Na2O and Na2O+:  Thermodynamics and Low-Lying Electronic States

High-level [up to RCCSD(T)/aug-cc-pV5Z‘//RCCSD/aug-cc-pVQZ‘] ab initio calculations are performed on Na2O and Na2O+ (where the ‘ indicates a nonstandard aug-cc-pVXZ basis set). The electronic ground-state equilibrium geometries are found to be linear in both cases. The heats of formation, ΔH f 298(Na2O) and ΔH f 298(Na2O+), are derived, with the recommended values being ΔH f 298(Na2O) = −5.5 ± 1.0 kcal mol-1 and ΔH f 298(Na2O+) = 106.7 ± 0.5 kcal mol-1, respectively. It is found that large basis sets, the RCCSD(T) method, and appropriate account of basis set superposition error (BSSE) are necessary in order to obtain reliable results. S 298 for Na2O and Na2O+ are also calculated giving 67 and 63 cal K-1 mol-1, respectively. The calculations also yield ΔH a 298(Na2O) = 115.5 ± 1 kcal mol-1 and D 0(NaO···Na) = 52 ± 1 kcal mol-1. The adiabatic ionization energy of Na2O is calculated as 4.86 ± 0.02 eV and corresponds to the process Na2O+(X2Πu) + e- ← Na2O(XΣg +); the corresponding vertical ionization energy is calculated to be 5.00 ± 0.02 eV. The ground state of Na2O+ is established as the 2Πu state, with the AΣu + state lying ca. 0.65 eV above. The low-lying triplet states of Na2O are briefly considered. The implication of the present results on the mass spectrometry of the vapor above Na2O is discussed.