Theoretical and experimental studies of the N2O− and N2O ground state potential energy surfaces. Implications for the O−+N2→N2O+e and other processes

Abstract

The ground state potential energy surface of the nitrous oxide negative ion is characterized and related to that of the neutral molecule by a synergetic theoretical–experimental approach. Ab initio multiconfiguration self-consistent-field/configuration interaction (MCSCF/CI) and other calculations for N2O−(X 2A′) yield the minimum energy geometry (ReNN, ReNO, AeNNO) = (1.222±0.05 Å, 1.375±0.10 Å, 132.7±2°), the vibrational frequencies (ν1,ν2,ν3) = (912±100 cm−1, 555±100 cm−1, 1666±100 cm−1), the dipole moment μ =2.42±0.3 D, and other properties. The N2O− molecular ion in the X 2A′ state is found to have a compact electronic wavefunction—one with very little diffuse character. The MCSCF/CI bending potential energy curve from 70° to 180° for the X 1Σ+(1 1A′) state of N2O as well as the bending curve for the X 2A′ state of N2O− are also reported. The dissociation energy D (N2–O−) =0.43±0.1 eV and, thus, the adiabatic electron affinity E.A.(N2O) =0.22±0.1 eV and the dissociation energy D (N–NO−) =5.1±0.1 eV are determined from beam–collision chamber experiments. Corrections are made for both the dispersion in the ion beam and the translational motion of each target gas. The combined theoretical and experimental results yield a vertical electron affinity V.E.A.(N2O) of −2.23±0.2 eV and enable the construction of angular dependent Morse functions to represent the neutral and ionic surfaces. This construction leads to the determination of the minimum intersection locus as (V*, R*NN, R*NO, A*NNO) = (0.67±0.1 eV, 1.18±0.05 Å, 1.28±0.10 Å, 154±3°). The predicted activation energy of this critical point with respect to the asymptote O−, N2—0.21±0.1 eV—and the position of the critical point with R*NN well outside of the N2 (v=0) outer turning point imply that the reaction O−+N2→N2O+e will be strongly facilitated by reagent vibrational excitation.