A b i n i t i o multiple root optimization MCSCF study of the C∞v/C s excitation spectra and potential energy surfaces of N2O

Abstract

This paper presents the results from extensive theoretical studies of the electronic structure for nitrous oxide. These studies have included the development of correlated ab initio MCSCF/CI wave functions for each of some 30 states, including MCSCF excited root/same symmetry calculations where necessary. Also, in the case of seven lower states, the potential energy hypersurfaces and dipole moment functions have been scanned using both MCSCF/CI and SCF wave functions. Vertical spectrum calculations are the most accurate and have been carried out at several levels of sophistication, including SCF, MCSCF, and MCSCF/CI correlation for DZ, DZ+diffuse, DZ+d, and DZ+d+diffuse one‐electron bases. From all of these calculations the adiabatic excitation spectrum is established and the behavior of a number of states upon dissociation under C_∞v and C_s symmetry is examined. Adiabatic correlation diagrams are then constructed to summarize many of the results and to relate them to the asymptotic spectra O(k^sL) +N₂ ( j^sΛ) and NO( j^sΛ) +N(k^sL). In the light of the present work, optical and electron impact data are interpreted and assigned. It is found that each feature observed in the 4–8 eV region is supported by an electronic transition (s) allowed only under C_s symmetry. These features derive such intensity as they have in consequence of vibrational motion of the molecule in its normal, linear ground state. In distinct contrast, the features observed above 8 eV are shown to involve vertical excitations allowed under C_∞v as well as C_s symmetry. Several of these transitions have not been assigned previously. For others, such as the all important D¹ Σ⁺ state, the present results provide the necessary hard evidence for making an assignment. Also reported are correlated ab initio, DZ+d treatments for the C_2v isomer 1 ¹A₁ (1 ¹A′) of normal nitrous oxide, and for the O–N₂ van der Waals interaction. The 1 ¹A₁, or ‘‘ring,’’ state is shown to be bound relative to O(¹D)+N₂ by some 1.2 eV. The O(³P)–N₂ long range 1 ³Π ab initio interaction potential is reasonably well fit by an exp‐6 functional form V(r)=37.757 exp(−r/0.566 569)−47.139 r⁻⁶ in a.u.