leading to CH2CHNC+H. The increased energy content of the reactants makes it possible to overcome the exit barrier of this channel and the CN rotational excitation could possibly enhance it by smearing the preference for the C-side attack. [Copyright &y& Elsevier]