Combined Crossed Molecular Beams, Kinetics, and Theoretical Studies of the Multichannel N(2D) + C6H5CH3 (Toluene) Reaction: Rate Constants, Primary Product Channels, and Branching Fractions

The elementary reaction N(2D) + C6H5CH3 (toluene), of potential relevance in the atmospheric chemistry of Titan, has been investigated using a combined experimental/theoretical approach. CMB (crossed-molecular-beam) experiments employing mass-spectrometric detection and TOF (time-of-flight) analysis at collision energy of 34.3 kJ/mol were used for exploring primary products, branching fractions (BFs), and reaction micromechanism. Kinetic experiments in a continuous supersonic flow reactor were used for determining rate constants as a function of temperature (50, 75, 127, 177, and 296 K). Synergistically, electronic structure computations of the C7H8N PES (potential energy surface) and related statistical (Rice-Ramsperger-Kassel-Marcus/Master-Equation, RRKM/ME) calculations of product BFs were carried out to facilitate interpretation of experiments and elucidate the reaction micromechanism. It is found that barrierless “bridge” additions of N(2D) to the different positions of the aromatic ring form seven-, six-, and five-membered ring isomeric C7H8N intermediates that lead to a variety of product channels. Rate constants are fast, ranging between 2.1 and 2.4 × 10–10 cm3 s–1 over the 50–296 K range. The BFs of channels leading to “ring-contraction” via HCN/C2H2/H eliminations and to “ring-expansion” via CH3 elimination were quantified. Statistical simulations of isomeric product BFs on the computed PES were performed in CMB conditions and at 94, 175, and 296 K. Agreement is noted between experimental and theoretical BFs. The results are expected to be useful for improved modeling of Titan’s atmosphere and to provide molecular-level insight into N-incorporation at methyl-substituted aromatic carbon sites, with potential relevance to plasma-assisted functionalization of carbon nanomaterials.