Crossed-Beam and Theoretical Studies of the O(3P, 1D) + Benzene Reactions: Primary Products, Branching Fractions, and Role of Intersystem Crossing

Unveiling the Reaction Mechanism of the N(2D) + Pyridine Reaction: Ring-Contraction versus 7-Membered-Ring Formation Channels

Despite the relevance of the reactions of the prototypical nitrogen-containing six-membered aromatic molecule (N-heterocyclic) of pyridine (C6H5N) in environmental science, astrochemistry, planetary science, prebiotic chemistry, and materials science, few experimental/theoretical studies exist on the bimolecular reactions involving pyridine and neutral atomic/molecular radicals. We report a combined experimental and theoretical study on the elementary reaction of pyridine with excited nitrogen atoms, N(2D), aimed at providing information about the primary reaction products and their branching fractions (BFs). From previous crossed molecular beam (CMB) experiments with mass-spectrometric detection and present synergistic calculations of the reactive potential energy surface (PES) and product BFs we have unveiled the reaction mechanism. It is found that the reaction proceeds via N(2D) barrierless addition to pyridine that, via bridged intermediates followed by N atom "sliding" into the ring, leads to 7-membered-ring structures. They further evolve, mainly via ring-contraction mechanisms toward 5-membered-ring radical products and, to a smaller extent, via H-displacement mechanisms toward 7-membered-ring isomeric products and their isomers. Using the theoretical statistical estimates, an improved fit of the experimental data previously reported has been obtained, leading to the following results for the dominant product channels: C4H4N (pyrrolyl) + HCN (BF = 0.61 ± 0.20), C3H3N2 (1H-imidazolyl/1H-pyrazolyl) + C2H2 (BF = 0.11 ± 0.06), and C5H4N2 (7-membered-ring molecules or pyrrole carbonitriles) + H (BF = 0.28 ± 0.10). The ring-contraction product channels C4H4N (pyrrolyl) + HCN, C3H3N2 (1H-imidazolyl) + C2H2, C3H3N2 (1H-pyrazolyl) + C2H2, and C5H5 (cyclopentadienyl) + N2 have statistical BFs of 0.54, 0.09, 0.11, and 0.07, respectively. Among the H-displacement channels, the cyclic-CHCHCHCHNCN + H channel and cyclic-CHCHCHCHCN2 + H are theoretically predicted to have a comparable BF (0.07 and 0.06, respectively), while the other isomeric 7-membered-ring molecule + H channel has a BF of 0.03. Pyrrole-carbonitriles and 1H-ethynyl-1H-imidazole (+ H) isomeric channels have an overall BF of 0.03. Implications for the chemistry of Saturn's moon Titan and prebiotic chemistry, as well as for understanding the N-doping of graphene or carbon nanotubes, are noted.

Spiers Memorial Lecture: New directions in molecular scattering

The field of molecular scattering is reviewed as it pertains to gas-gas as well as gas-surface chemical reaction dynamics. We emphasize the importance of collaboration of experiment and theory, from which new directions of research are being pursued on increasingly complex problems. We review both experimental and theoretical advances that provide the modern toolbox available to molecular-scattering studies. We distinguish between two classes of work. The first involves simple systems and uses experiment to validate theory so that from the validated theory, one may learn far more than could ever be measured in the laboratory. The second class involves problems of great complexity that would be difficult or impossible to understand without a partnership of experiment and theory. Key topics covered in this review include crossed-beams reactive scattering and scattering at extremely low energies, where quantum effects dominate. They also include scattering from surfaces, reactive scattering and kinetics at surfaces, and scattering work done at liquid surfaces. The review closes with thoughts on future promising directions of research.

Crossed molecular beam experiments and theoretical simulations on the multichannel reaction of toluene with atomic oxygen

Despite extensive experimental and theoretical studies on the kinetics of the O(3P) + C7H8 (toluene) reaction and a pioneering crossed molecular beam (CMB) investigation, the branching fractions (BFs) of the CH3C6H4O(methylphenoxy) + H, C6H5O(phenoxy) + CH3, and spin-forbidden C5H5CH3 (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(3P,1D) + 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 CH3-elimination channel dominates (BF = 0.69 ± 0.22) in the O(3P) 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(3P) + 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.

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.

A combined crossed molecular beam and theorerical study of the O(3P,1D) + acrylonitrile (CH2CHCN) reactions and implications for combustion and extraterrestrial environments

Acrylonitrile (CH₂CHCN) is ubiquitous in space (molecular clouds, solar-type star forming regions, and circumstellar envelopes) and is also abundant in the upper atmosphere of Titan. The reaction O(³P) + CH₂CHCN can be of relevance in the chemistry of the interstellar medium because of the abundance of atomic oxygen. The oxidation of acrylonitrile is also important in combustion as the thermal decomposition of pyrrolic and pyridinic structures present in fuel-bound nitrogen generates many nitrogen-bearing compounds, including acrylonitrile. Despite its relevance, limited information exists on this reaction. We report a combined experimental and theoretical investigation of the reactions of acrylonitrile with both ground ³P and excited ¹D atomic oxygen. From product angular and time-of-flight distributions in crossed molecular beam experiments with mass spectrometric detection at a collision energy, E_c, of 31.4 kJ mol^-1, we have identified the primary reaction products and determined their branching fractions (BFs). Theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) were performed to interpret the experimental results and elucidate the reaction mechanism. Adiabatic statistical calculations of product BFs for the decomposition of the main triplet and singlet intermediates have been carried out. Combining the experimental and theoretical results, we conclude that the O(³P) reaction leads to two main product channels: (i) CH₂CNH (ketenimine) + CO (dominant with a BF of 0.87 ± 0.05), formed via efficient intersystem crossing from the entrance triplet PES to the underlying singlet PES, and (ii) HCOCHCN + H (minor, with a BF of 0.13 ± 0.05), occurring adiabatically on the triplet PES. Our study suggests the inclusion of this reaction as a possible destruction pathway of CH₂CHCN and a possible formation route of CH₂CNH in the interstellar medium. The O(¹D) + CH₂CHCN reaction mainly leads to the formation of CH₂CNH + CO adiabatically on the singlet PES. This result can improve models related to the chemistry of interstellar ice and cometary comas, where O(¹D) reactions can play a role. Overall, our results are expected to be useful for improving the models of combustion and extraterrestrial environments.

OH(2Π) + C2H4 Reaction: A Combined Crossed Molecular Beam and Theoretical Study

The reaction between the ground-state hydroxyl radical, OH(²Π), and ethylene, C₂H₄, has been investigated under single-collision conditions by the crossed molecular beam scattering technique with mass-spectrometric detection and time-of-flight analysis at the collision energy of 50.4 kJ/mol. Electronic structure calculations of the underlying potential energy surface (PES) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of product branching fractions on the derived PES for the addition pathway have been performed. The theoretical results indicate a temperature-dependent competition between the anti-/syn-CH₂CHOH (vinyl alcohol) + H, CH₃CHO (acetaldehyde) + H, and H₂CO (formaldehyde) + CH₃ product channels. The yield of the H-abstraction channel could not be quantified with the employed methods. The RRKM results predict that under our experimental conditions, the anti- and syn-CH₂CHOH + H product channels account for 38% (in similar amounts) of the addition mechanism yield, the H₂CO + CH₃ channel for ∼58%, while the CH₃CHO + H channel is formed in negligible amount (<4%). The implications for combustion and astrochemical environments are discussed.

Reactions O(3P, 1D) + HCCCN(X1Σ+) (Cyanoacetylene): Crossed-Beam and Theoretical Studies and Implications for the Chemistry of Extraterrestrial Environments

Cyanoacetylene (HCCCN), the first member of the cyanopolyyne family (HC_nN, where n = 3, 5, 7, ...), is of particular interest in astrochemistry being ubiquitous in space (molecular clouds, solar-type protostars, protoplanetary disks, circumstellar envelopes, and external galaxies) and also relatively abundant. It is also abundant in the upper atmosphere of Titan and comets. Since oxygen is the third most abundant element in space, after hydrogen and helium, the reaction O + HCCCN can be of relevance in the chemistry of extraterrestrial environments. Despite that, scarce information exists not only on the reactions of oxygen atoms with cyanoacetylene but with nitriles in general. Here, we report on a combined experimental and theoretical investigation of the reactions of cyanoacetylene with both ground ³P and excited ¹D atomic oxygen and provide detailed information on the primary reaction products, their branching fractions (BFs), and the overall reaction mechanisms. More specifically, the reactions of O(³P, ¹D) with HCCCN(X¹Σ⁺) have been investigated under single-collision conditions by the crossed molecular beams scattering method with mass spectrometric detection and time-of-flight analysis at the collision energy, E_c, of 31.1 kJ/mol. From product angular and time-of-flight distributions, we have identified the primary reaction products and determined their branching fractions (BFs). Theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) were performed to assist the interpretation of the experimental results and clarify the reaction mechanism. Adiabatic statistical calculations of product BFs for the decomposition of the main triplet and singlet intermediates have also been carried out. Merging together the experimental and theoretical results, we conclude that the O(³P) reaction is characterized by a minor adiabatic channel leading to OCCCN (cyanoketyl) + H (experimental BF = 0.10 ± 0.05), while the dominant channel (BF = 0.90 ± 0.05) occurs via intersystem crossing to the underlying singlet PES and leads to formation of ¹HCCN (cyanomethylene) + CO. The O(¹D) reaction is characterized by the same two channels, with the relative CO/H yield being slightly larger. Considering the recorded reactive signal and the calculated entrance barrier, we estimate that the rate coefficient for reaction O(³P) + HC₃N at 300 K is in the 10^-12 cm³ molec^-1 s^-1 range. Our results are expected to be useful to improve astrochemical and photochemical models. In addition, they are also relevant in combustion chemistry, because the thermal decomposition of pyrrolic and pyridinic structures present in fuel-bound nitrogen generates many nitrogen-bearing compounds, including cyanoacetylene.

Intersystem crossing in the entrance channel of the reaction of O(3P) with pyridine

Two quantum effects can enable reactions to take place at energies below the barrier separating reactants from products: tunnelling and intersystem crossing between coupled potential energy surfaces. Here we show that intersystem crossing in the region between the pre-reactive complex and the reaction barrier can control the rate of bimolecular reactions for weakly coupled potential energy surfaces, even in the absence of heavy atoms. For O(3P) plus pyridine, a reaction relevant to combustion, astrochemistry and biochemistry, crossed-beam experiments indicate that the dominant products are pyrrole and CO, obtained through a spin-forbidden ring-contraction mechanism. The experimental findings are interpreted-by high-level quantum-chemical calculations and statistical non-adiabatic computations of branching fractions-in terms of an efficient intersystem crossing occurring before the high entrance barrier for O-atom addition to the N-atom lone pair. At low to moderate temperatures, the computed reaction rates prove to be dominated by intersystem crossing.

Crossed-beam and theoretical studies of multichannel nonadiabatic reactions: branching fractions and role of intersystem crossing for O(3P)+1,3-butadiene

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...