Ab initio and reaction dynamic calculations for three-atom reactions using potential energy surface and reaction coordinate calculations using relativistic effective core potentials.

Abstract

Potential energy surfaces are generated for the reactions of Ca + $H_2$ → CaH + H, Ti + NO → TiO + N, Ti + $O_2$ → TiO + O, Sc+NO → ScO+N, Sc + O$_2$ → ScO + O, LiH + H → Li + $H_2$ using ab initio method. Six $^1$A` potential energy surfaces of the Ca + $H_2$ system have been calculated using an effective-core potential and a core-polarization potential for Ca atom. The 2 $^1A`$ surface, made from Ca(4s3d, $^1D$), has a very stable intermediate, and crosses with the ground state (1 $^1A`$) potential energy surface in $C_{2v}$ geometry. A diabatic coupling between these two surfaces can result in the CaH product. For the reaction between transition metals and oxidant molecules, charge transfer from metal to oxidant molecule occurred and energy barrier is present, coming from the ionic-neutral potential energy surface crossing. The end-on attack appeared to be the most efficient for the reaction of Ti + NO → TiO + N and Sc+NO → ScO+N, while the side-on attack is the most efficient for the reaction of $Ti + O_2 → TiO + O$ and $Sc + O_2 → ScO + O$. Trajectory calculations on the interpolated potential energy surface which was made by IMLS/Shepard interpolation scheme illustrate that the initial vibrational content of LiH plays a rather minor role in the sense that the reactive cross section depends little on it, and the reactive cross section has maximum value, when the initial collision energy is about 0.1 eV. The endothermic reaction of $Li + H_2 → LiH + H$ requires highly excited $H_2$ molecules for it to occur at low collision energy (ν≥ 4), and it is more efficient to put the same amount energy into the $H_2$ vibration energy rather than translational energy. IRC and DRC calculations were performed and their results are analyzed by vibration modes of reactants and products. Our works show that reaction mechanisms and some essential features can be predicted by analyzing the potential surfaces or reaction dynamic calculations.