ChemInform Abstract: UV‐Absorption Spectrum of the Methylperoxy Radical and the Kinetics of Its Disproportionation Reaction at 300 K.

Abstract

Molecular modulation spectroscopy combined with ultraviolet spectroscopic techniques have been used to observe the behavior of the CH 3 O 2 radicals generated in the gas phase by near-ultraviolet modulated photolysis of flowing Cl 2 -CH 4 -O 2 mixture. The kinetics of the disproportionation reaction (1) (1(a)) CH 3 O 2 +CH 3 O 2 →CH 3 O+CH 3 O+O 2 (1(b)) →CH 2 O+CH 3 OH+O 2 (1(c)) →CH 3 OOCH 3 +O 2 and the absorption cross-sections of CH 3 O 2 were measured by computer fitting of the modulated absorption traces obtained in the wavelength range 220 to 270 nm at 300 K and 240 torr. The rate constant for the elementary self-reaction k 1 =k 1(a) +k 1(b) +k 1(c) was determined to be (3.61±0.55)×10 −13 cm 3 molecules −1 s −1 . The parameter k obs /σ (where k obs is the observed apparent second-order rate constant) was measured from the decay curves in the dark phase of the modulated photolysis period in the wavelength range 230-260 nm, and had a value 1.16×10 5 cm s −1 at 250 nm. At 250 nm the absorption cross-section was determined as σ(CH 3 O 2 )=4.14×10 −18 cm 2 molecule −1 , leading to a value of k obs =(4.8±0.5)×10 −13 cm 3 molecule −1 s −1 . In addition, the absorption spectrum of CH 3 O 2 was measured in the range 210-295 nm using diode array spectroscopy. A detailed review of all previous studies concerning the kinetics and spectrum of the CH 3 O 2 radical is presented, and a recommended spectrum, representing an average from selected recent studies, is proposed