MRCI study of the lowest electronic states of O₂H⁺

Abstract

The lowest electronic states of O₂H⁺ are investigated using ab initio calculations at the internally contracted multireference configuration interaction (MRCI) level. For this purpose, all the CSFs built with the full valence space have been selected as multireferential space and all the valence electrons have been correlated. With this approach, four electronic states, ³A^″, ¹A^″', ¹A^″ and ¹A^′ have been found under 22000cm⁻¹. They can be related to the X³Σ_g ⁻, a ¹Δ_g and b ¹Σ⁺ _g states of molecular oxygen. The bent triplet A^″ has been identified as the molecular ground electronic state although a singlet A′ state lies close at 1500 cm⁻¹. The proximity of these two states can explain the observed magnetic properties of O₂H⁺. In order to characterize the calculated states, several electric properties of O₂H⁺ have been evaluated with MRCO. The four potential energy surfaces have been generated by fitting the MRCI energies of 83 structures to products of Morse oscillators and Fourier functions. Relative energies up to 13000 cm⁻¹ have been included in the fitting. The equilibrium geometries of X³A^″ and a ¹A′, determined from the fitted functions, appear more or less identical, although they show different electric properties.