Kinetics of C2(a3Πu) radical reactions with alkanes by LIF

Abstract

The reactions of C2(a3Piu) radicals with a series of alkanes have been studied at room temperature and 6.5 torr total pressure using the pulsed laser photolysis/laser-induced fluorescence technique. C2(a3Piu) radicals were generated by photolysis of C2Cl4 with the focused output from the fourth harmonic of a Nd: YAG laser at 266 nm. The relative concentration of C2(a3Piu) radicals was monitored on the (0,0) band of the C2(d3Pig <-- a3Piu) transition at 516.5 nm by laser-induced fluorescence. From the analysis of the relative concentration-time behavior of C2(a3Piu) under pseudofirst-order conditions, the rate constants for the reactions of C2(a3Piu) with alkanes (C1-C8) were determined. The rate constant increases linearly with the increasing of the number of CH2 groups in the alkanes. The experimental results indicate that the reaction of C2(a3Piu) with small alkanes (C1-C8) follows the typical hydrogen abstraction process. Based on the correlation of the experimental results with the bond dissociation energy of the alkanes, the reactions of C2(a3Piu) with small alkanes likely proceed via the mechanism of hydrogen abstraction.

An experimental study of the reaction kinetics of C2(X1Σg+) with hydrocarbons (CH4, C2H2, C2H4, C2H6 and C3H8) over the temperature range 24–300 K: Implications for the atmospheres of Titan and the Giant Planets

“…The energy difference between the lowest vibrational levels of these two states is only 610 cm −1 or 880 K. While many experimental studies have been carried out to measure the rate coefficients of the removal of 3 C 2 by hydrocarbons (Donnelly and Pasternack, 1979;Reisler et al, 1979Reisler et al, , 1980Pasternack et al, 1980Pasternack et al, , 1981Becker et al, 2000;Huang et al, 2004Huang et al, , 2005, data concerning the reactivity of 1 C 2 are scarcer. These were obtained in the early eighties and were only available at room temperature (Pasternack and McDonald, 1979;Reisler et al, 1980) and above (Pitts et al, 1982).…”

Direct ab initio dynamics study of the reaction of C2(A3Πu) with CH4

“…To our knowledge, the relevant data available in a wide temperature regime remains incomplete and uncertain [15] and there is no available theoretical study of this reaction despite of its importance. Therefore, detailed theoretical kinetic studies, including insights gained from electronic structure, can be especially useful in interpreting the reaction profiles and understanding the reactivity and selectivity of these systems and thorough study on this fundamental reaction can provide important clues into the behavior of larger systems.…”

“…They reported the alkane size dependence of the reactivity: the reactivity of 3 C 2 with the first three lighter alkanes is higher by several orders of magnitude compared to the big size of the alkane. However, the rate constant of 3 C 2 with the lightest alkane methane is too small to be determined accurately below room temperature [15,19]. The differences are large between measured rate constants [6,7,15,23].…”

“…However, the rate constant of 3 C 2 with the lightest alkane methane is too small to be determined accurately below room temperature [15,19]. The differences are large between measured rate constants [6,7,15,23]. On the contrary to the reactivity of the triplet state, previous experimental kinetic studies demonstrated that the reactions of 1 C 2 with hydrocarbons are found to be significantly rapid even in very low temperature range 24-300 K, and the rate coefficients are typically one order of magnitude higher than that of 3 C 2 with saturated hydrocarbons, except for CH 4 [5,6,8,21].…”

“…These studies predicted the lower reactivity of triplet state compared to the singlet one and they found that the reactivity is dependent on the nature of the molecule and more specifically, the number of p bonds which indicates the electrophilic character of the triplet state. In the past decades, the gas phase reaction of 3 C 2 with a selected series of saturated alkanes (CH 4 , C 2 H 6 , C 3 H 8 , n-C 4 H 10 , i-C 4 H 10 , and n-C 6 H 14 ) was performed by Sims and co-workers [19], Chen and co-workers [15,20], and Pitts and co-workers [4,[6][7][8]. They reported the alkane size dependence of the reactivity: the reactivity of 3 C 2 with the first three lighter alkanes is higher by several orders of magnitude compared to the big size of the alkane.…”