Rovibrational States of N₃^– and CO₂ Up to High J: A Theoretical Study Beyond fc-CCSD(T)

An accurate near-equilibrium potential energy surface (PES) has been constructed for the azide ion (N₃^–) on the basis of coupled cluster calculations up to CCSDTQ (Kállay, M.; Surján, P. R. J. Chem. Phys. 2001, 115, 2945.), with contributions from inner-shell correlation and special relativity being taken into account as well. A larger number of rovibrational states has been investigated by variational calculations with Watson’s isomorphic Hamiltonian for linear molecules. Analogous calculations for CO₂ demonstrate the high quality of this type of calculations. The G_v values of the symmetric stretching and bending vibration of ¹⁴N₃^– are predicted to be ν₁ = 1307.9 cm^–1 and ν₂ = 629.3 cm^–1, with an uncertainty of ca. 1 cm^–1. Fermi resonance is less pronounced for the lower polyads of ¹⁴N₃^– compared with ¹²C¹⁶O₂ but is as strong as in CO₂ for the lowest diad of isotopologue 15–14–15. The band origin of the antisymmetric stretching vibration of ¹⁴N₃^– is calculated to be ν₃ = 1986.4 cm^–1, only 0.1 cm^–1 lower than the experimental value. The corresponding vibrational transition dipole moment is predicted to be as large as μ = 0.476 D, 46% higher than calculated for CO₂. The perturbed combination tone (01¹1), which was accessible through diode laser IR spectroscopy, undergoes anharmonic interaction with at least two other vibrational states.