Structures and Thermochemistry of the Alkali Metal Monoxide Anions, Monoxide Radicals, and Hydroxides

The geometries, enthalpies of formation (ΔH o 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 o 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 2Σ+ 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 1Σ+ for KO−. The 1Σ+ state of KO− is indicated by the calculated T 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 o f for KO−.