Temperature dependence of the rate constants for the reactions of C 2 H with C 2 H 2 , H 2 , and D 2

Mitsuo Koshi*, Koichi Fukuda, Kenshu Kamiya, Hiroyuki Matsui

*Corresponding author for this work

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Abstract

Reactions of the ethynyl (C 2 H) radical with C 2 H 2 , H 2 , and D 2 were studied over the temperature range 298-438 K by time-resolved mass spectrometry. The rate of the reaction with C 2 H 2 was followed by measuring the appearance rate of C 4 H 2 . The rates of the reactions with H 2 and D 2 were deduced by measuring the dependence of C 4 H 2 production (arising from the reaction of C 2 H with the C 2 H 2 precursor) on the partial pressure of added H 2 or D 2 . The rate constants for the reaction C 2 H + C 2 H 2 → C 4 H 2 + H were also measured following reflected shock waves by monitoring H atom resonant absorption at 121.6 nm. In both experiments, the C 2 H radical was generated by ArF (193-nm) laser photolysis of C 2 H 2 . A rate constant of (1.5 ± 0.3) × 10 -10 cm 3 molecule -1 s -1 was obtained for reaction 1 without any temperature dependence at T = 298-2177 K. The results for the reactions C 2 H + H 2 → C 2 H 2 + H and C 2 H + D 2 → C 2 HD + D could be represented by the Arrhenius expressions, k 2 = (1.8 ± 1.0) × 10 -11 exp(-(1090 ± 299)/T) and k 3 = (1.4 ± 0.8) × 10 -11 exp(-(1377 ± 301)/T) cm 3 molecule -1 s -1 , over the range of T = 298-438 K. The classical barrier height for reactions 2 and 3 was estimated to be 2 kcal/mol on the basis of conventional transition-state theory. The isotope effects on reactions 2 and 3 calculated with Wigner tunneling correction were in good agreement with the present results.

Original languageEnglish
Pages (from-to)9839-9843
Number of pages5
JournalJournal of physical chemistry
Volume96
Issue number24
DOIs
StatePublished - 1 Nov 1992

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