MCSCF potential energy surface for the high barrier adiabatic 1 4Σ− pathway of the O+(4S)+N2(X 1Σg+) →NO+(X 1Σ+)+N(4S) reaction

Abstract

The collinear ⁴Σ⁻ pathway for the state‐specific O⁺(⁴S)+N₂(X ¹Σ_g⁺) →NO⁺(X ¹Σ⁺)+N(⁴S) reaction has been surveyed with ab initio calculations. A ninety‐nine configuration, fifteen orbital multiconfiguration self‐consistent field (MCSCF) wave function, involving the use of a double‐zeta plus polarization one‐electron basis, was developed for the long range ⁴Σ⁻ state. This long range ⁴Σ⁻ state has the character of O⁺+N₂ for long R_NO, or of N+NO⁺ for long R_NN, and is for most geometries, the lowest, or 1 ⁴Σ⁻, state. The ab initio exothermicity computed with the present wave function is 0.93 eV, compared to an accurate experimental value of 1.10 eV. The saddle‐point in the energy surface is 8.0 eV above O⁺+N₂, with critical values of R*_NN=1.48±0.02 Å and R*_NO =1.38±0.02 Å. These values are 0.38 and 0.32 Å greater than the equilibrium bond lengths of N₂(X ¹Σ_g⁺) and NO⁺(X ¹Σ⁺). The present wave function reproduces the experimental bond lengths of these two diatomics to within 0.01 Å when the third atom is removed to 500 a.u. (264.6 Å). Preceding the barrier on the O⁺+N₂ side is an appreciable polarization well. With the present wave function the attributes of this well are D° (N₂–O⁺) =0.38 eV, R_NN^E=1.108 Å, R_NO^e =2.350 Å, ϑ^e_NNO=180°, k^e_NN=24.679 md/Å, k^e_NO=0.295 md/Å, k_ϑ=0.0366 md/Å. There is also a smaller polarization well on the N+NO⁺ side. Although the ion–atom interaction is more stable at C_s the latter well is bound by 0.02 eV at 180° with R^e_NN=3.34 Å, and R^e_NO=1.07 Å.