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

The geometries, enthalpies of formation (Delta H-f(0)), 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 Delta H-f(0) values for the ground states at 298 K are as follows: 8.5 ((3)Pi LiO-), 48.5 ((2)Pi LiO+), -243.4 ((1)Sigma(+) LiOH), 34.2 ((3)Pi NaO), 86.4 ((2)Pi NaO+), -190.8 ((1)Sigma(+) NaOH), 15.1 ((1)Sigma(+) KO-), 55.9 ((2)Sigma(+) KO+), and -227.0 ((1)Sigma(+) KOH) kJ mol(-1). While the LiO+ and NaO+ radicals have (2)Pi ground states, for KO+, the (2)Sigma(+) and (2)Pi electronic states lie very close in energy, with our best estimate being a preference for the (2)Sigma(+) state by 1.1 kJ mol(-1) at 0 K. In a similar manner, the ground state for MO- changes from (3)Pi for LiO- and NaO- to (1)Sigma(+) for KO-. The (1)Sigma(+) 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 (1)Sigma(+) Delta H-f(0), for KO-.