KINETIC AND MECHANISTIC STUDY OF THE SELF-REACTION OF CH3OCH2O2 RADICALS AT ROOM-TEMPERATURE

The UV absorption spectrum and kinetics of the self reaction of CH3OCH2O2 at 298 K have been studied using both the modulated photolysis of Cl2/CH3OCH3/O2/N2 mixtures and the pulse radiolysis of SF6/CH3OCH3/O2 mixtures. The spectrum, characterized in the range 200-290 nm, is in good agreement with the single published determination.8 The observed second-order removal kinetics of CH3OCH2O2, k5obs, were found to be sensitive to both the variation of total pressure (17-760 Torr) and the composition of the reaction mixtures: 2CH3OCH2O2 --> 2CH3OCH2O + O2 (5a); --> CH3OCHO + CH3OCH2OH + O2 (5b). The kinetic studies and a detailed product investigation using long path length FTIR spectroscopy (T = 295 K; Cl2/CH3OCH3/O2/N2 system) provide evidence to support a mechanism involving the rapid thermal decomposition of CH3-OCH2O by H atom ejection occurring in competition with the reaction with O2: CH3OCH2O(+M)-->CH3OCHO + H (+M) (6); CH3OCH2O + O2 --> CH3OCHO + HO2 (4). The complications in the measured values of k5obs in the present studies, and those reported previously,8 are believed to occur as a direct result of formation of H atoms from reaction 6. Accordingly, a pressure-independent value of k5 = (2.1 +/- 0.3) X 10(-12) cm3 molecule-1 s-1 is derived for the elementary rate coefficient at 298 K, with identical values of the branching ratio alpha = k5a/k5 = 0.7 +/- 0.1 determined independently from the FTIR product studies and the modulated photolysis experiments. As part of this work, the rate coefficient for the reaction of Cl atoms with CH3OCH2Cl was found to be (2.9 +/- 0.2) x 10(-11) cm3 molecule(-1) s-1.