A Quantum Study on the Reaction between C(3P) and Acetylene

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.

The interstellar chemistry of C3H and C3H2 isomers

We report the detection of linear and cyclic isomers of C3H and C3H2 towards various starless cores and review the corresponding chemical pathways involving neutral (C3Hx with x=1,2) and ionic (C3Hx+ with x = 1,2,3) isomers. We highlight the role of the branching ratio of electronic Dissociative Recombination (DR) reactions of C3H2+ and C3H3+ isomers showing that the statistical treatment of the relaxation of C3H* and C3H2* produced in these DR reactions may explain the relative c,l-C3H and c,l-C3H2 abundances. We have also introduced in the model the third isomer of C3H2 (HCCCH). The observed cyclic-to-linear C3H2 ratio vary from 110 ± 30 for molecular clouds with a total density around 1×104 molecules.cm-3 to 30 ± 10 for molecular clouds with a total density around 4×105 molecules.cm-3, a trend well reproduced with our updated model. The higher ratio for low molecular cloud densities is mainly determined by the importance of the H + l-C3H2 → H + c-C3H2 and H + t-C3H2 → H + c-C3H2 isomerization reactions.

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.

Quantum Chemical Evaluation of the Astrochemical Significance of Reactions between S Atom and Acetylene or Ethylene

Addition-elimination reactions of S atom in its 3P ground state with acetylene (C2H2) and ethylene (C2H4) were characterized with both molecular orbital and density functional theory calculations employing correlation consistent basis sets in order to assess the likelihood that either reaction might play a general role in astrochemistry or a specific role in the formation of S2 (X3Sigmag-) via a mechanism proposed by Saxena, P. P.; Misra, A. Mon. Not. R. Astron. Soc. 1995, 272, 89. The acetylene and ethylene reactions proceed through C2H2S (3A' ') and C2H4S (3A' ') intermediates, respectively, to yield HCCS (2Pi) and C2H3S (2A'). Substantial barriers were found in the exit channels for every combination of method and basis set considered in this work, which effectively precludes hydrogen elimination pathways for both S + C2H2 and S + C2H4 in the ultracold interstellar medium where only very modest barriers can be surmounted and processes without barriers tend to predominate. However, if one or both intermediates are formed and stabilized efficiently under cometary or dense interstellar cloud conditions, they could serve as temporary reservoirs for the S atom and participate in reactions such as S + C2H2S --> S2 + C2H2 or S + C2H4S --> S2 + C2H4. For formation and stabilization to be efficient, the reaction must possess a barrier height small enough to be surmountable at low temperatures yet large enough to prevent redissociation to reactants. Barrier heights computed with B3LYP and large basis sets are very low, but more rigorous QCISD(T) and RCCSD(T) results indicate that the barrier heights are closer to 3-4 kcal/mol. The calculations therefore indicate that S + C2H2 or S + C2H4 could contribute to the formation of S2 in comets and may serve as a means to gauge coma temperature. The energetics of the ethylene reaction are more favorable.

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.

Ab initio based potential energy surfaces and Franck-Condon analysis of ionization thresholds of cyclic-C3H and linear-C3H

We report a Franck-Condon analysis in reduced dimensionality of the ionization thresholds of linear(l)-C3H and cyclic(c)-C3H using MP2-based potential energy surfaces and CCSD(T)/aug-cc-pVTZ calculations of electronic energies at selected geometries. The potential energy surfaces are fits to tens of thousands of MP2/aug-cc-pVTZ energies for the neutral and cation systems. These fits properly describe the invariance of the potential with respect to all permutations of the three C atoms. The realism of the potential surfaces is assessed by comparing stationary-point structures, energies, and normal-mode frequencies with previous high-level ab initio calculations. Several key vibrational modes in this ionization process are located at saddle points and so a numerical approach to obtain the Franck-Condon factors for those modes is done. On the basis of this analysis combined with a simple harmonic treatment of the energies of the remaining modes and key electronic energy differences obtained with CCSD(T)/aug-cc-pVTZ calculations, we find the threshold ionization energy of l-C3H to be 9.06 eV and for c-C3H we estimate the threshold to be in the range 9.70-9.76 eV. We estimate these values are accurate to within +/-0.05 eV.

Quantum study on the branching ratio of the reaction NO2+OH

A reduced dimensionality (RD) approximation is developed for the title reaction which treats the angle of approach of the hydroxyl radical to the nitrogen dioxide molecule and the radial distance between the two species explicitly. All other degrees of freedom are treated adiabatically. Electronic structure calculations at the complete active space self-consistent field level are used to fit a potential energy surface (PES) in these two coordinates. Within this RD model the adiabatic capture centrifugal sudden approximation is used to calculate the high pressure limit rate constant. A correction for reflection from the PES due to rotationally nonadiabatic transitions is applied using the wave packet capture approximation. The branching ratio for the title reaction is calculated for the atmospherically significant temperature range of 200-400 K at 20 Torr without distinguishing between the conformers of HOONO. The result is k(HOONO)k(HNO(3) )=0.051 at 20 Torr and 300 K, which is in good agreement with the measured branching ratio between cis-cis-HOONO and nitric acid. This suggests that most of the different conformers of HOONO were converted to the most stable cis-cis conformer on the time scale of the measurements made.

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.

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.

Quasiclassical trajectory calculations of the reaction C+C2H2→l-C3H, c-C3H+H, C3+H2 using full-dimensional triplet and singlet potential energy surfaces

Full-dimensional, density functional theory (B3LYP/6-311g(d,p))-based potential energy surfaces (PESs) are reported and used in quasi-classical calculations of the reaction of C with C(2)H(2). For the triplet case, the PES spans the region of the reactants, the complex region (with numerous minima and saddle points) and the products, linear(l)-C(3)H+H, cyclic(c)-C(3)H+H and c-(3)C(3)+H(2). For the singlet case, the PES describes the complex region and products l-C(3)H+H, c-C(3)H+H and l-(1)C(3)+H(2). The PESs are invariant under permutation of like nuclei and are fit to tens of thousands of electronic energies. Energies and harmonic frequencies of the PESs agree well the DFT ones for all stationary points and for the reactant and the products. Dynamics calculations on the triplet PES find both l-C(3)H and c-C(3)H products, with l-C(3)H being dominant at the energies considered. Limited unimolecular reaction dynamics on the singlet PES find both products in comparable amounts as well as the C(3)+H(2) product.

Quantum Dynamics Study on the Product Branching for the C(3P) + C2H2 Reaction: cyclic-C3H versus linear-C3H

Reduced-dimensionality quantum reactive scattering calculations for the C(3P) + C2H2 reaction have been carried out in order to understand the product branching dynamics of cyclic-C3H + H and linear-C3H + H. Our model treats only two degrees of freedom but can explicitly describe both of the C3H isomer product channels. The lowest triplet potential energy surface has been obtained by the hybrid density-functional method at the B3LYP/6-31G(d,p) level of theory. The calculated reaction probabilities were found to be dominated by resonance consistent with the complex-formation potential, and the results show that cyclic-C3H is preferentially formed via the cyclic-C3H2 intermediate produced by insertion of C(3P) into the CC bond. We have found that the isomerization from the cyclic-C3H2 to linear-C3H2 intermediate is suppressed by a barrier separating potential wells corresponding to these two intermediates. It has also been found that the energy dependence of the calculated total reaction cross section is in good agreement with the result of crossed molecular beam experiments.

Gas phase electronic spectra of two C5H5 radical isomers

Gas phase electronic spectra of two C5H5 radical isomers have been observed in the 440-470 nm spectral region. The technique was a mass selective resonant two-color two-photon ionization coupled to a supersonic plasma source. Structures, relative energies and vertical electronic transition energies of six isomers of C5H5 have been calculated. Based on an analysis of the rotational profiles of the observed bands and theoretical calculations, the spectra are assigned as the A 2A" <-- X 2A" electronic transition of isomer 1 and A 2A2 X 2B1 of 6 with origin band at 461.8 nm and 456.1 nm, respectively. Isomer 1, 1,3-vinylpropargyl, has Cs symmetry, while 6, a planar zig-zig chain with one hydrogen on each carbon, has C2v symmetry.

Isomeric Structures and Visible Electronic Spectrum of the C7H3 Radicals

The 2-(buta-1,3-diynyl)cycloprop-2-yl-1-ylidene radical, a new three-membered ring chain with Cs symmetry, has been detected by electronic spectroscopy in the gas phase. The experimental investigation used a mass selective resonant two color two photon ionization technique coupled to a supersonic plasma source. Structures and relative stability energies of eight isomers of the C7H3 radical have been calculated. Based on the rotational analysis and the theoretical calculations, the observed spectrum is assigned as an 2A" <-- X2A' electronic transition of this exotic chemical species. This result shows that such a plasma source is a powerful tool to investigate intermediates involved in hydrocarbon chemistry as in flames.

Quantum scattering calculations on chemical reactions

This review discusses recent quantum scattering calculations on bimolecular chemical reactions in the gas phase. This theory provides detailed and accurate predictions on the dynamics and kinetics of reactions containing three atoms. In addition, the method can now be applied to reactions involving polyatomic molecules. Results obtained with both time-independent and time-dependent quantum dynamical methods are described. The review emphasises the recent development in time-dependent wave packet theories and the applications of reduced dimensionality approaches for treating polyatomic reactions. Calculations on over 40 different reactions are described.