Saddle point region of the a 4A″ first excited state of N2O

The potential energy surface for the a 4A″ state of N2O+ has been scanned at 120° over the space 1.8⩽RNN⩽3.2 a.u. and 1.8⩽RNO⩽3.0 a.u. at the ab initio SCF and CI levels with double-zeta plus polarization one-electron basis. This surface exhibits a saddle topology cresting at RNNsp=2.29 a.u. and RNNsp=2.51 a.u. in the CI calculations. Within the high interaction region, the minimum energy pathway proceeds smoothly from RNN≃Re(N2), at RNO=3.0 a.u., through a remarkably elongated saddle-point region to RNO≃Re(NO+), at RNN=3.2 a.u. A bending curve for RNN=RNO=2.2 a.u. starting from ϑNNO=120° and encompassing angles up to 180° and down to 90° has shown that (a) the Cs(C∞v) state correspondence is a A″(1 4Π) and (b) the purely angular barrier to the collinear configuration is 2.6 eV, relative to a most stable saddle-point angle of ϑNNsp= 118°.These results are discussed in relation to the electronic structure of N2O+, especially the X 2Π and A 2Σ+ states; and the O+(N2, N) NO+ reaction. Two principle conclusions are established on the strength of the SCF/CI results: (a) the a 4A″ state is the first adiabatically excited state of the N2O+ molecular ion and (b) the potential energy hypersurface for this state provides a smooth, adiabatic, low-barrier Cs pathway for the ion-molecule reaction. Incorporation of the MCSCF/CI results described below leads to the establishment of a lower limit to the a 4A″ potential barrier to reaction: 3.9 kcal/mol. It is apparently the availability of this novel pathway which causes the O+(4S)+N2→NO++N reaction to play such a critical role in ionospheric chemistry. The excitation energies 1 4Π←X 2Π and A 2Σ+←X 2Π are obtained by optimizing MCSCF and MCSCF/CI wave functions in a double-zeta plus polarization one-electron basis for each of these three states at the N2O(X 1Σ+) geometry. The resulting Te(A←X) transition energy is calculated to be 4.17 eV, compared to the zero-point corrected spectroscopic value of 3.42 eV. The Te(1 4Π←X 2Π) spacing is predicted to be 6.56 eV with an estimated uncertainty of 0.75 eV. No Te(1 4Π←X win 02Π) experimental value is available for comparison.