Reaction N(²D) + CH₂CCH₂ (Allene): An Experimental and Theoretical Investigation and Implications for the Photochemical Models of Titan

Abstract

We report on a combined experimental and theoretical investigation of the N( 2 D) + CH 2 CCH 2 (allene) reaction of relevance in the atmospheric chemistry of Titan. Experimentally, the reaction was investigated (i) under singlecollision conditions by the crossed molecular beams (CMB) scattering method with mass spectrometric detection and time-offlight analysis at the collision energy (E c ) of 33 kJ/mol to determine the primary products and the reaction micromechanism and (ii) in a continuous supersonic flow reactor to determine the rate constant as a function of temperature from 50 to 296 K. Theoretically, electronic structure calculations of the doublet C 3 H 4 N 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( 2 D) to one of the two equivalent carbon− carbon double bonds of CH 2 CCH 2 , followed by the formation of several cyclic and linear isomeric C 3 H 4 N intermediates that can undergo unimolecular decomposition to bimolecular products with elimination of H, CH 3 , HCN, HNC, and CN. The kinetic experiments confirm the barrierless nature of the reaction through the measurement of rate constants close to the gas-kinetic rate at all temperatures. Statistical estimates of product branching fractions (BFs) on the theoretical PES were carried out under the conditions of the CMB experiments at room temperature and at temperatures (94 and 175 K) relevant for Titan. Up to 14 competing product channels were statistically predicted with the main ones at E c = 33 kJ/mol being formation of cyclic-CH 2 C(N)CH + H (BF = 87.0%) followed by CHCCHNH + H (BF = 10.5%) and CH 2 CCNH + H (BF = 1.4%) the other 11 possible channels being negligible (BFs ranging from 0 to 0.5%). BFs under the other conditions are essentially unchanged. Experimental dynamical information could only be obtained on the overall H-displacement channel, while other possible channels could not be confirmed within the sensitivity of the method. This is also in line with theoretical predictions as the other possible channels are predicted to be negligible, including the HCN/HNC + C 2 H 3 (vinyl) channels (overall BF < 1%). The dynamics and product distributions are dramatically different with respect to those observed in the isomeric reaction N( 2 D) + CH 3 CCH (propyne), where at a similar E c the main product channels are CH 2 NH (methanimine) + C 2 H (BF = 41%), c-C(N)CH + CH 3 (BF = 32%), and CH 2 CHCN (vinyl cyanide) + H (BF = 12%). Rate coefficients (the recommended value is 1.7 (±0.2) × 10 −10 cm 3 s −1 over the 50−300 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 abundance (including any new products formed) as a function of the altitude.

Kinetics, Products, and Mechanisms Study of the Atmospheric Degradation of (E)-4-Methoxy-3-buten-2-one with Hydroxyl Radicals

“…As seen from Tables S1–S5, the T1 diagnostic values of closed-shell species are less than 0.02, and those of open-shell species are less than 0.044, indicating that the single-reference method is reliable for energies in chemical accuracy (within 4.2 kJ/mol) for the target reactions. It has been shown that the CCSD­(T) method is able to properly describe the values of energy in several systems. − Unless mentioned otherwise, the final energies reported below are calculated using the dual-level methodology CCSD­(T)/6-311++G­(d,p)//BH&HLYP /6-311++G­(d,p) + 0.9335 × ZPE, which is abbreviated hereafter as CCSD­(T)//BH&HLYP for the sake of brevity.…”

Design of de Laval nozzles for gas-phase molecular studies in uniform supersonic flow

“…In some experiments, a significant difference in the maximum amplitude of the exponential decay is observed between kinetics with and without reactants, 50,94 or when increasing the co-reagent. 121…”

“…It is also noteworthy that a non-zero intercept seems less observed in the most analogous experiments but in cold cells 139 like at room temperature. 38,67,[140][141][142][143] But it is true that one can also notice sometimes experiments reported a non-zero intercept in cryogenic cells 79 as well as room temperature 94,[141][142][143][144][145][146] despite it is only a small intercept most of the time. At room temperature, bell-shaped curves probed on products have been also observed.…”

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

“…The theoretical investigation of the reaction between N( 2 D) and C 6 H 6 was performed adopting a well-established computational strategy previously used for the investigation of many other bimolecular reactions. 26,31,[34][35][36][37][38][39][68][69][70] The PES of the N( 2 D) + C 6 H 6 system was first characterized by locating the lowest stationary points (minima and transition states) at the B3LYP level of theory in conjunction with the 6-311+G** basis set. 71,72 The optimization of all the stationary points was then refined by the use of the same hybrid density functional with a larger basis set, i.e.…”

“…To examine the effects of the present experimental determinations on the chemistry of Titan's atmosphere, we included the N( 2 D) + benzene reaction in the 1D-photochemical model described in Dobrijevic et al,92 which treats the chemistry of neutrals and cations (anions are not considered in this study as they have only a minor influence on the overall chemistry), and the coupling between them from the lower atmosphere to the ionosphere. Details of the recent updates of the chemical scheme are presented in Benne et al,93 with a subsequent update in Vanuzzo et al36 Two different simulations were performed during this investigation. The first simulation represents the nominal model where the N( 2 D) + benzene reaction is neglected.…”