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

The collinear 4Σ− pathway for the state-specific O+(4S)+N2(X 1Σg+) →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 RNO, or of N+NO+ for long RNN, 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 1Σg+) 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, RNNE=1.108 Å, RNOe =2.350 Å, ϑeNNO=180°, keNN=24.679 md/Å, keNO=0.295 md/Å, kϑ=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 Cs the latter well is bound by 0.02 eV at 180° with ReNN=3.34 Å, and ReNO=1.07 Å.