Kinetics of the reaction ethynyl radical + oxygen from 193 to 350 K using laser flash kinetic infrared absorption spectroscopy

Abstract

Rate coefficients for the reaction C2H + 0 2 are measured from 193 to 350 K by using transient absorption spectroscopy with an infrared color center laser. Ethynyl radicals are produced by pulsed laser photolysis of C2H2 in a temperature variable flow cell and a tunable color center laser probesthe transient removal of C2H (R 22+(0,0,0)) in absorption. The rate coefficient has a slight negative temperature dependence over the range 193-350 K which can be expressed as kol = (1.5 f 0.3) X 10-l' exp[(230 f 36)/T] cm3 molecule-' s-l. The lack of a pressure dependence under our experimental conditions strongly suggests that the HCCOO* complex is short lived with respect to encountering a collision and rapidly dissociates to products without competition from collisional stabilization to HCCOO or collisionally induced dissociation to reactants.

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

“…16,17,[22][23][24] In the present work we report LPD-CL measurements of the rate constants for the reactions of C 2 H with NO and H 2 in a temperature region between 295 and 440 K.…”

The near-infrared spectrum of ethynyl radical

“…SF 6 has often also been added to test gas mixtures to accelerate vibrational relaxation, although several workers have reported that it was found to have no effect on ground state kinetic spectroscopy of C 2 H and was therefore left out. 16,19,20,23 Until future experiments are able to provide a direct monitor of the vibrational relaxation, we can only echo a summary statement from Laufer and Fahr 15 in their 2004 review of the reactions and kinetics of ethynyl radicals from acetylene photodissociation: "the role of vibrational excitation in the subsequent chemistry is not clear. "…”

“…[12][13][14] In support of the extensive reaction network modeling in multiple environments, rate constants have been measured for reactions of ethynyl radicals with various partners over a range of temperatures. [15][16][17][18][19][20][21][22][23][24][25] Following the astronomical observation of the ethynyl radical, 9 the gas-phase microwave spectrum was recorded and the molecular structure determined in the 1980's. [26][27][28] Prior to this, laboratory observations had been limited to work in low temperature matrices.…”

“…Hsu et al 46,47 For kinetic spectroscopy studies, vibronic relaxation prior to reaction is typically tested by varying the pressure or choice of an inert buffer to verify insensitivity of the observed reaction rates. 16,17,19,20,23 Direct measurements of hot band relaxation 35 are preferable, but not always feasible, to confirm the timescale of vibrational thermalization.…”

Dark state vibronic coupling in the Ã(2Π) ←X̃(2Σ+) band of ethynyl radical via high resolution infrared absorption spectroscopy

“…6 As a natural corollary, there has been a large amount of research effort focused on ethynyl radical. Experimentally, there have been many studies aimed at measuring rate coefficients for reactions of C 2 H with a variety of neutral molecules, [7][8][9][10][11][12][13] particularly since it is likely these reactions contribute initially to the nucleation of small gas-phase molecules to form particulate-sized species. Spectroscopically, studies aimed at understanding the vibrationless ground state 4,[14][15][16] have confirmed it to be of 2 S + symmetry, which differs qualitatively from larger C n H (n 4 4) radicals expected to have 2 P ground states.…”