Theoretical study of the reaction mechanism and kinetics of the OH plus trimethyl orthoformate ((CH3O)3CH) + O2 reaction

The profiles of potential energy surface (PES) of Trimethyl orthoformate (TMOF) with OH radicals under atmospheric conditions have been studied by performing M06-2X-GD3/6-311 + +G(d,p) approach for geometry optimization and ab initio method QCISD (T)/6-311 + +G(d,p) for energy calculations. The reaction of the OH radicals with TMOF in the presence of O-2 is found to form mainly OH radicals, 2,2-dimethoxy-1,3-dioxetane, dimethyl carbonate [(CH3O)(2)CO] and HC(O)H. Regeneration of OH radicals for the TMOF + OH + O-2 reaction is verified in the present calculations, which is consistent with the experimental observations. Based on the PES of the TMOF + OH reaction, temperature-dependent rate coefficients and branching ratios at 298-1500 K are determined using the conventional transition state theory. At 298 K, the calculated individual rate coefficients for H-abstraction from the -CH- site and the CH3O- site of TMOF are 3.90 x 10(-12) and 1.58 x 10(-12) cm(3) molecule(-1) s(-1), respectively. It is found that similar to 71% of the TMOF + OH reaction takes place through the -CH- site H-abstraction and similar to 29% of the TMOF + OH reaction belongs to the CH3O- site H-abstraction. The overall rate coefficient displays opposite temperature dependencies at low and high temperatures, viz., negative at low temperatures and positive at high temperatures. In the temperature range of 298-1500 K, the calculated overall rate coefficients can be expressed by the three-parameter Arrhenius equation: k (298-1500K) = 1.02 x 10(-13) x (T/298)(3.4) x exp(9885/RT) cm(3) molecule(-1) s(-1).