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Despite extensive experimental and theoretical studies on the kinetics of the O( 3 P) + C 7 H 8 (toluene) reaction and a pioneering crossed molecular beam (CMB) investigation, the branching fractions (BFs) of the CH 3 C 6 H 4 O(methylphenoxy) + H, C 6 H 5 O(phenoxy) + CH 3 , and spin-forbidden C 5 H 5 CH 3 (methylcyclopentadiene) + CO product channels remain an open question, which has hampered the proper inclusion of this important reaction in the chemical modelling of various chemical environments. We report a CMB study with universal soft electron-ionization mass-spectrometric detection of the reactions O( 3 P, 1 D) + toluene at the collision energy of 34.7 kJ mol -1 . From CMB data we have inferred the reaction dynamics and quantified the BFs of the primary products and the role of intersystem crossing (ISC). The CH 3 -elimination channel dominates (BF = 0.69 ± 0.22) in the O( 3 P) reaction, while the H-displacement and CO-formation channels are minor (BF = 0.22 ± 0.07 and 0.09 ± 0.05, respectively), with ISC accounting for more than 50% of the reactive flux. Synergistic transition-state theory (TST)-based master equation simulations including nonadiabatic TST on ab initio coupled triplet/singlet potential energy surfaces were employed to compute the product BFs and assist in the interpretation of the CMB results. In the light of the good agreement between the theoretical predictions for the O( 3 P) + toluene reaction and the CMB results as well as the absolute rate constant as a function of temperature ( T ) (from literature), the so-validated computational methodology was used to predict channel-specific rate constants as a function of T at 1 atm.

We report on a combined experimental and theoretical investigation of the N( 2 D) + C 6 H 6 (benzene) reaction, which is of relevance in the aromatic chemistry of the atmosphere of Titan. Experimentally, the reaction was studied (i) under single-collision conditions by the crossed molecular beams (CMB) scattering method with mass spectrometric detection and time-of-flight analysis at the collision energy ( E c ) of 31.8 kJ mol -1 to determine the primary products, their branching fractions (BFs), and the reaction micromechanism, and (ii) in a continuous supersonic flow reactor to determine the rate constant as a function of temperature from 50 K to 296 K. Theoretically, electronic structure calculations of the doublet C 6 N potential energy surface (PES) were performed to assist the interpretation of the experimental results and characterize the overall reaction mechanism. The reaction is found to proceed 6 barrierless addition of N( via barrierless addition of N( 2 D) to the aromatic ring of C 6 H 6 , followed by formation of several cyclic (five-, six-, and seven-membered ring) and linear isomeric C 6 N intermediates that can undergo unimolecular decomposition to bimolecular products. Statistical estimates of product BFs on the theoretical PES were carried out under the conditions of the CMB experiments and at the temperatures relevant for Titan's atmosphere. In all conditions the ring-contraction channel leading to C 6 N intermediates that can undergo unimolecular decomposition to bimolecular products. Statistical estimates of product BFs on the theoretical PES were carried out under the conditions of the CMB experiments and at the temperatures relevant for Titan's atmosphere. In all conditions the ring-contraction channel leading to C 5 H 5 (cyclopentadienyl) + HCN is dominant, while minor contributions come from the channels leading to o -C 6 N ( 5 -N-cycloheptatriene radical) + H, C o -N-cycloheptatriene radical) + H, C 4 N (pyrrolyl) + C 4 N (pyrrolyl) + C 2 H 2 (acetylene), C 5 H 5 CN (cyano-cyclopentadiene) + H, and p -C 6 N + H. Rate constants (which are close to the gas kinetic limit at all temperatures, with the recommended value of 2.19 ± 0.30 × 10 5 N + H. Rate constants (which are close to the gas kinetic limit at all temperatures, with the recommended value of 2.19 ± 0.30 × 10 -10 cm 3 s -1 over the 50-296 K range) and BFs have been used in a photochemical model of Titan's atmosphere to simulate the effect of the title reaction on the species abundances as a function of the altitude.

Atomic oxygen reactions can contribute significantly to the oxidation of unsaturated aliphatic and aromatic hydrocarbons. The reaction mechanism is started by electrophilic O atom addition to the unsaturated bond(s) to form "chemically activated" triplet oxy-intermediate(s), which can evolve adiabatically on the triplet potential energy surface (PES) and nonadiabatically via intersystem crossing on the singlet PES, forming intermediates that undergo unimolecular decomposition to a variety of bimolecular product channels. Here, we apply a combined crossed molecular beam (CMB)-theoretical approach to the study of the O( 3 P) + 1,3-butadiene reaction. Although the kinetics of this reaction have been extensively investigated, little is known about the primary products and their branching fractions (BFs). In the present work, a total of eight product channels were observed and characterized in a CMB experiment at a collision energy of 32.6 kJ mol -1 . Synergic ab initio transition-state theory-based master equation simulations coupled with nonadiabatic transition-state theory on coupled triplet/singlet PESs were employed to compute the product BFs and assist the interpretation of the CMB experimental results. The good agreement found between the theoretical predictions and CMB experiments supported the use of the adopted methodology for the prediction of channel-specific rate constants as a function of temperature and pressure suitable to be used for the kinetic modeling of 1,3-butadiene oxidation and of systems where 1,3-butadiene is an important intermediate.

By exploiting the potentialities of a recently implemented grid empowered molecular simulator based on the combination of collaborative interoperable service oriented computing and the usage of high performance - high throughput technologies, the results of crossed molecular beam experiments have been virtually simulated and compared with the real (measured) laboratory data for the reactive system OH + CO. The direct comparison of theoretically predicted laboratory angular distributions with experimental raw data avoids possible uncertainties associated with the analysis of crossed beam experiments, in which trial centre-of-mass functions are tested until the best-fit of the experimental data is achieved. To make such a comparison as accurate as possible, the rotational distributions of the OH radicals employed in previous crossed beam experiments have been characterized by laser-induced-fluorescence. The capability of performing massive calculations using grid-distributed technologies has allowed the running of quasiclassical trajectory calculations for all the initial rotational states of the OH radicals present in the beam (from the ground rotational state N(OH) = 1 up to N(OH) = 10) on three different potential energy surfaces and the comparison of related outcomes.

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.

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 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 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.