Combined crossed beam and theoretical studies of the N(2D) + C2H4 reaction and implications for atmospheric models of Titan

The dynamics of the H displacement channels in the reaction N((2)D) + C(2)H(4) have been investigated by the crossed molecular beam technique with mass spectrometric detection and time-of-flight analysis at two different collision energies (17.2 and 28.2 kJ/mol). The interpretation of the scattering results is assisted by new electronic structure calculations of stationary points and product energetics for the C(2)H(4)N ground state doublet potential energy surface. RRKM statistical calculations have been performed to derive the product branching ratio under the conditions of the present experiments and of the atmosphere of Titan. Similarities and differences with respect to a recent study performed in crossed beam experiments coupled to ionization via tunable VUV synchrotron radiation are discussed (Lee, S.-H.; et al. Phys. Chem. Chem. Phys.2011, 13, 8515-8525). Implications for the atmospheric chemistry of Titan are presented.

Intersystem crossing and dynamics in O(3P) + C2H4 multichannel reaction: experiment validates theory

The O((3)P) + C(2)H(4) 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 + CH(2)CHO, H + CH(3)CO, H(2) + CH(2)CO, CH(3) + HCO, CH(2) + CH(2)O) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-ionization 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 cross-sections. 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, CH(3) + HCO and H + CH(2)CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.

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.

Combined crossed molecular beam and theoretical studies of the N(2D) + CH4 reaction and implications for atmospheric models of Titan

The dynamics of the H-displacement channel in the reaction N((2)D) + CH(4) has been investigated by the crossed molecular beam (CMB) technique with mass spectrometric detection and time-of-flight (TOF) analysis at five different collision energies (from 22.2 up to 65.1 kJ/mol). The CMB results have identified two distinct isomers as primary reaction products, methanimine and methylnitrene, the yield of which significantly varies with the total available energy. From the derived center-of-mass product angular and translational energy distributions the reaction micromechanisms, the product energy partitioning and the relative branching ratios of the competing reaction channels leading to the two isomers 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 CH(4)N ground state doublet potential energy surface. Differently from previous theoretical studies, both insertion and H-abstraction pathways have been found to be barrierless at all levels of theory employed in this work. A comparison between experimental results on the two isomer branching ratio and RRKM estimates, based on the new electronic structure calculations, confirms the highly nonstatistical nature of the N((2)D) + CH(4) reaction, with the production of the CH(3)N isomer dominated by dynamical effects. The implications for the chemical models of the atmosphere 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.

Observation of organosulfur products (thiovinoxy, thioketene and thioformyl) in crossed-beam experiments and low temperature rate coefficients for the reaction S(1D) + C2H4

The reaction between excited sulfur atoms, S((1)D), and the simplest alkene molecule, ethene, has been investigated in a complementary fashion in (a) crossed-beam dynamic experiments with mass spectrometric detection and time-of-flight (TOF) analysis at a collision energy of 37.0 kJ mol(-1), (b) low temperature kinetic experiments ranging from room temperature down to 23 K, and (c) electronic structure calculations of stationary points and product energetics on the C(2)H(4)S singlet potential energy surface. The rate coefficients for total loss of S((1)D) are found to be very large (ca. 4 x 10(-10) cm(3) molecule(-1) s(-1)) down to very low temperature indicating that the overall reaction is barrier-less. From laboratory angular and TOF distributions at different product masses, three competing reaction channels leading to H + CH(2)CHS (thiovinoxy), H(2) + CH(2)CS (thioketene), and CH(3) + HCS (thioformyl) have been unambiguously identified and their dynamics characterized. Branching ratios have also been estimated. These studies, which exploit the capability of producing intense supersonic beams of sulfur S((3)P,(1)D) atoms and measuring rate coefficients down to very low temperature, offer considerable promise for further dynamical investigations of other sulfur atom reactions of particular relevance to combustion and atmospheric chemistry.

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.

Gas-phase reactions in extraterrestrial environments: laboratory investigations by crossed molecular beams

We have investigated gas-phase reactions of N((2)D) with the most abundant hydrocarbons in the atmosphere of Titan by the crossed molecular beam technique. In all cases, molecular products containing a novel CN bond are formed, thus suggesting possible routes of formation of gas-phase nitriles in the atmosphere of Titan and primordial Earth. The same approach has been recently extended to the study of radical-radical reactions, such as the reaction of atomic oxygen with the CH(3) and C(3)H(5) radicals. Products other than those already considered in the modeling of planetary atmospheres and interstellar medium have been identified.

Formation of the 2,4-pentadiynyl-1 radical (H2CCCCCH, X2B1) in the crossed beams reaction of dicarbon molecules with methylacetylene

The chemical dynamics to synthesize the 2,4-pentadiynyl-1 radical, HCCCCCH(2)(X(2)B(1)), via the neutral-neutral reaction of dicarbon with methylacetylene, was examined in a crossed molecular beams experiment at a collision energy of 37.6 kJ mol(-1). The laboratory angular distribution and time-of-flight spectra of the 2,4-pentadiynyl-1 radical and its fragmentation patterns were recorded at m/z = 63-60 and m/z = 51-48. Our findings suggest that the reaction dynamics are indirect and dictated by an initial attack of the dicarbon molecule to the pi electron density of the methylacetylene molecule to form cyclic collision complexes. The latter ultimately rearranged via ring opening to methyldiacetylene, CH(3)-C triple bond C-C triple bond C-H. This structure decomposed via atomic hydrogen emission to the 2,4-pentadiynyl-1 radical; here, the hydrogen atom was found to be emitted almost parallel to the total angular momentum as suggested by the experimentally observed sideways scattering. The overall reaction was strongly exoergic by 182 +/- 10 kJ mol(-1). The identification of the resonance-stabilized free 2,4-pentadiynyl-1 radical represents a solid background for the title reaction to be included into more refined reaction networks modeling the chemistry of circumstellar envelopes and also of sooting combustion flames.

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

The dynamics of the C + C2H2 reaction has been investigated using two crossed molecular beam apparatus of different concepts. Differential cross sections have been obtained for the C(3PJ) + C2H2(X1sigmag+) --> l/c-C3H + H(2S1/2) reaction in experiments conducted with pulsed supersonic beams and variable beam crossing angle configuration at two relative translational energies ET = 0.80 and 3.5 kJ mol(-1). H(2S1/2) atoms were detected by time-of-flight mass spectrometry with sequential excitation to the 2PJ(o) state using a laser beam tuned at the Lyman-alpha transition around 121.567 nm and ionisation by a second laser beam at 364.7 nm. Doppler-Fizeau spectra of the H atoms were recorded with the Lyman-alpha laser beam parallel to the relative velocity vector of the reagents. These spectra could be fitted using a forward convolution process including two contributions. The recoil energy distribution functions of both contributions were taken as statistical, with total energies corresponding to a reaction exoergicity deltaH0(o) = -11 kJ mol(-1) for the major one, assigned to the c-C3H + H path, and -1.5 kJ mol(-1) for the minor one, assigned to the l-C3H + H path. The angular distribution was taken as also statistical (uniform) for the minor contribution but somewhat backward peaked for the major one. Differential cross sections have been obtained for the three energetically allowed and competitive C(3PJ) + C2H2(X1sigmag+) --> l/c-C3H + H(2S1/2) and C(3PJ) + C2H2(X1sigmag+) --> C3(X1sigmag+) + H2(X1sigmag+) reaction channels in experiments conducted with supersonic continuous beams under 45 degrees crossing angle configuration using "soft" electron-ionisation mass spectrometry time-of-flight detection at ET = 3.5 and 18.5 kJ mol(-1). From measurements of angular and time-of-flight distributions at the mass-to-charge ratios m/z = 37 and 36, product angular and translational energy distributions have been determined in the centre-of-mass system for both linear- and cyclic-C3H isomer formation as well as for C3 production. The variations of the dynamics and product branching ratios with collision energy have been characterized. The ratios c-C3H/l-C3H and C3/C3H from the C(3P) reactions have been both found to decrease with increasing ET. Formation of C3(X1sigmag+) from the C(3P) reaction has been rationalized in terms of intersystem crossing between triplet and singlet C3H2 potential energy surfaces. There is good agreement between the results at ET = 3.5 kJ mol(-1) obtained with the two different crossed molecular beam techniques for the C(3PJ) + C2H2(X1sigmag+) --> l/c-C3H + H(2S1/2) channels. An estimate of the exoergicity of the C(3PJ) + C2H2(X1sigmag+) --> c-C3H + H (2S1/2) pathway from the extent of the translational energy release corroborates the value of deltaH0(o) = -11 kJ mol(-1) obtained from the Doppler-Fizeau measurements. The overall results have been discussed in the light of the available theoretical information on the relevant triplet and singlet C3H2 potential energy surfaces, and compared with the results of previous related kinetic and dynamic work as well as of theoretical calculations of the reaction dynamics.

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.

Experimental and Theoretical Differential Cross Sections for the N(2D) + H2 Reaction

In this paper, we report a combined experimental and theoretical study on the dynamics of the N(2D) + H2 insertion reaction at a collision energy of 15.9 kJ mol(-1). Product angular and velocity distributions have been obtained in crossed beam experiments and simulated by using the results of quantum mechanical (QM) scattering calculations on the accurate ab initio potential energy surface (PES) of Pederson et al. (J. Chem. Phys. 1999, 110, 9091). Since the QM calculations indicate that there is a significant coupling between the product angular and translational energy distributions, such a coupling has been explicitly included in the simulation of the experimental results. The very good agreement between experiment and QM calculations sustains the accuracy of the NH2 ab initio ground state PES. We also take the opportunity to compare the accurate QM differential cross sections with those obtained by two approximate methods, namely, the widely used quasiclassical trajectory calculations and a rigorous statistical method based on the coupled-channel theory.

Dynamics of the C(D1)+D2 reaction: A comparison of crossed molecular-beam experiments with quasiclassical trajectory and accurate statistical calculations

In this paper we report a combined experimental and theoretical study on the dynamics of the insertion reaction C((1)D)+D(2) 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.

Dynamics of the O(3P) + C2H4Reaction: Identification of Five Primary Product Channels (Vinoxy, Acetyl, Methyl, Methylene, and Ketene) and Branching Ratios by the Crossed Molecular Beam Technique with Soft Electron Ionization

The crossed molecular beam scattering technique with soft electron ionization (EI) is used to disentangle the complex dynamics of the polyatomic O(3P) + C2H4 reaction, which is of great relevance in combustion and atmospheric chemistry. Exploiting the newly developed capability of attaining universal product detection by using soft EI, at a collision energy of 54.0 kJ mol(-1), five different primary products have been identified, which correspond to the five exoergic competing channels leading to CH2CHO(vinoxy) + H, CH3CO(acetyl) + H, CH3(methyl) + HCO(formyl), CH2(methylene) + HCHO(formaldehyde), and CH2CO(ketene) + H2. From laboratory product angular and velocity distributions, center-of-mass product angular and translational energy distributions and the relative branching ratios for each channel have been obtained, affording an unprecedented characterization of this important reaction.

Crossed beam studies of the reactions of atomic oxygen in the ground 3P and first electronically excited 1D states with hydrogen sulfide

The reactions of both ground, (3)P, and electronically excited, (1)D, oxygen atoms with hydrogen sulfide, H(2)S, have been investigated by means of the crossed molecular beams method with mass spectrometric detection at different collision energies. Amongst the possible reaction channels those leading to HSO+H for the O((3)P) reaction and to HSO/HOS+H and SO+H(2) for the O((1)D) reaction have been identified and investigated. The dynamics of the channels leading to HSO/HOS+H are elucidated for the reactions of both states and the trend with increasing the collision energy analyzed. Noteworthily, the formation of SO+H(2) products appears to be an open channel for the O((1)D) reaction, at least for the highest collision energy investigated (11.8 kcal/mol). Finally, the recent experimental and theoretical estimates of the enthalpy of formation of the HSO radical have been critically analyzed to evaluate their conformity with the present experimental data.

Differential cross sections from quantum calculations on coupled Ab initio potential energy surfaces and scattering experiments for Cl(2P)+H2 reactions

To assess the relative reactivity of the spin-orbit excited state of atomic Cl with molecular hydrogen, we have measured differential cross sections using an atomic Cl beam with a known concentration of the ground and excited spin-orbit states. These are compared with the first determination of the cross sections from quantum mechanical scattering calculations on a set of coupled ab initio potential energy surfaces. The comparison suggests that these surfaces may underestimate the degree of rotational excitation of the HCl products and that the excited spin-orbit state plays a minor role in the reaction.

Quantum effects in the differential cross Ssctions for the insertion reaction N(2D) + H2

The quantum (QM) scattering theory has been difficult to apply to the family of insertion reactions and the approximate quasiclassical trajectory (QCT) method or statistical calculations were mostly applied. In this Letter, we compare the experimental differential cross sections for the title insertion reaction with the results of QM and QCT calculations on an ab initio potential energy surface. The QM results reproduce well the crossed beam experiment, while a small, but significant, difference in the QCT ones points to quantum effects, possibly the occurrence of tunneling through the combined potential and centrifugal barrier.

Crossed beam studies of elementary reactions of N and C atoms and CN radicals of importance in combustion

The dynamics of some elementary reactions of N(2D), C(3P,1D) and CN(X2 sigma +) of importance in combustion have been investigated by using the crossed molecular beam scattering method with mass spectrometric detection. The novel capability of producing intense, continuous beams of the radical reagents by a radio-frequency discharge beam source was exploited. From angular and velocity distribution measurements obtained in the laboratory frame, primary reaction products have been identified and their angular and translational energy distributions in the center-of-mass system, as well as branching ratios, have been derived. The dominant N/H exchange channel has been examined in the reaction N(2D) + CH4, which is found to lead to H + CH2NH (methylenimine) and H + CH3N (methylnitrene); no H2 elimination is observed. In the reaction N(2D) + H2O the N/H exchange channel has been found to occur via two competing pathways leading to HNO + H and HON + H, while formation of NO + H2 is negligible. Formation of H + H2CCCH (propargyl) is the dominant pathway, at low collision energy (Ec), of the C(3P) + C2H4 reaction, while at high Ec formation of the less stable C3H3 isomers (cyclopropenyl and/or propyn-1-yl) also occurs; the H2 elimination channel is negligible. The H elimination channel has also been found to be the dominant pathway in the C(3P,1D) + CH3CCH reaction leading to C4H3 isomers and, again, no H2 elimination has been observed to occur. In contrast, both H and H2 elimination, leading in comparable ratio to C3H + H and C3(X1 sigma g+) + H2(X1 sigma g+), respectively, have been observed in the reaction C(3P) + C2H2(X1 sigma g+). The occurrence of the spin-forbidden molecular pathway in this reaction, never detected before, has been rationalized by invoking the occurrence of intersystem crossing between triplet and singlet manifolds of the C3H2 potential energy surfaces. The reaction CN(X2 sigma +) + C2H2 has been found to lead to internally excited HCCCN (cyanoacetylene) + H. For all the reactions the dynamics have been discussed in the light of recent theoretical calculations on the relevant potential energy surfaces. Previous, lower resolution studies on C and CN reactions carried out using pulsed beams are noted. Finally, throughout the paper the relevance of these results to combustion chemistry is considered.

A combined crossed molecular beam and ab initio investigation of C2 and C3 elementary reactions with unsaturated hydrocarbons--pathways to hydrogen deficient hydrocarbon radicals in combustion flames

Crossed molecular beam experiments on dicarbon and tricarbon reactions with unsaturated hydrocarbons acetylene, methylacetylene, and ethylene were performed to investigate the dynamics of channels leading to hydrogen-deficient hydrocarbon radicals. In the light of the results of new ab initio calculations, the experimental data suggest that these reactions are governed by an initial addition of C2/C3 to the pi molecular orbitals forming highly unsaturated cyclic structures. These intermediates are connected via various transition states and are suggested to ring open to chain isomers which decompose predominantly by displacement of atomic hydrogen, forming C4H, C5H, HCCCCCH2, HCCCCCCH3, H2CCCCH and H2CCCCCH. The C2(1 sigma g+) + C2H4 reaction has no entrance barrier and the channel leading to the H2CCCCH product is strongly exothermic. This is in strong contrast with the C3(1 sigma g+) + C2H4 reaction as this is characterized by a 26.4 kJ mol-1 threshold to form a HCCCCCH2 isomer. Analogous to the behavior with ethylene, preliminary results on the reactions of C2 and C3 with C2H2 and CH3CCH showed the H-displacement channels of these systems to share many similarities such as the absence/presence of an entrance barrier and the reaction mechanism. The explicit identification of the C2/C3 vs. hydrogen displacement demonstrates that hydrogen-deficient hydrocarbon radicals can be formed easily in environments like those of combustion processes. Our work is a first step towards a systematic database of the intermediates and the reaction products which are involved in this important class of reactions. These findings should be included in future models of PAH and soot formation in combustion flames.

The formation of nitriles in hydrocarbon-rich atmospheres of planets and their satellites: laboratory investigations by the crossed molecular beam technique

Crossed molecular beam experiments of cyano radicals, CN(X(2)Sigma(+)), reacting with unsaturated hydrocarbons have been performed to investigate synthetic routes to nitriles formation in hydrocarbon-rich atmospheres of planets and their moons. We have verified that all cyano radical reactions with acetylene, ethylene, methylacetylene, allene, benzene, and dimethylacetylene proceed without entrance barrier, have exit barriers well below the energy of the reactant molecules, and are strongly exothermic. The identification of the CN versus H atom exchange channel makes these reactions compelling candidates to synthesize unsaturated nitriles in solar system environments. Some of these nitriles, hitherto unobserved in our solar system, now represent an ideal target to be detected in the future Cassini-Huygens mission to Titan.