Nadia Balucani

Crossed beam studies of four‐atom reactions: The dynamics of OH+CO

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.

The dynamics of the reaction OH + D2 → HOD + D: Crossed beam experiments and quantum mechanical scattering calculations on ab initio potential energy surfaces

Abstract The dynamics of the reaction OH + D 2 → HOD + D has been investigated in a crossed molecular beam experiment at a collision energy of 6.3 kcal/mol. From laboratory product angular and velocity distribution measurements, center-of-mass product translational energy and angular distributions were determined. The HOD angular distribution is strongly backward scattered (with respect to the OH direction), which reflects a direct rebound dynamics, and the average fraction of total available energy released into translation is 0.34, indicating a high degree of product internal excitation. These results are compared with the predictions of two different ab initio potential energy surfaces within quantum mechanical scattering calculations based on the rotating bond approximation. It is concluded that a new surface, obtained from large scale high-quality electronic structure calculations, represents a significant improvement with respect to the previous Walch-Dunning-Schatz-Elgersma potential energy surface.

Crossed molecular beams and quasiclassical trajectory studies of the reaction O(1D)+H2(D2)

The dynamics of the reactions O(1D)+H2→OH+H and O(1D)+D2→OD+D have been investigated in crossed molecular beam experiments with mass spectrometric detection at the collision energies of 1.9 and 3.0 kcal/mol, and 5.3 kcal/mol, respectively. From OH(OD) product laboratory angular and velocity distribution measurements, center-of-mass product translational energy and angular distributions were derived. The angular distributions are nearly backward–forward symmetric with a favored backward peaking which increases with collision energy. About 30% of the total available energy is found to be channeled into product translational energy. The results are compared with quasiclassical trajectory calculations on a DIM (diatomic-in-molecules) potential energy surface. Related experimental and theoretical works are noted. Insertion via the 1 1A′ ground state potential energy surface is the predominant mechanism, but the role of a second competitive abstraction micromechanism which should evolve on one of (or both) the ...

Dynamics of the Simplest Chlorine Atom Reaction: An Experimental and Theoretical Study

Angular distributions and time-of-flight spectra for the reaction Cl + H2 → HCl + H obtained from a high-resolution, crossed-molecular beam experiment were compared to differential cross sections calculated by both converged quantum mechanical scattering and quasi-classical trajectory methods. Good agreement was found between the experimental results and each theoretical prediction. The results demonstrate that excellent agreement can be obtained between state-of-the-art simulations and experiments for the detailed dynamical properties of this prototype chlorine atom reaction.

Crossed molecular beam reactive scattering: from simple triatomic to multichannel polyatomic reactions

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(1D), N(2D), O(1D) – with H2(D2), 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(3P) and C(3P), 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. Contents PAGE 1. Introduction 110 2. Experimental: improved crossed molecular beams apparatus 113  2.1. Soft electron-ionisation detection 114  2.2. Product angular and velocity distributions 116  2.3. Crossed molecular beam experiments with variable beam crossing angle 118  2.4. Supersonic sources of radical beams 120 3. Triatomic insertion reactions 120  3.1. Reaction C(1D) + H2 122  3.2. Reaction N(2D) + H2 126  3.3. Reaction O(1D) + H2 129 4. Polyatomic multichannel reactions 133  4.1. Reaction O(3P) + C2H2 134   4.1.1. Product angular and TOF distributions 135   4.1.2. Determination of branching ratios 138   4.1.3. Determination of product ionisation energies 138  4.2. Reaction C(3P) + C2H2 138   4.2.1. H and H2 elimination channels, and branching ratios 140   4.2.2. Ionisation energy of c/l-C3H radicals 145  4.3. Reaction O(3P) + C2H4 145   4.3.1. Observation of all product channels and branching ratios 146  4.4. Reaction C(3P) + C2H4 151   4.4.1. H-elimination channels 151   4.4.2. C–C bond fission channels 154 5. Summary and outlook 154 Acknowledgements 157 References 158

Comparative dynamics of Cl(2P) and O(3P) interactions with a hydrocarbon surface

The dynamics of the interactions of atomic chlorine with the surface of a saturated hydrocarbon liquid, squalane, were investigated and compared to the results of an earlier study on analogous oxygen-atom interactions. Beams of continuous supersonic chlorine atoms were directed onto a squalane surface, and the volatile products, Cl and HCl, were observed by mass spectrometry as a function of incident angle, final angle, and incident Cl-atom energy. Both the Cl and HCl time-of-flight (from the surface to the detector) distributions revealed thermal and hyperthermal interaction channels, in analogy to the dynamical behavior of the O and OH signals observed in the previous study. The thermal HCl product may arise from two mechanisms: (i) desorption of trapped HCl product and (ii) reaction of trapped Cl atoms to form thermal HCl, which subsequently desorbs. In contrast, the reaction of O atoms with squalane led to a thermal OH signal, which could only come from desorption of trapped OH. The hyperthermal HCl s...

Dynamics of the reaction O(1D) + HCl → ClO + H from crossed-beam experiments

Abstract The angular velocity distribution of the ClO product from the reaction O( 1 D) + HCl at 12.2 kcal/mol collision energy has been obtained in a crossed-molecular-beam study. The product center-of-mass angular distribution is found to be almost backward—forward symmetric, with backward scattering being slightly favored, from which it is deduced that part of the reaction occurs via a long-lived complex and part via direct abstraction of the halogen atom. The derived large fraction (≈ 43%) of energy released into translation indicates the existence of a barrier in the exit channel. A lower limit of the branching ratio between ClO + H and OH + Cl channels is derived and is compared to recent bulk results.

Formation of complex organic molecules in cold objects: the role of gas-phase reactions

While astrochemical models are successful in reproducing many of the observed interstellar species, they have been struggling to explain the observed abundances of complex organic molecules. Current models tend to privilege grain surface over gas phase chemistry in their formation. One key assumption of those models is that radicals trapped in the grain mantles gain mobility and react on lukewarm (>30 K) dust grains. Thus, the recent detections of methyl formate (MF) and dimethyl ether (DME) in cold objects represent a challenge and may clarify the respective role of grain surface and gas phase chemistry. We propose here a new model to form DME and MF with gas phase reactions in cold environments, where DME is the precursor of MF via an efficient reaction overlooked by previous models. Furthermore, methoxy, a precursor of DME, is also synthetized in the gas phase from methanol, which is desorbed by a non-thermal process from the ices. Our new model reproduces fairy well the observations towards L1544. It also explains, in a natural way, the observed correlation between DME and MF. We conclude that gas phase reactions are major actors in the formation of MF, DME and methoxy in cold gas. This challenges the exclusive role of grain-surface chemistry and favours a combined grain-gas chemistry.

Exploring the reaction dynamics of nitrogen atoms: A combined crossed beam and theoretical study of N(2D)+D2→ND+D

In the first successful reactive scattering study of nitrogen atoms, the angular and velocity distribution of the ND product from the reaction N(2D)+D2 at 5.1 and 3.8 kcal/mol collision energies has been obtained in a crossed molecular beam study with mass spectrometric detection. The center-of-mass product angular distribution is found to be nearly backward–forward symmetric, reflecting an insertion dynamics. About 30% of the total available energy goes into product translation. The experimental results were compared with those of quasiclassical trajectory calculations on an accurate potential energy surface obtained from large scale ab initio electronic structure computations. Good agreement was found between the experimental results and the theoretical predictions.

Crossed beam reaction of cyano radicals with hydrocarbon molecules. IV. Chemical dynamics of cyanoacetylene (HCCCN; X 1Σ+) formation from reaction of CN(X 2Σ+) with acetylene, C2H2(X 1Σg+)

The chemical reaction dynamics to form cyanoacetylene, HCCCN (X 1Σ+), via the radical–neutral reaction of cyano radicals, CN(X 2Σ+;ν=0), with acetylene, C2H2(X 1Σg+), are unraveled in crossed molecular beam experiments at two collision energies of 21.1 and 27.0 kJ mol−1. Laboratory angular distributions and time-of-flight spectra of the HCCCN product are recorded at m/e=51 and 50. Experiments were supplemented by electronic structure calculations on the doublet C3H2N potential energy surface and RRKM investigations. Forward-convolution fitting of the crossed beam data combined with our theoretical investigations shows that the reaction has no entrance barrier and is initiated by an attack of the CN radical to the π electron density of the acetylene molecule to form a doublet cis/trans HCCHCN collision complex on the 2A′ surface via indirect reactive scattering dynamics. Here 85% of the collision complexes undergo C–H bond rupture through a tight transition state located 22 kJ mol−1 above the cyanoacetylen...