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

Abstract

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 (E_c) 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.