Ab initio calculation of the ground (1A′) potential energy surface and theoretical rate constant for the Si+O2→SiO+O reaction

The entrance channel of the Si+O2→SiO+O reaction has been investigated for collinear and perpendicular approach of the silicon atom to the O2 molecule by ab initio electronic structure calculations using the multireference configuration interaction (MRCI) method and Davidson correction (MRCI+Q). Results show that the reaction can proceed through the ground singlet (1A′) and first triplet (3A′) electronic states at low temperatures. The ground A′1 three-dimensional potential energy surface (PES) which correlates the Si(3P)+O2(X 3Σg−) reactants to the SiO(X 1Σ+)+O(1D) products was computed at the MRCI+Q level of theory using the Woon and Dunning cc-pVTZ basis sets. The reaction was found barrierless and three minima have been characterized on the A′1 PES with energy ordering: linear OSiO(1Σg+)<triangular OSiO(1A1)<linear SiOO(1Σ+). About 2500 ab initio data points have been fitted to a many body expansion using the method of Aguado and Paniagua, with a global root-mean-square of 1.49 kcal/mol. The analytical A′1 PES has been used to determine the thermal rate constants in the temperature range 15–300 K by quasiclassical trajectory calculations. Comparison with experimental results shows a quite good agreement for temperature dependence of the rate constants when the spin–orbit structure of the reactants is taken into account. The rate constants are also compared with earlier results of adiabatic capture calculations. The excellent agreement between both theoretical results for temperatures above 50 K points out an increasing contribution of the first triplet state to reactivity when temperature increases.