K. P. Lim

The thermal decomposition of C2H5I

The high-temperature thermal dissociation of C2H5I has been characterized in this study. Kinetics and overall yield experiments were performed over the temperature range, 946–2046 K, using the atomic resonance absorption spectrometric technique (ARAS) for the temporal detection of both product H and I atoms behind reflected shock waves. The C2H5I decomposition proceeds by both C-I fission and HI elimination. Rate constants for the C-I fission process, measured over the temperature and density ranges, 946–1303 K and 0.82–4.4×1018 cm−3, respectively, can be represented to within ±37% by the firstorder expression: k=6.34×109 exp(−15,894 K/T) s−1. Overall yield data for atomic product gave a branching ratio for C-I fission of (0.87±0.11) suggesting that 13% of the reaction proceeds through molecular HI elimination. This conclusion is consistent with earlier studies that showed C-I fission to be the dominant dissociation channel. The temperature and pressure dependences of the dissociation rate constants and the yield data have been described theoretically using three formulations of unimolecular rate theory. The best description was obtained with a full Masters equation analysis. However, all three calculations confirm that the HI-elimination pathway is lower lying than the C-I fission process by ≈3 kcal mol−1.

Multipass optical detection in reflected shock waves: Application to OH radicals

An UV multipass optical absorption method to increase the sensitivity for radical species detection has been developed for high temperature chemical kinetics experiments in a shock tube. The specific illustration is for OH radicals in the reflected shock wave regime. With a resonance lamp source, 12 optical passes were found to give a sufficient signal‐to‐noise ratio for a large range of [OH]. Two different calibration procedures using the reaction systems H2/O2 and C2H5I/NO2 were used, and a curve of growth was determined. The measured absorbance (ABS), was found to be dependent on both temperature and [OH]. The results can be expressed in a modified Beer’s law form as,(ABS)=9.49×10−12T−0.5281[OH]0.8736.Using this curve of growth, the absorbance data from the above kinetics experiments were converted to concentration profiles. These were fully modeled with previously established mechanisms, giving excellent fits. The multipass method is compared to earlier systems that used both resonance lamp and laser ...

Thermal decomposition of CF3I using I-atom absorption

I-atom atomic resonance absorption spectrometry (ARAS) has been developed and applied to measure the thermal decomposition rate constant for CF 3 I (+M) → CF 3 + I (+M). The I-atom curve-of-growth (λ = 183 nm) was determined using this reaction, and, for [I] ⩽3 × 10 12 molecules cm −3 , (ABS) = 1.9215 × 10 −13 [I], yielding σ = 1.933 × 10 −14 cm 2 . Measured rate constants can be expressed by k 1 = 3.24 × 10 −9 exp(−17286 K/T) cm 3 molecule −1 s −1 (±56%, 1033 ⩽ T ⩽ 1285 K). RRKM theory has been applied to rationalize this result.