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

Abstract

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.

Figure 1

Schematic representation of the potential energy surface for the reaction N(²D) + CH₂CHCN with energies evaluated at the CCSD(T)/aug-cc-pVTZ level of theory (see text), considering only N(²D) addition to the double bond. The structure of the heavier coproduct from the four main channels, all accompanied by the H coproduct, is shown as well as the structure of the main intermediates. Thick, color coded (green and blue) solid lines indicate the four pathways leading to the underlined main products according to our RRKM estimates (see text).

Figure 2

Schematic representation of the potential energy surface for the reaction N(²D) + CH₂CHCN with energies evaluated at the CCSD(T)/aug-cc-pVTZ level of theory (see text). In this figure, we have considered the attack of the N(²D) atom to the triple bond of CN and to the lone pair of the nitrogen atom.

Figure 3

(Top): LAB angular distribution of the C₃H₂N₂ product detected at m/z = 65 (C₃HN₂⁺) for the N(²D) + vinyl cyanide reaction at E_c = 31.4 kJ/mol. The solid black curve represents the calculated distribution when using the best-fit CM functions shown in Figure 5. (Bottom): Velocity vector (Newton) diagram of the experiment. The radius of each circle represents the maximum velocity that the indicated product can attain in the center-of-mass (CM) reference frame if all available energy is channeled into product recoil energy (see text).

Figure 4

Time-of-flight distributions at m/z = 65 at the indicated LAB angles for the N(²D) + vinyl cyanide reaction. Empty circles: experimental data. Solid line: simulated TOF spectra when using the best-fit CM functions depicted in Figure 5.

Figure 5

Best-fit CM product angular, T(θ), (top) and translational energy, P(E_T^′), (bottom) distributions for the N(²D) + C₂H₃CN reaction. The shaded areas represent the error bars determined for the best-fit CM functions. The vertical lines in the graph of P(E_T^′) indicate the total energy (E_tot = E_c – ΔH₀⁰) of the six different, most exothermic H-displacement isomeric channels (b), (c), (d), (e), (g), and (h), in which the heavy coproduct of the H atom has general formula C₃H₂N₂.

Figure 6

B3LYP optimized geometries (distances in black in Å, angles in blue in degrees) of the minima identified along the PES for the reaction N(²D) + vinyl cyanide CH₂CHCN.

Figure 7

B3LYP optimized geometries (distances in black in Å, angles in blue in degrees) of the main transition states identified along the PES for the reaction N(²D) + CH₂CHCN.

Figure 8

B3LYP optimized geometries (distances in black in Å, angles in blue in degrees) of vinyl cyanide and of the four main molecular products identified along the PES for the reaction N(²D) + CH₂CHCN.