Ab Initio and DFT Direct Dynamics Studies on the Reaction Path and Rate Constant of the Hydrogen Abstraction Reaction: SiH3F + H → SiH2F + H2

Abstract

Ab initio and density functional theory (DFT) direct dynamics methods have been used to study the title reaction, and the results of the two methods have been fully compared. First, electronic structure information, including geometries, gradients, and force constants (Hessians), are calculated using ab initio UQCISD and DFT BHLYP methods with the 6-311+G** basis set. Furthermore, the energies of some selected points along the minimum-energy path (MEP) are improved by a series of single-point G2//QCISD and PMP4/6-311+G(3df,2p)//BHLYP calculations. Then, the changes of the geometries, generalized normal-mode vibratioanal frequencies, potential energies, and total curvature along the MEP of the two methods are discussed and compared. Finally, the reaction rate constants within 200−3000 K are calculated by canonical variational transition-state theory with the small-curvature tunneling correction (CVT/SCT) method. The results show that the DFT method is good enough in comparison with the ab initio method for the reaction to obtain satisfactory reaction rate constants. The variational effect is small, and in the lower-temperature range, the small-curvature tunneling effect is important for the reaction. Fluorine substitution decreases the reactivity of the Si−H bond toward H atom attack, and the decrease in k/n mainly stems from a corresponding increase in activation energy. The reaction rate constant based on the scaled PMP4/6-311+G(3df,2p)//BHLYP potential energy curve is in the excellent agreement with the experimental value at 293 K, and the rate constants within 200−3000 K are fitted to be a three-parameter expression: k = 3.3 × 105T2.60 exp(−737/T) cm3 mol-1 s-1.