Absolute Rate Coefficients of the Reactions of C₂H with NO and H₂ between 295 and 440 K

Abstract

The rate coefficients of the title reactions were measured at 295 < T < 440 K and at He pressures in the 2-20 Torr range by a laser photodissociation/chemiluminescence technique (LPD/CL), which was validated earlier. Ethynyl radicals were generated by 193 nm excimer laser photolysis of C 2 H 2 (or C 2 HCF 3 ), and the real-time pseudo-first-order decay of thermalized C 2 H was monitored by the luminescence of CH(A 2 ∆) produced by their reaction with O 2 , present in large excess. The temperature dependence of the rate coefficient k(C 2 H+NO), reported for the first time, can be expressed by k(T) ) (1.0 ( 0.2) × 10 -10 exp[-(287 ( 65)/T] cm 3 molecule -1 s -1 ; the k(T)295 K) value of (3.9 ( 0.4) × 10 -11 is independent of pressure (2-10 Torr) and agrees with literature data at 20 Torr. It is argued that the fast C 2 H + NO reaction is an associationelimination process, probably forming HCN + CO or CN + HCO (or H + CO), and is of major importance to NO-reburning chemistry in fuel-rich hydrocarbon flames. The k(C 2 H+H 2 ) result at 295 K, of (6.9 ( 0.7) × 10 -13 , supports the higher of the literature values rather than the slightly lower data (of about 5 × 10 -13 ). The ab initio transition state theory k(T) expression of Harding et al., 18 with the barrier height and the closely correlated ω 6 ‡ TS bending frequency adjusted to fit the average of all k(C 2 H+H 2 ) measurements at 295 K, thus resulting in k(T) ) 1.31 × 10 -18 T 2.39 exp(-174/T) cm 3 molecule -1 s -1 , is found to provide a close representation (standard deviation 21%) of all three available sets of k(T) measurements at higher temperatures, including the present data in the 295-440 K range. Combination with the equilibrium constant leads to k(C 2 H 2 +H)(T) ) 1.50 × 10 -13 T 1.32 exp(-15400/T), implying values at flame temperatures almost 20 times higher than recent recommendations.

Formation of simple organic molecules in inner T Tauri disks

“…We include neutral-neutral and ion-molecule bimolecular reactions, three body processes and thermal dissociations. Important reactions involved in the build up of small organic molecules at high temperatures are those of H 2 with radicals such as C 2 and C 2 H. These reactions have been studied in the laboratory over a relatively wide temperature range: C 2 + H 2 → C 2 H + H between 295 and 493 K (Pitts et al 1982) and C 2 H + H 2 → C 2 H 2 + H in the range 178-440 K (Peeters et al 1996;Opansky & Leone 1996). They have moderate activation barriers of about 1400 K, which make them very slow in the cold interstellar medium, although at high temperatures they become rapid enough to control the abundance of C-bearing species.…”

High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

“…Skokov and Wheeler and co-workers 32 performed a similar study using density functional theory (DFT). Significant work to reconcile experimentally observed rates 33,34 with theoretical values for the reactions of ethynyl radical with other small molecules has been done by Nguyen and co-workers. [35][36][37] While numerous experimental and theoretical databases exist for the computation of heats of formation of simple hydrogen abstraction reactions, systematic and comprehensive high-accuracy studies of the reaction barriers (especially for reactions involving the ethynyl radical) are rare.…”

A modified potential energy surface for the C2H+H2↔C2H2+H reaction and a theoretical study on its rate constants

“…The linear transition state was located and the barrier height was found to be 4.0 kcal/mol. According to the recent conclusions [5,7,13,[19][20][21], lower barrier heights of 2.0-2.5 kcal/mol may reproduce the experimental rate constants better over a broad range of temperatures. Based on the properties of the saddle point reported by Harding et al [18], Wang and Bowman [23] have constructed the first global PES of the C 2 H + H 2 system and accomplished a quantum dynamics calculation.…”

“…The C 2 H + H 2 M C 2 H 2 + H reaction has been extensively studied in the past few decades. Experimentally, the rate constants have been measured with various methods over wide temperature range of 178-4650 K [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Theoretically, the ab initio calculations have been carried out by a few groups [18][19][20][21][22].…”