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

Abstract

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 O2 is found to form mainly OH radicals, 2,2-dimethoxy-1,3-dioxetane, dimethyl carbonate [(CH3O)2CO] and HC(O)H. Regeneration of OH radicals for the TMOF + OH + O2 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 × 10−12 and 1.58 × 10−12 cm3 molecule−1 s−1, respectively. It is found that ∼71% of the TMOF + OH reaction takes place through the CH site H-abstraction and ∼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 (298–1500 K) = 1.02 × 10−13 × (T/298)3.4 × exp(9885/RT) cm3 molecule−1 s−1.