Raffaele Petrucci

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.

Crossed-beam universal-detection reactive scattering of radical beams characterized by laser-induced-fluorescence: the case of C2 and CN

Continuous supersonic beams of dicarbon (C2) and cyano (CN) radicals have been generated by a high-pressure radio-frequency discharge beam source starting from dilute mixtures in rare gases of suitable precursor molecules. Their internal quantum state distributions have been characterized by laser-induced-fluorescence (LIF) in a new crossed molecular beam-laser apparatus. These supersonic beams have been used to study the reactive scattering of C2 and CN radicals with unsaturated hydrocarbons. This paper reports here on the C2 and CN radical beam characterization by LIF and on dynamics studies of the reactions CN + C2H2 (acetylene) and CN + CH3CCH (methylacetylene) by the crossed molecular beam scattering technique with universal mass spectrometric detection and time-of-flight analysis. The role of CN rovibrational excitation on the dynamics of the CN + C2H2 reaction is discussed with reference to previous dynamics and kinetics studies. These reactions are of interest in the chemistry of planetary atmospheres (Titan) and the interstellar medium as well as in combustion.

Experimental and theoretical studies of the O(3P) + C2H4 reaction dynamics: collision energy dependence of branching ratios and extent of intersystem crossing.

The reaction of O((3)P) with C(2)H(4), 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 E(c) 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 + CH(2)CHO, H + CH(3)CO, CH(3) + HCO, CH(2) + H(2)CO, and H(2) + CH(2)CO. These experiments extend our previous CMB work at higher collision energy (E(c) ∼ 13 kcal∕mol) and when the results are combined with the literature branching ratios from kinetics experiments at room temperature (E(c) ∼ 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((3)P) + C(2)H(4) 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 + CH(2)CHO, CH(3) + HCO, and CH(2) + H(2)CO. 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.

Crossed-beam dynamics, low-temperature kinetics, and theoretical studies of the reaction S(1D) + C2H4.

The reaction between sulfur atoms in the first electronically excited state, S((1)D), and ethene (C(2)H(4)) has been investigated in a complementary fashion in (a) crossed-beam dynamic experiments with mass spectrometric detection and time-of-flight (TOF) analysis at two collision energies (37.0 and 45.0 kJ mol(-1)), (b) low temperature kinetics experiments ranging from 298 K down to 23 K, and (c) electronic structure calculations of stationary points and product energetics on the C(2)H(4)S singlet and triplet potential energy surfaces. 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 temperatures indicating that the overall reaction is barrierless. 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. Product branching ratios have also been estimated. Interpretation of the experimental results on the reaction kinetics and dynamics is assisted by high-level theoretical calculations on the C(2)H(4)S singlet potential energy surface. RRKM (Rice-Ramsperger-Kassel-Marcus) estimates of the product branching ratios using the newly developed singlet potential energy surface have also been performed and compared with the experimental determinations.

Combined crossed beam and theoretical studies of the C(1D) + CH4 reaction.

The reaction involving atomic carbon in its first electronically excited state (1)D 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((1)D) into one of the C-H bonds of methane leading to the formation of the intermediate HCCH(3) (methylcarbene or ethylidene), which either decomposes directly into the products C(2)H(3) + H or C(2)H(2) + H(2) or isomerizes to the more stable ethylene, which in turn dissociates into C(2)H(3) + H or H(2)CC + H(2). The experimental results indicate that the H-displacement and H(2)-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((1)D) + CH(4) 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((2)D) + CH(4), O((1)D) + CH(4) and S((1)D) + CH(4) are also commented on.

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.

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.

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.

Crossed molecular beam studies of C(3P,1D) and C2(X1Σ+g,a3Πu) reactions with acetylene

The crossed molecular beam method with mass spectrometric detection is a versatile technique to study bimolecular reactions under single collision conditions, thus permitting us to elucidate the chemical dynamics and—in the case of polyatomic reactions—the nature of the primary products and their branching ratios. A review of the recent progress that has been made in the understanding of gas-phase carbon chemistry is given by illustrating some of the key results on the reactions C(3P,1D) and C2(X1Σ+g,a3Πu) with acetylene, one of the most important hydrocarbons in applied processes and natural environments. The results are discussed in the light of the available theoretical information on the relevant triplet and singlet C3H2 and C4H2 potential energy surfaces, and compared with the results of previous experimental work. The improvements in our experimental apparatus that have been made to accomplish the present results are highlighted and the possibility of extending the same approach to the study of other polyatomic reactions is emphasized.