Thermal decomposition of CH3I using I-atom absorption

The recently developed I-atom atomic resonance absorption spectrometric (ARAS) technique has been used to study the thermal decomposition kinetics of CH3I over the temperature range, 1052–1820 K. Measured rate constants for CH3I(+Kr)→CH3+I(+Kr) between 1052 and 1616 K are best expressed by k(±36%)=4.36×10−9 exp(−19858 K/T) cm3 molecule−1 s−1. Two unimolecular theoretical approaches were used to rationalize the data. The more extensive method, RRKM analysis, indicates that the dissociation rates are effectively second-order, i.e., the magnitude is 61–82% of the low-pressure-limit rate constants over 1052–1616 K and 102–828 torr. With the known E0=ΔH00=55.5 kcal mole −1, the optimized RRKM fit to the ARAS data requires (ΔE)down=590 cm−1.