The sodium superoxide radical: X̃ 2A 2 and à 2B 2 potential energy surfaces

Abstract

The two lowest electronic states of NaO 2 have been studied using ab initio methods, including RHF SCF, CISD, Davidson corrected CISD, UHF SCF, UMP2-UMP4, CASSCF, and CISD on CASSCF based natural orbitals, with basis sets ranging in quality from Na(11s7p/7s5p), O(9s5pld/5s3pld) to Na(13s10p2dlf/7s6p2dlf), O(11s7p2dlf/6s4p2dlf), i.e. from TZ+d to TZ2P+f+R. Total and relative energies, geometries, vibrational frequencies, and dipole moments of stationary points on the X̃ 2A 2 and à 2B 2 surfaces have been determined. A C 2v minimum is proposed for X̃ 2A 2 NaO 2 with r e (OO)=1.335 and r e(NaO)=2.10 Å. The analogous structure for the à 2B 2 state is predicted to have r e(OO)=1.34 and r e(NAO)=2.13 Å with an adiabatic excitation energy of T e=8.5 kcal mol −1. Linear, 2ΠNaOO, which correlates to the 2A 2 and 2B 2 minima via C s transition states, is predicted to be a shallow minimum lying 17 kcal mol −1 above the X̃ 2A 2 structure but with a barrier to rearrangement of less than 1 kcal mol −1. The dissociation energy of the ground state of NaO 2 is deduced to be D 0=38 kcal mol −1, a value substantially lower than recent experiment estimates.