Reactions O(³P, ¹D) + HCCCN(X¹Σ⁺) (Cyanoacetylene): Crossed-Beam and Theoretical Studies and Implications for the Chemistry of Extraterrestrial Environments

Abstract

Cyanoacetylene (HCCCN), the first member of the cyanopolyyne family (HC n N, where n = 3, 5, 7, ...), is of particular interest in astrochemistry being ubiquitous in space (molecular clouds, solar-type protostars, protoplanetary disks, circumstellar envelopes, and external galaxies) and also relatively abundant. It is also abundant in the upper atmosphere of Titan and comets. Since oxygen is the third most abundant element in space, after hydrogen and helium, the reaction O + HCCCN can be of relevance in the chemistry of extraterrestrial environments. Despite that, scarce information exists not only on the reactions of oxygen atoms with cyanoacetylene but with nitriles in general. Here, we report on a combined experimental and theoretical investigation of the reactions of cyanoacetylene with both ground 3P and excited 1D atomic oxygen and provide detailed information on the primary reaction products, their branching fractions (BFs), and the overall reaction mechanisms. More specifically, the reactions of O(3P, 1D) with HCCCN(X1Σ+) have been investigated under single-collision conditions by the crossed molecular beams scattering method with mass spectrometric detection and time-of-flight analysis at the collision energy, E c, of 31.1 kJ/mol. From product angular and time-of-flight distributions, we have identified the primary reaction products and determined their branching fractions (BFs). Theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) were performed to assist the interpretation of the experimental results and clarify the reaction mechanism. Adiabatic statistical calculations of product BFs for the decomposition of the main triplet and singlet intermediates have also been carried out. Merging together the experimental and theoretical results, we conclude that the O(3P) reaction is characterized by a minor adiabatic channel leading to OCCCN (cyanoketyl) + H (experimental BF = 0.10 ± 0.05), while the dominant channel (BF = 0.90 ± 0.05) occurs via intersystem crossing to the underlying singlet PES and leads to formation of 1HCCN (cyanomethylene) + CO. The O(1D) reaction is characterized by the same two channels, with the relative CO/H yield being slightly larger. Considering the recorded reactive signal and the calculated entrance barrier, we estimate that the rate coefficient for reaction O(3P) + HC3N at 300 K is in the 10–12 cm3 molec–1 s–1 range. Our results are expected to be useful to improve astrochemical and photochemical models. In addition, they are also relevant in combustion chemistry, because the thermal decomposition of pyrrolic and pyridinic structures present in fuel-bound nitrogen generates many nitrogen-bearing compounds, including cyanoacetylene.

A combined crossed molecular beam and theorerical study of the O(3P,1D) + acrylonitrile (CH2CHCN) reactions and implications for combustion and extraterrestrial environments

“…For the system involving ethylene, pulsed CMB studies at E c = 6 kcal mol À1 and 3 kcal mol À1 with VUV photoionization detection 93,94 revealed that the contribution of O( 1 D) cannot be neglected and its role was also confirmed in the CMB experiment performed at E c = 35.1 kJ mol À1 by our research group. 90 The second reason is that the system involving atomic oxygen and cyanoacetylene 91 as reference because the two systems, having a comparable entrance potential energy barrier on the triplet PES and having been studied at nearly equal collision energy, have been investigated by using an oxygen beam generated under the same conditions (in the experimental study of the O( 3 P, 1 D) + ethylene reactions the diameter of the nozzle was different) and because the reaction pathway under analysis is the same, i.e., a CO-forming channel (in the experiment involving ethylene, however, the CO-forming channel was not observed to occur 90,93,94 ). Analyzing the TOF spectra, it should be noted that the fingerprint of the CH 2 CNH + CO channel from the reaction of O( 3 P) and O( 1 D) is clearer at Y = 321 than at the angle Y = 441 (that is, very near to the center of mass angle of 451), because at Y CM the relative contribution of the heavy coproduct of the H-displacement channel (1f) is strongly amplified for kinematic reasons (by the CM -LAB transformation Jacobian 60 ), being scattered over a much narrower LAB angular range (see the Newton diagram in Fig.…”

“…of the products is the same of that of the reagents (v r ), the T(y) function is backward-forward symmetric with a maximum at y = 01 and y = 1801. [95][96][97][98][99] Regarding the O( 1 D) + CH 2 CHCN reaction, the T(y) function for the CO-forming channel (2b) was found to be strongly forward biased, as in the case of the O( 1 D) + HCCCN (cyanoacetylene) reaction, 91 indicating an osculating complex mechanism. 97 Regarding the product translational energy distributions, the P E 0 T À Á functions of the two reactive channels of the O( 3 P) + acrylonitrile reaction exhibit a peak well away from zero (at B44 kJ mol À1 for both channels), an indication that the two reactive channels have a sizeable exit potential energy barrier.…”

“…91 The higher reactivity of O( 3 P) when reacting with acrylonitrile compared to the system involving cyanoacetylene is a direct consequence of the lower entrance potential energy barrier on the triplet PES, which was calculated to be 7 kJ mol À1 (at the CCSD(T)/CBS level of theory) for the O( 3 P) + acrylonitrile reaction and 9 kJ mol À1 (at the CCSD(T)/CBS level of theory) for the O( 3 P) + cyanoacetylene reaction. 91…”

Crossed molecular beam experiments and theoretical simulations on the multichannel reaction of toluene with atomic oxygen

“…Although in the past decades the O( 3 P) + toluene reaction had been extensively studied, the characterization of its detailed mechanism and dynamics, its primary product distribution and role with C2, C3, and C4 unsaturated hydrocarbons, [1][2][3][5][6][7]12,[15][16][17][18][19][20] with benzene, 8,9 pyridine, 11 and the simples unsaturated nitriles. 67,68 It is also a gateway towards the investigation of other O( 3 P) reactions with more complex aromatics, including the prototype of PAHs, naphthalene. 1b).…”

Atmospheric Degradation of 3-Methoxy-1-propanol by Reaction with Hydroxyl Radicals and Chlorine Atoms

“…The electronic partition function of the Cl• atoms was evaluated by considering the splitting of 881 cm –1 between the ground 2 P 3/2 and excited 2 P 1/2 electronic states of Cl• atoms . The CTST method developed by Singleton and Cvetanovic and Alvarez-Idaboy et al , has been successfully employed to a lot of similar reaction systems. − …”