Crossed-beam studies on the dynamics of the C + C2H2 interstellar reaction leading to linear and cyclic C3H + H and C3 + H2

A crossed molecular beams and computational study on the unusual reactivity of banana bonds of cyclopropane (c-C3H6; ) through insertion by ground state carbon atoms (C(3Pj))

The mechanism and chemical dynamics of the reaction of ground electronic state atomic carbon C(³P_j) with cyclopropane c-C₃H₆ have been explored by combining crossed molecular beams experiments with electronic structure calculations of the pertinent triplet C₄H₆ potential energy surface and statistical computations of product branching ratios under single-collision conditions. The experimental findings suggest that the reaction proceeds via indirect scattering dynamics through triplet C₄H₆ reaction intermediate(s) leading to C₄H₅ product(s) plus atomic hydrogen via a tight exit transition state, with the overall reaction exoergicity evaluated as 231 ± 52 kJ mol^-1. The calculations indicate that C(³P_j) can easily insert into one of the three equivalent C-C 'banana' bonds of cyclopropane overcoming a low barrier of only 2 kJ mol^-1 following the formation of a van der Waals reactant complex stabilized by 15 kJ mol^-1. The carbon atom insertion into one of the six C-H bonds is also feasible via a slightly higher barrier of 5 kJ mol^-1. These results highlight an unusual reactivity of cyclopropane's banana C-C bonds, which behave more like unsaturated C-C bonds with a π-character than saturated σ C-C bonds, which are known to be generally unreactive toward the ground electronic state atomic carbon such as in ethane (C₂H₆). The statistical theory predicts the overall product branching ratios at the experimental collision energy as 50% for 1-butyn-4-yl, 33% for 1,3-butadien-2-yl, i-C₄H₅, and 11% for 1,3-butadien-1-yl, n-C₄H₅, with i-C₄H₅ (230 kJ mol^-1 below the reactants) favored by the C-C insertion providing the best match with the experimentally observed reaction exoergicity. The C(³P_j) + c-C₃H₆ reaction is predicted to be a source of C₄H₅ radicals under the conditions where its low entrance barriers can be overcome, such as in planetary atmospheres or in circumstellar envelopes but not in cold molecular clouds. Both i- and n-C₄H₅ can further react with acetylene eventually producing the first aromatic ring and hence, the reaction of the atomic carbon with c-C₃H₆ can be considered as an initial step toward the formation of benzene.

Reaction Dynamics Study of the Molecular Hydrogen Loss Channel in the Elementary Reactions of Ground-State Silicon Atoms (Si(3P)) With 1- and 2-Methyl-1,3-Butadiene (C5H8)

The bimolecular gas-phase reactions involving ground-state atomic silicon (Si; ³P) and 1- and 2-methyl-1,3-butadiene were studied via crossed molecular beam experiments. Our data revealed indirect scattering dynamics through long-lived SiC₅H₈ collision complex(es) along with molecular hydrogen loss pathways, leading to facile formation of SiC₅H₆ isomer(s). We propose that the reactions of silicon with 1- and 2-methyl-1,3-butadiene possess reaction dynamics in an analogy to the silicon-1,3-butadiene system. This leads to cyclic methyl-substituted 2-methylene-1-silacyclobutene isomers via nonadiabatic reaction dynamics through intersystem crossing (ISC) from the triplet to the singlet surface in overall exoergic reactions through tight exit transition states and molecular hydrogen loss. Our study also suggests that the methyl group-although a spectator from the chemical viewpoint-can influence the disposal of the angular momentum into the rotational excitation of the final product.

Valence shell threshold photoelectron spectroscopy of C3Hx (x = 0–3)

We present the photoelectron spectra of C₃H_x (x = 0–3) formed in a microwave discharge flow-tube reactor by consecutive H abstractions from C₃H₄ (C₃H_x + F → C₃H_x−1 + HF (x = 1–4)), but also from F + CH₄ schemes by secondary reactions.

On the formation of resonantly stabilized C5H3 radicals--a crossed beam and ab initio study of the reaction of ground state carbon atoms with vinylacetylene

The formation of polycyclic aromatic hydrocarbons in combustion environments is linked to resonance stabilized free radicals. Here, we investigated the reaction dynamics of ground state carbon atoms, C((3)P(j)), with vinylacetylene at two collision energies of 18.8 kJ mol(-1) and 26.4 kJ mol(-1) employing the crossed molecular beam technique leading to two resonantly stabilized free radicals. The reaction was found to be governed by indirect scattering dynamics and to proceed without an entrance barrier through a long-lived collision complex to reach the products, n- and i-C(5)H(3) isomers via tight exit transition states. The reaction pathway taken is dependent on whether the carbon atom attacks the π electron density of the double or triple bond, both routes have been compared to the reactions of atomic carbon with ethylene and acetylene. Electronic structure/statistical theory calculations determined the product branching ratio to be 2:3 between the n- and i-C(5)H(3) isomers.

Formation of nitriles and imines in the atmosphere of Titan: combined crossed-beam and theoretical studies on the reaction dynamics of excited nitrogen atoms N(2D) with ethane

The dynamics of the H-displacement channels in the reaction N(2D) + C2H6 have been investigated by the crossed molecular beam technique with mass spectrometric detection and time-of-flight analysis at two different collision energies (18.0 and 31.4 kJ mol(-1)). From the derived center-of-mass product angular and translational energy distributions the reaction micromechanisms and the product energy partitioning have been obtained. The interpretation of the scattering results is assisted by new ab initio electronic structure calculations of stationary points and product energetics for the C2H6N ground state doublet potential energy surface. C-C bond breaking and NH production channels have been theoretically characterized and the statistical branching ratio derived at the temperatures relevant for the atmosphere of Titan. Methanimine plus CH3 and ethanimine plus H are the main reaction channels. Implications for the atmospheric chemistry of Titan are discussed.

Crossed-beam and theoretical studies of the S(1D) + C2H2 reaction

The reaction dynamics of excited sulfur atoms, S((1)D), with acetylene has been investigated by the crossed-beam scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis at the collision energy of 35.6 kJ mol(-1). These studies have been made possible by the development of intense continuous supersonic beams of S((3)P,(1)D) atoms. From product angular and TOF distributions, center-of-mass product angular and translational energy distributions are derived. The S((1)D) + C(2)H(2) reaction is found to lead to formation of HCCS (thioketenyl) + H, while the only other energetically allowed channels, those leading to CCS((3)Sigma(-), (1)Delta) + H(2), are not observed to occur to an appreciable extent. The dynamics of the H-elimination channel is discussed and elucidated. The interpretation of the scattering results is assisted by synergic high-level ab initio electronic structure calculations of stationary points and product energetics for the C(2)H(2)S ground-state singlet potential energy surface. In addition, by exploiting the novel capability of performing product detection by means of a tunable electron-impact ionizer, we have obtained the first experimental information on the ionization energy of thioketenyl radical, HCCS, as synthesized in the reactive scattering experiment. This has been complemented by ab initio calculations of the adiabatic and vertical ionization energies for the ground-state radical. The theoretically derived value of 9.1 eV confirms very recent, accurate calculations and is corroborated by the experimentally determined ionization threshold of 8.9 +/- 0.3 eV for the internally warm HCCS produced from the title reaction.

Elementary reactions and their role in gas-phase prebiotic chemistry

The formation of complex organic molecules in a reactor filled with gaseous mixtures possibly reproducing the primitive terrestrial atmosphere and ocean demonstrated more than 50 years ago that inorganic synthesis of prebiotic molecules is possible, provided that some form of energy is provided to the system. After that groundbreaking experiment, gas-phase prebiotic molecules have been observed in a wide variety of extraterrestrial objects (including interstellar clouds, comets and planetary atmospheres) where the physical conditions vary widely. A thorough characterization of the chemical evolution of those objects relies on a multi-disciplinary approach: 1) observations allow us to identify the molecules and their number densities as they are nowadays; 2) the chemistry which lies behind their formation starting from atoms and simple molecules is accounted for by complex reaction networks; 3) for a realistic modeling of such networks, a number of experimental parameters are needed and, therefore, the relevant molecular processes should be fully characterized in laboratory experiments. A survey of the available literature reveals, however, that much information is still lacking if it is true that only a small percentage of the elementary reactions considered in the models have been characterized in laboratory experiments. New experimental approaches to characterize the relevant elementary reactions in laboratory are presented and the implications of the results are discussed.

Probing the dynamics of polyatomic multichannel elementary reactions by crossed molecular beam experiments with soft electron-ionization mass spectrometric detection

In this Perspective we highlight developments in the field of chemical reaction dynamics. Focus is on the advances recently made in the investigation of the dynamics of elementary multichannel radical-molecule and radical-radical reactions, as they have become possible using an improved crossed molecular beam scattering apparatus with universal electron-ionization mass spectrometric detection and time-of-flight analysis. These improvements consist in the implementation of (a) soft ionization detection by tunable low-energy electrons which has permitted us to reduce interfering signals originating from dissociative ionization processes, usually representing a major complication, (b) different beam crossing-angle set-ups which have permitted us to extend the range of collision energies over which a reaction can be studied, from very low (a few kJ mol(-1), as of interest in astrochemistry or planetary atmospheric chemistry) to quite high energies (several tens of kJ mol(-1), as of interest in high temperature combustion systems), and (c) continuous supersonic sources for producing a wide variety of atomic and molecular radical reactant beams. Exploiting these new features it has become possible to tackle the dynamics of a variety of polyatomic multichannel reactions, such as those occurring in many environments ranging from combustion and plasmas to terrestrial/planetary atmospheres and interstellar clouds. By measuring product angular and velocity distributions, after having suppressed or mitigated, when needed, the problem of dissociative ionization of interfering species (reactants, products, background gases) by soft ionization detection, essentially all primary reaction products can be identified, the dynamics of each reaction channel characterized, and the branching ratios determined as a function of collision energy. In general this information, besides being of fundamental relevance, is required for a predictive description of the chemistry of these environments via computer models. Examples are taken from recent on-going work (partly published) on the reactions of atomic oxygen with acetylene, ethylene and allyl radical, of great importance in combustion. A reaction of relevance in interstellar chemistry, as that of atomic carbon with acetylene, is also discussed briefly. Comparison with theoretical results is made wherever possible, both at the level of electronic structure calculations of the potential energy surfaces and dynamical computations. Recent complementary CMB work as well as kinetic work exploiting soft photo-ionization with synchrotron radiation are noted. The examples illustrated in this article demonstrate that the type of dynamical results now obtainable on polyatomic multichannel radical-molecule and radical-radical reactions might well complement reaction kinetics experiments and hence contribute to bridging the gap between microscopic reaction dynamics and thermal reaction kinetics, enhancing significantly our basic knowledge of chemical reactivity and understanding of the elementary reactions which occur in real-world environments.

Unraveling the dynamics of the C(3P,1D) + C2H2 reactions by the crossed molecular beam scattering technique

A detailed investigation of the dynamics of the reactions of ground- and excited-state carbon atoms, C(3P) and C(1D), with acetylene is reported over a wide collision energy range (3.6-49.1 kJ mol-1) using the crossed molecular beam (CMB) scattering technique with electron ionization mass spectrometric detection and time-of-flight (TOF) analysis. We have exploited the capability of (a) generating continuous intense supersonic beams of C(3P, 1D), (b) crossing the two reactant beams at different intersection angles (45, 90, and 135 degrees ) to attain a wide range of collision energies, and (c) tuning the energy of the ionizing electrons to low values (soft ionization) to suppress interferences from dissociative ionization processes. From angular and TOF distribution measurements of products at m/z=37 and 36, the primary reaction products of the C(3P) and C(1D) reactions with C2H2 have been identified to be cyclic (c)-C3H + H, linear (l)-C3H + H, and C3 + H2. From the data analysis, product angular and translational energy distributions in the center-of-mass (CM) system for both the linear and cyclic C3H isomers as well as the C3 product from C(3P) and for l/c-C3H and C3 from C(1D) have been derived as a function of collision energy from 3.6 to 49.1 kJ mol-1. The cyclic/linear C3H ratio and the C3/(C3 + c/l-C3H) branching ratios for the C(3P) reaction have been determined as a function of collision energy. The present findings have been compared with those from previous CMB studies using pulsed beams; here, a marked contrast is noted in the CM angular distributions for both C3H- and C3-forming channels from C(3P) and their trend with collision energy. Consequently, the interpretation of the reaction dynamics derived in the present work contradicts that previously proposed from the pulsed CMB studies. The results have been discussed in the light of the available theoretical information on the relevant triplet and singlet C3H2 ab initio potential energy surfaces (PESs). In particular, the branching ratios for the C(3P) + C2H2 reaction have been compared with the available theoretical predictions (approximate quantum scattering calculations and quasiclassical trajectory calculations on ab initio triplet PESs and, very recent, statistical calculations on ab initio triplet PESs as well as on ab initio triplet/singlet PESs including nonadiabatic effects, that is, intersystem crossing). While the experimental branching ratios have been corroborated by the statistical predictions, strong disagreement has been found with the results of the dynamical calculations. The astrophysical implications of the present results have been noted.

Mean potential phase space theory of chemical reactions

A nonconventional application of phase space theory to the insertion reactions A+H(2), with A=C((1)D) and S((1)D), is presented. Instead of approximating the potential energies of interaction between separated fragments by their isotropic long-range contributions, as in the original theory, the latter are replaced by the accurate potential energies averaged with respect to Jacobi angles. The integral and differential cross sections obtained from this mean potential phase space theory (MPPST) turn out to be in very satisfying agreement with the benchmark predictions of the time-independent and time-dependent statistical quantum methods. The formal and numerical simplicity of MPPST with respect to any approach combining statistical assumptions and dynamical calculations makes it a promising tool for studying indirect polyatomic reactions.

Prediction of product branching ratios in the C(P3)+C2H2→l-C3H+H∕c-C3H+H∕C3+H2 reaction using ab initio coupled clusters calculations extrapolated to the complete basis set combined with Rice-Ramsperger-Kassel-Marcus and radiationless transition theories

Ab initio CCSD(T) calculations of intermediates and transition states on the singlet and triplet C3H2 potential energy surfaces extrapolated to the complete basis set limit are combined with statistical computations of energy-dependent rate constants of the C(3P)+C2H2 reaction under crossed molecular beam conditions. Rice-Ramsperger-Kassel-Marcus theory is applied for isomerization and dissociation steps within the same multiplicity and radiationless transition and nonadiabatic transition state theories are used for singlet-triplet intersystem crossing rates. The calculated rate constants are utilized to predict product branching ratios. The results demonstrate that, in qualitative agreement with available experimental data, c-C3H+H and C3+H2 are the most probable products at low collision energies, whereas l-C3H+H becomes dominant at higher Ec above approximately 25 kJ/mol.

Investigating the Chemical Dynamics of the Reaction of Ground-State Carbon Atoms with Acetylene and Its Isotopomers

We investigated the multichannel reaction of ground-state carbon atoms with acetylene, C2H2 (X1Sigmag+), to form the linear and cyclic C3H isomers (atomic hydrogen elimination pathway) as well as tricarbon plus molecular hydrogen. The experiments were conducted under single-collision conditions at three different collision energies between 8.0 kJ mol-1 and 31.0 kJ mol-1. Our studies were complemented by crossed molecular beam experiments of carbon with three isotopomers C2D2(X1Sigmag+), C2HD (X1Sigma+), and 13C2H2 (X1Sigmag+) to clarify a potential intersystem crossing (ISC), the effect of the symmetry of the reaction intermediates on the center-of-mass angular distributions, the collision energy-dependent branching ratios of the atomic versus molecular hydrogen elimination pathways, and deuterium-enrichment processes. The results are discussed in light of recent electronic structure and dynamics calculations.

Classical treatment of molecular collisions: striking improvement of the description of recoil energy distributions using Gaussian weighted trajectories

The Gaussian weighting (GW) procedure, recently used in the classical treatment of molecular collisions, is a practical way of taking into account quantization of product vibrational actions. The goal of this brief communication is to show that the GW procedure may drastically improve the predictions of the recoil energy distribution between final fragments, an observable frequently measured in molecular beam experiments.

On the theory of complex-forming chemical reactions: effect of parity conservation on the polarization of differential cross sections

For complex-forming triatomic reactions such as the prototypical insertion reactions intensively studied in the last few years, quantum mechanical differential cross sections (DCS) present sharp forward/backward polarization peaks when the reagent rotational angular momentum quantum number j is zero. Moreover, the size of the peaks decreases rapidly with increasing j values so that for j = 3, they are no longer visible. In contrast, the polarization peaks are always missing in the classical mechanical DCSs. Apart from the peaks, however, the quantum and classical DCSs are usually in good agreement. In a recent rapid communication, we showed that the fundamental reason for the previous differences in the quantum and classical scenarios is that parity conservation leads in quantum mechanics to an angular momentum constraint without equivalent in classical mechanics. We also proposed a parity-restoring approximation leading to an accurate semi-classical description of the peaks. While only the main lines of the demonstration were given in the communication, we report here the whole developments. We also analyse why the peaks disappear when the reagent diatom is rotationally excited. As a by-product of the previous developments, we finally discuss the possibility of a general statistico-dynamical semiclassical approach.

Crossed beam studies of radical–radical reactions: O(3P) + C3H5(allyl)

The dynamics of the radical-radical reaction O((3)P) + C(3)H(5) has been investigated by means of the crossed molecular beam technique with mass spectrometric detection at a collision energy of 73.0 kJ mol(-1); the reaction mechanism of the H-displacement channel has been elucidated, while experimental evidence of the occurrence of one or more C-C bond-breaking channels at this collision energy has been obtained.