Dynamics of the N(2D) + D2 Reaction from Crossed-Beam and Quasiclassical Trajectory Studies

An experimental and theoretical investigation of the N(2D) + C6H6 (benzene) reaction with implications for the photochemical models of Titan

We report on a combined experimental and theoretical investigation of the N(2D) + C6H6 (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 (Ec) 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 C6H6N 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 via barrierless addition of N(2D) to the aromatic ring of C6H6, followed by formation of several cyclic (five-, six-, and seven-membered ring) and linear isomeric C6H6N 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 C5H5 (cyclopentadienyl) + HCN is dominant, while minor contributions come from the channels leading to o-C6H5N (o-N-cycloheptatriene radical) + H, C4H4N (pyrrolyl) + C2H2 (acetylene), C5H5CN (cyano-cyclopentadiene) + H, and p-C6H5N + 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 cm3 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.

The N(2D) + CH2CHCN (Vinyl Cyanide) Reaction: A Combined Crossed Molecular Beam and Theoretical Study and Implications for the Atmosphere of Titan

The reaction of electronically excited nitrogen atoms, N(²D), with vinyl cyanide, CH₂CHCN, has been investigated under single-collision conditions by the crossed molecular beam (CMB) scattering method with mass spectrometric detection and time-of-flight (TOF) analysis at the collision energy, E_c, of 31.4 kJ/mol. Synergistic electronic structure calculations of the doublet potential energy surface (PES) have been performed to assist in the interpretation of the experimental results and characterize the overall reaction micromechanism. Statistical (Rice–Ramsperger–Kassel–Marcus, RRKM) calculations of product branching fractions (BFs) on the theoretical PES have been carried out at different values of temperature, including the one corresponding to the temperature (175 K) of Titan’s stratosphere and at a total energy corresponding to the E_c of the CMB experiment. According to our theoretical calculations, the reaction is found to proceed via barrierless addition of N(²D) to the carbon–carbon double bond of CH₂=CH–CN, followed by the formation of cyclic and linear intermediates that can undergo H, CN, and HCN elimination. In competition, the N(²D) addition to the CN group is also possible via a submerged barrier, leading ultimately to N₂ + C₃H₃ formation, the most exothermic of all possible channels. Product angular and TOF distributions have been recorded for the H-displacement channels leading to the formation of a variety of possible C₃H₂N₂ isomeric products. Experimentally, no evidence of CN, HCN, and N₂ forming channels was observed. These findings were corroborated by the theory, which predicts a variety of competing product channels, following N(²D) addition to the double bond, with the main ones, at E_c = 31.4 kJ/mol, being six isomeric H forming channels: c-CH(N)CHCN + H (BF = 35.0%), c-CHNCHCN + H (BF = 28.1%), CH₂NCCN + H (BF = 26.3%), c-CH₂(N)CCN(cyano-azirine) + H (BF = 7.4%), trans-HNCCHCN + H (BF = 1.6%), and cis-HNCCHCN + H (BF = 1.3%), while C–C bond breaking channels leading to c-CH₂(N)CH(2H-azirine) + CN and c-CH₂(N)C + HCN are predicted to be negligible (0.02% and 0.2%, respectively). The highly exothermic N₂ + CH₂CCH (propargyl) channel is also predicted to be negligible because of the very high isomerization barrier from the initial addition intermediate to the precursor intermediate able to lead to products. The predicted product BFs are found to have, in general, a very weak energy dependence. The above cyclic and linear products containing an additional C–N bond could be potential precursors of more complex, N-rich organic molecules that contribute to the formation of the aerosols on Titan’s upper atmosphere. Overall, the results are expected to have a significant impact on the gas-phase chemistry of Titan’s atmosphere and should be properly included in the photochemical models.

Gas-phase formation of silicon monoxide via non-adiabatic reaction dynamics and its role as a building block of interstellar silicates

Silicon monoxide (SiO) is classified as a key precursor and fundamental molecular building block to interstellar silicate nanoparticles, which play an essential role in the synthesis of molecular building blocks connected to the Origins of Life. In the cold interstellar medium, silicon monoxide is of critical importance in initiating a series of elementary chemical reactions leading to larger silicon oxides and eventually to silicates. To date, the fundamental formation mechanisms and chemical dynamics leading to gas phase silicon monoxide have remained largely elusive. Here, through a concerted effort between crossed molecular beam experiments and electronic structure calculations, it is revealed that instead of forming highly-stable silicon dioxide (SiO₂), silicon monoxide can be formed via a barrierless, exoergic, single-collision event between ground state molecular oxygen and atomic silicon involving non-adiabatic reaction dynamics through various intersystem crossings. Our research affords persuasive evidence for a likely source of highly rovibrationally excited silicon monoxide in cold molecular clouds thus initiating the complex chain of exoergic reactions leading ultimately to a population of silicates at low temperatures in our Galaxy.

Gas-Phase Preparation of Subvalent Germanium Monoxide (GeO, X1+) via Non-Adiabatic Reaction Dynamics in the Exit Channel

The subvalent germanium monoxide (GeO, X¹Σ⁺) molecule has been prepared via the elementary reaction of atomic germanium (Ge, ³P_j) and molecular oxygen (O₂, X³Σ_g^-) with each reactant in its electronic ground state by means of single-collision conditions. The merging of electronic structure calculations with crossed beam experiments suggests that the formation of germanium monoxide (GeO, X¹Σ⁺) commences on the singlet surface through unimolecular decomposition of a linear singlet collision complex (GeOO, i1, C_∞v, ¹Σ⁺) via intersystem crossing (ISC) yielding nearly exclusively germanium monoxide (GeO, X¹Σ⁺) along with atomic oxygen in its electronic ground state [p1, O(³P)]. These results provide a sophisticated reaction mechanism of the germanium-oxygen system and demonstrate the efficient "heavy atom effect" of germanium in ISC yielding (nearly) exclusive singlet germanium monoxide and triplet atomic oxygen compared to similar systems (carbon dioxide and dinitrogen monoxide), in which non-adiabatic reaction dynamics represent only minor channels.

A Combined Experimental and Theoretical Study on the Formation of the 2-Methyl-1-silacycloprop-2-enylidene Molecule via the Crossed Beam Reactions of the Silylidyne Radical (SiH; X(2)Π) with Methylacetylene (CH3CCH; X(1)A1) and D4-Methylacetylene (CD3CCD; X(1)A1)

The bimolecular gas-phase reactions of the ground-state silylidyne radical (SiH; X(2)Π) with methylacetylene (CH3CCH; X(1)A1) and D4-methylacetylene (CD3CCD; X(1)A1) were explored at collision energies of 30 kJ mol(-1) under single-collision conditions exploiting the crossed molecular beam technique and complemented by electronic structure calculations. These studies reveal that the reactions follow indirect scattering dynamics, have no entrance barriers, and are initiated by the addition of the silylidyne radical to the carbon-carbon triple bond of the methylacetylene molecule either to one carbon atom (C1; [i1]/[i2]) or to both carbon atoms concurrently (C1-C2; [i3]). The collision complexes [i1]/[i2] eventually isomerize via ring-closure to the c-SiC3H5 doublet radical intermediate [i3], which is identified as the decomposing reaction intermediate. The hydrogen atom is emitted almost perpendicularly to the rotational plane of the fragmenting complex resulting in a sideways scattering dynamics with the reaction being overall exoergic by -12 ± 11 kJ mol(-1) (experimental) and -1 ± 3 kJ mol(-1) (computational) to form the cyclic 2-methyl-1-silacycloprop-2-enylidene molecule (c-SiC3H4; p1). In line with computational data, experiments of silylidyne with D4-methylacetylene (CD3CCD; X(1)A1) depict that the hydrogen is emitted solely from the silylidyne moiety but not from methylacetylene. The dynamics are compared to those of the related D1-silylidyne (SiD; X(2)Π)-acetylene (HCCH; X(1)Σg(+)) reaction studied previously in our group, and from there, we discovered that the methyl group acts primarily as a spectator in the title reaction. The formation of 2-methyl-1-silacycloprop-2-enylidene under single-collision conditions via a bimolecular gas-phase reaction augments our knowledge of the hitherto poorly understood silylidyne (SiH; X(2)Π) radical reactions with small hydrocarbon molecules leading to the synthesis of organosilicon molecules in cold molecular clouds and in carbon-rich circumstellar envelopes.

VECTOR CORRELATIONS AND PRODUCT POLARIZATIONS IN THE N(2D) + D2 → ND + D REACTIVE SYSTEM

The vector correlations between products and reagents of the N (2D) + D 2 reaction are investigated by employing quasi-classical trajectory (QCT) calculation on the accurate DMBE potential energy surface (PES) of the 2A″ state. Stereo-dynamic quantities, including the four generalized polarization-dependent differential cross-sections (PDDCSs), the angular distribution P(θr), the dihedral-angle distribution P(φr), as well as the product rotational angular distribution in the polar form of P(θr, φr), are calculated in the center-of-mass (CM) frame. The results indicate that the product rotational angular momentum j′ not only aligns along the y-axis, but also orients to the negative direction of the y-axis. The isotope effect in the context of chemical stereo-dynamics and influences of different versions of ground-state PESs on vector correlations are shown and discussed.

INFLUENCE OF VIBRATIONAL EXCITATION AND COLLISION ENERGY ON THE STEREO DYNAMICS OF THE REACTIONS: N(4S)+H2(v = 0-3,j = 0) → NH(X3Σ-)+H

Quasi-classical trajectory (QCT) calculations have been carried out to study the stereodynamics of the title reactions, using the double many-body expansion (DMBE) potential energy surface (PES) constructed by Poveda [Poveda LA, Varandas AJC, Phys. Chem. Chem. Phys.7:2867, 2005]. Vector correlations, such as the distributions of the polarization-dependent differential cross-sections (PDDCSs), the angular distributions of P(θr), P(ϕr), P(θr,ϕr), and the product alignment parameter P2( cos θr) are reported within the energy range of 25–140 kcal mol-1. The influences of the collision energy and the initial state-selected vibrational excitation are discussed in detail. In addition, Validity of the current QCT calculations is also examined and compared with the revelant results reported by Pascual et al. [Pascual RZ, Schatz GC, Lendvay G, Troya D, J. Phys. Chem. Aਠ106:4125, 2002].

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.

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.

Dynamics of insertion reactions of H2 molecules with excited atoms

Recent progress in the study of insertion reactions of hydrogen molecules with excited atoms is reviewed in this article. In particular, the dynamics of the reaction of O(1D), N(2D), C(1D), and S(1D) with H2 and its isotopomers, which have received a great deal of attention over the past decade, are examined in detail. All of these systems have in common the existence of several potential energy surfaces (PES) correlating with the reagents' states, and consequently, they can give rise to reaction following different adiabatic and nonadiabatic pathways. The main contribution, however, arises from their ground singlet PESs which feature the existence of deep wells with small or null barriers for insertion. Accordingly, these reactions proceed mainly via formation of relatively long-lived collision complexes and display an overall nearly statistical behavior. In spite of their similarities, the various reactions have peculiar characteristics caused by important differences of their respective PESs. The contribution of excited PES to the global reactivity, which has also become an important issue and a challenge both for theory and experiment, is also examined. The different theoretical approaches are discussed in the text, along with the experimental results obtained by a variety of techniques. The recent exact quantum treatments of these reactive systems together with the development of a rigorous statistical model have contributed to a very accurate description which in many cases matches very well the detailed measurements. The quasi-classical trajectory (QCT) method has also provided a fairly accurate description of the reaction dynamics for these systems. In particular, the analysis in terms of collision times has yielded interesting clues about the reaction mechanisms.

Influence of rotation and isotope effects on the dynamics of the N(D2)+H2 reactive system and of its deuterated variants

Integral cross sections and thermal rate constants have been calculated for the N((2)D)+H(2) reaction and its isotopic variants N((2)D)+D(2) and the two-channel N((2)D)+HD by means of quasiclassical trajectory and statistical quantum-mechanical model methods on the latest ab initio potential-energy surface [T.-S. Ho et al., J. Chem. Phys. 119, 3063 (2003)]. The effect of rotational excitation of the diatom on the dynamics of these reactions has been investigated and interesting discrepancies between the classical and statistical model calculations have been found. Whereas a net effect of reagent rotation on reactivity is always observed in the classical calculations, only a very slight effect is observed in the case of the asymmetric N((2)D)+HD reaction for the statistical quantum-mechanical method. The thermal rate constants calculated on this Potential-Energy Surface using quasiclassical trajectory and statistical model methods are in good agreement with the experimental determinations, although the latter are somewhat larger. A reevaluation of the collinear barrier of the potential surface used in the present study seems timely. Further theoretical and experimental studies are needed for a full understanding of the dynamics of the title reaction.

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.

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.