Structures and thermochemistry of the alkali metal monoxide anions, monoxide radicals, and hydroxides

The geometries, enthalpies of formation (ΔH° _f), separations of electronic states, electron affinities, gas-phase acidities, and bond dissociation energies associated with the alkali metal monoxide anions (MO ^-), monoxide radicals (MO ^.), and hydroxides (MOH) (M = Li, Na, and K) have been investigated using single-reference and multireference variants of the WnC procedures. Our best estimates of the ΔH° _f values for the ground states at 298 K are as follows: 8.5 ( ³π LiO ^-), 48.5 ( ²π LiO), -243.4 ( ¹σ ⁺ LiOH), 34.2 ( ³π NaO ^-), 86.4 ( ²π NaO ^.), -190.8 ( ¹σ ⁺ NaOH), 15.1 ( ¹σ ⁺ KO ^-), 55.9 ( ²σ ⁺ KO ^.), and -227.0 ( ¹σ+ KOH) kJ mol ^-1. While the LiO ^. and NaO ^. radicals have ²π ground states, for KO ^., the ²π+ and ²π electronic states lie very close in energy, with our best estimate being a preference for the ²σ ⁺ state by 1.1 kJ mol ^-1 at 0 K. In a similar manner, the ground state for MO ^- changes from ³π for LiO ^- and NaO ^- to ¹π ⁺ for KO ^-. The ¹σ ⁺ state of KO ^- is indicated by the calculated ₁ diagnostic and the SCF contribution to the total atomization energy to have a significant degree of multireference character. This leads to a difference of more than 100 kJ mol ^-1 between the single-reference W2C and multireference W2C-CAS-ACPF and W2C-CAS-AQCC estimates for the ¹σ ⁺ δH° _f for KO _-.