Kinetics of the reactions of isopropoxy radicals with NO, NO2, and O2

Abstract

A fluorescence excitation spectrum of (CH3)2CHO (isopropoxy radical) is reported following photolysis of isopropyl nitrite at 355 nm. Rate constants for the reaction of isopropoxy with NO, NO2, and O2 have been measured as a function of pressure (1–50 Torr) and temperature (25–110°C) by monitoring isopropoxy radical concentrations using laser-induced fluorescence. We have obtained the following Arrhenius expressions for the reaction of isopropoxy with NO and O2 respectively: (1.22±0.28)×10−11 exp[(+0.62±0.14 kcal)/RT]cm2/s and (1.51±0.70)×10−14 exp[(−0.39±0.28)kcal/RT]cm3/s where the uncertainties represent 2σ. The results with NO2 are more complex, but indicate that reaction with NO2 proceeds more rapidly than with NO contrary to previous reports. The pressure dependence of the thermal decomposition of the isopropoxy radical was studied at 104 and 133°C over a 300 Torr range using nitrogen as a buffer gas. The reaction is in the fall-off region over the entire range. Upper limits for the reaction of isopropoxy with acetaldehyde, isobutane, ethylene, and trimethyl ethylene are reported. We have performed the first LIF study of the isopropoxy radical. Arrhenius parameters were measured for the reaction of i-PrO with O2, NO, NO2, using direct radical measurement techniques. All reactions are in their high-pressure limits at a few Torr of pressure. The rate constant for the reactions of i-PrO with NO and NO2 reactions exhibit a small negative activation energy. Studies of the i-PrO + NO2 reaction produce data which indicate that O(3P) reacts rapidly with i-PrO. Unimolecular decomposition studies of i-PrO indicate that the reaction is in the fall-off region between 1 and 300 Torr of N2 and the high-pressure limit is above 1 atmosphere of N2.