Direct kinetic study of methoxy radical reacting with NO 2 and O2 and deuterium kinetic isotope effects

Alkoxy radicals (RO•) are important intermediates in the photooxidation of volatile organic compounds (VOCs) in the troposphere. The competition between their three fates (unimolecular decomposition, unimolecular isomerization, and reaction with O2) greatly impacts the formation of hazardous tropospheric ozone (O3) and secondary organic aerosols (SOAs). To date, direct kinetic studies of RO• + O2 have been limited to alkoxy radicals derived from select C1-C7 alkanes and two halogenated alkanes. The rate constants of O2 reactions are unknown for alkoxy radicals derived from oxygenated VOCs or non-alkane hydrocarbons. This makes it difficult to build or validate structure-activity relations (SARs) for the reactions of alkoxy radicals with O2.;The kinetics and mechanism of the methoxy + O2 reaction is the prototype for other RO• + O2 reactions. I investigated the temperature-dependent kinetics and deuterium isotope effects of the reaction of methoxy + O2 at pressures up to 700 Torr of N2 and over 250 -- 333 K. By combining my absolute rate measurement for CH 3O• + NO2 with our group's relative rate measurement for CH3O• + O2/NO2, we obtained k O2 as 1.3--5+0.9 x 10--14 exp[-(663 +/- 144)/T] cm3 molecule --1 sec--1, corresponding to 1.4 x 10--15 cm3 molecule-1 sec--1 at 298 K. The rate constant at 298 K is in excellent agreement with previous work, but the observed temperature dependence is less than previously reported. The deuterium isotope effect, kCH3O•+O2 / kCD3O•+O2, can be expressed in Arrhenius form as 1.6--0.9+2.1 x exp[(312 +/- 255)/T], which provides insights into the effect of tunneling on the CH3O + O2 rate constant.;The reaction of methoxy + NO2 is a typical radical-radical recombination reaction in the atmosphere. Pressure dependent kinetic data for this reaction provides information to investigate collisional energy transfer. I studied the pressure dependent (30 -- 700 Torr of N2) rate constant for CH3O• (CD3O•) + NO2 over 250 -- 333 K. The low pressure limiting rate constants are k CH3O+NO20 = 4.29--0.37+0.40 x 10--29 (T/298)-(1.65 +/- 1.11) and kCD3O+NO20 =9.97--0.91 +1.00 x 10--29 (T/298)-(4.79 +/- 0.92) cm6 molecule--2 sec--1 respectively. The high pressure limiting rate constants are given by kCH3O+NO2infinity = (1.95 +/- 0.03) x 10--11 (T/298)-(1.13+/- 0.18) and kCD3O+NO2infinity = (1.91 +/- 0.02) x 10--11 (T/298)-(1.11 +/- 0.09) cm3 molecule--1 sec--1 respectively. The rate constants for the two isotopologues track each other closely as the high pressure limit is approached.