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

Benjamin Mintz, Bun Chan, Michael B. Sullivan, Thomas Buesgen, Anthony P. Scott, Steven R. Kass, Leo Radom and Angela K. Wilson

Publication Date (Web): August 3, 2009

J. Phys. Chem. A, 2009, 113, 9501-9510.

ABSTRACT:

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 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⁻.

DOI: 10.1021/jp9034826