The Electronic Structure of Boron, Aluminum, and Scandium Monoxides: BO, AlO, and ScO

The diatomics BO, AlO, and ScO play roles in the combustion of the parent atoms. Potential energy curves for the lowest-lying spin-free (ΛS) and spin-orbit (Ω) states of gas-phase BO, AlO, and ScO were calculated with the SA-CASSCF/SO-icMRCI+Q/aug-cc-pwCVnZ-DK method (n = Q for ScO and n = 5 for BO and AlO). The spectroscopic parameters obtained at the icMRCI+Q level are consistent with the available experimental data, and new data for the spin-orbit states are presented. At the CCSD(T) level, the use of reference PW91 orbitals resulted in a significant improvement of the spectroscopic parameters for AlO and small improvements for BO and ScO. The dissociation energies were predicted at the icMRCI+Q and Feller-Peterson-Dixon (FPD) levels, including CCSDT, CCSDTQ, and spin-orbit effects. For BO, AlO, and ScO, the FPD D₀ (D_e) values are 192.4 (195.1), 120.2 (121.6), and 158.2 (159.6) kcal/mol, respectively. NBO analysis shows that the ionic character increases in going from BO to AlO and slightly decreases in going from AlO to ScO, with two highly polarized π bonds and one σ bond. A pronounced multireference character is predicted for AlO at the CCSD(T) and SA-CASSCF/icMRCI+Q levels. The lowest excited states of BO and AlO are mostly formed from excitations on the 1π orbitals. For ScO, excitations on the 2σ orbital are the most significant to form the lowest excited states.