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Thermal decomposition of CH 3 I revisited: Consistent calibration of I‐atom concentrations behind shock waves with dual I‐/H‐ARAS
Author(s) -
Weber Isabelle,
Olzmann Matthias
Publication year - 2019
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.21260
Subject(s) - chemistry , radical , atom (system on chip) , calibration , yield (engineering) , decomposition , thermal decomposition , analytical chemistry (journal) , hydrocarbon , absorption (acoustics) , hydrogen atom , atomic physics , thermodynamics , group (periodic table) , organic chemistry , physics , optics , computer science , embedded system , quantum mechanics
Abstract Iodinated hydrocarbons are often used as precursors for hydrocarbon radicals in shock‐tube experiments. The radicals are produced by C─I bond fission reaction, and their formation can be followed through time‐resolved monitoring of the complementary I‐atom concentrations, for example, by I‐atom resonance absorption spectroscopy (I‐ARAS). This very sensitive technique requires, however, an independent calibration. As a very clean source of I atoms, CH 3 I is particularly well suited as calibration system for I‐ARAS presumed the yield of I atoms and the rate coefficient of I‐atom formation from CH 3 I are known with sufficient accuracy. But if the formation of I atoms from CH 3 I by I‐ARAS is to be characterized, an independent calibration system is required. In this study, we propose a cross‐calibration approach for I‐ARAS based on the simultaneous time‐resolved monitoring of I and H atoms by ARAS in C 2 H 5 I pyrolysis experiments. For this reaction system, it can be shown that at sufficiently short reaction times very similar amounts of I and H atoms are formed (difference <1%). As calibration of H‐ARAS, with mixtures of N 2 O and H 2 , is a well‐established technique, we calibrated I‐atom absorption–time profiles with respect to simultaneously recorded H‐atom concentration–time profiles. Using this approach, we investigated the thermal decomposition of CH 3 I in the temperature range 950–2050 K behind reflected shock waves at two different nominal pressures ( p ∼ 0.4 and 1.6 bar, bath gas: Ar). From the obtained absolute I‐atom concentration–time profiles at temperatures T < 1250 K, we inferred a second‐order rate coefficient k ( T ) = (1.7 ± 0.7) × 10 15 exp(–20020 K/ T ) cm 3 mol –1 s –1 for the reaction CH 3 I + Ar → CH 3 + I + Ar. A small mechanism to describe the pyrolysis of CH 3 I under shock‐tube conditions is presented and discussed.