The Reaction N(²D) + CH₃CCH (Methylacetylene): A Combined Crossed Molecular Beams and Theoretical Investigation and Implications for the Atmosphere of Titan

Abstract

The reaction of excited nitrogen atoms N( 2 D) with CH 3 CCH (methylacetylene) was investigated 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 31.0 kJ/mol. Synergistic electronic structure calculations of the doublet potential energy surface (PES) were performed to assist the interpretation of the experimental results and characterize the overall reaction micromechanism. Theoretically, the reaction is found to proceed via a barrierless addition of N( 2 D) to the carbon–carbon triple bond of CH 3 CCH and an insertion of N( 2 D) into the CH bond of the methyl group, followed by the formation of cyclic and linear intermediates that can undergo H, CH 3 , and C 2 H elimination or isomerize to other intermediates before unimolecularly decaying to a variety of products. Kinetic calculations for addition and insertion mechanisms and statistical (Rice-Ramsperger-Kassel-Marcus) computations of product branching fractions (BFs) on the theoretical PES were performed at different values of total energy, including the one corresponding to the temperature (175 K) of Titan’s stratosphere and that of the CMB experiment. Up to 14 competing product channels were statistically predicted, with the main ones, at E c = 31.0 kJ/mol, being the formation of CH 2 NH (methanimine) + C 2 H (ethylidyne) (BF = 0.41), c -C(N)CH + CH 3 (BF = 0.32), CH 2 CHCN (acrylonitrile) + H (BF = 0.12), and c -CH 2 C(N)CH + H (BF = 0.04). Of the 14 possible channels, seven correspond to H displacement channels of different exothermicity, for a total H channel BF of ∼0.25 at E c = 31.0 kJ/mol. Experimentally, dynamical information could only be obtained about the overall H channels. In particular, the experiment corroborates the formation of acrylonitrile + H, which is the most exothermic of all 14 reaction channels and is theoretically calculated to be the dominant H-forming channel (BF = 0.12). The products containing a novel C–N bond could be potential precursors to form other nitriles (C 2 N 2 , C 3 N) or more complex organic species containing N atoms in planetary atmospheres, such as those of Titan and Pluto. Overall, the results are expected to have a potentially significant impact on the understanding of the gas-phase chemistry of Titan’s atmosphere and the modeling of that atmosphere.

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

“…The resulting beam velocity and speed ratio were 2283 m/s and 5.4, respectively. In our previous experiments on the reactions of N atoms with saturated and unsaturared hydrocarbons, − we verified that the presence of N­( 4 S) and N­( 2 P) in the nitrogen beam did not affect the experimental outcomes, because the rate coefficients of the N­( 4 S) and N­( 2 P) reactions with saturated and unsaturated hydrocarbons are much smaller than those of N­( 2 D). ,, In addition, the extra energy carried by the 2 P state was never necessary to fit the experimental data. Analogous to the case of N­( 2 D) + C 2 H 4 , we expect that the reactions involving N­( 4 S) and N­( 2 P) + vinyl cyanide do not contribute to the reactive signal.…”

“…RRKM calculations for the N­( 2 D) + CH 2 CHCN reaction were performed using a code implemented in our group for this purpose. ,, The microcanonical rate constant k ( E ) for a specific reaction at a specific total energy is given by the expression where N ( E ) represents the sum of states at the transition state at energy E , ρ­( E ) is the density of states of the reactant, and h is Planck’s constant. N ( E ) is obtained by integrating the relevant density of states up to energy E , and the rigid rotor/harmonic oscillator model is assumed.…”

“…RRKM calculations for the N( 2 D) + CH 2 CHCN reaction were performed using a code implemented in our group for this purpose. 29,30,92 The microcanonical rate constant k(E) for a specific reaction at a specific total energy is given by the expression k E ( )…”

“…Our experimental results have been accompanied by electronic structure calculations of the underlying potential energy surface (PES), and RRKM (Rice–Ramsperger–Kassel–Marcus) estimates of the product branching fractions (BFs) have been carried out under the conditions of our experiments and at the temperatures of relevance for Titan. The reactions N­( 2 D) + CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , and C 3 H 4 isomers (methylacetylene and allene (unpublished results)) − as well as N­( 2 D) + small aromatic compounds (pyridine, benzene, and toluene) − have been investigated. In a few cases, that is, for the N­( 2 D) reactions with H 2 and H 2 O, more sophisticated dynamical calculations have been performed and the detailed reaction mechanism elucidated. − …”

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

“… 9 − 20 When reacting with hydrocarbons, molecular products possessing a novel C–N bond are formed. 9 − 14 , 18 − 20 Recent kinetic experiments performed with the CRESU technique at the relevant temperature for the conditions of the upper atmosphere of Titan have revealed that the rate constants for several important N( 2 D) reactions are considerably larger 21 , 22 or smaller 23 than those determined by previous experiments above 200 K. 24 − 26 The inclusion of these data in updated versions of a photochemical model of the atmosphere of Titan have demonstrated the importance of this approach in making the model more accurate. 21 − 23…”