The electronic structure of LaO: Ligand field versus ab initio calculations

Abstract

The potentially pathological example of LaO has been chosen to test the application of ligand field theory (LFT) to metal monoxides. The test consists of a comparison of closed-shell ligand LFT (CSLLFT) results (in which a 2+/2− ionic M2+O2− model is a priori postulated with a point-charge ligand) against ab initio multiconfiguration self-consistent-field–multireference configuration interaction (MCSCF-MRCI) results (in which no ionicity is assumed a priori and an internal structure is allowed for the ligand). Special care has been devoted to the determination of a La3+ pseudopotential and its associated atomic basis set in order to keep the ab initio model close to the LFT one, yet at the same time capable of clearly exhibiting the consequences and the importance of the restrictions imposed in the CSLLFT model. The ab initio calculations reveal that the effective (Mulliken) ionicity in LaO is not La2+O2− but quite close to La+O−. Despite this, the (2+/2−) ionic CSLLFT model leads to the correct orbital occupations in the ground state but this model cannot account for the significant covalency contribution via the nominal 2pσ and 2pπ oxygen orbitals. The CSLLFT calculations correctly reproduce the excitation energies of the four lowest-lying observed states of LaO: these states are calculated to within 2000 cm−1 and globally better than via ab initio calculations. However, the (2+/2−) ionic CSLLFT model is shown to ignore the existence of a manifold of low-lying quartet (and doublet) states of (1+/1−) ionicity that might be relevant for interpreting features of the spectrum. This result exemplifies the need for developing, beyond the first attempts made in this direction, a computationally manageable open-shell ligand theory for the frequently encountered case of predominantly ionic structures with an open-shell ligand.

High-Resolution Spectroscopy of TaS in the Visible and Near-Infrared Regions

“…The ground state and the low-lying excited doublet states in LaS can be derived from the following configurations using the scheme of molecular orbitals given above: X 4 19*). The same types of states from similar configurations are also true for LaO (1,5).…”

“…For the first two elements La and Hf in the 5d transition period the spectra of LaO, HfO, and HfS are well known, and the ground state and the low-lying excited states of these molecules can be classified and derived from a simple scheme of molecular orbitals (5,6). In Ref.…”

Electron Affinities of the Early Lanthanide Monoxide Molecules

“…The results are summarized in Table I. [31,39,55], i.e., the transition from the ground vibronic state of the LaO − to the ground vibronic state of LaO. The ligand field theory predicated that the ground state of neutral LaO molecule has an electronic configuration of σ 2 σ * 1 f 0 (Table I) [22], in which the σ * 1 f 0 part is derived from the free La 2+ 6s4f 0 configuration.…”

The permanent electric dipole moments for the A 2Π and B 2Σ+ states and the hyperfine interactions in the A 2Π state of lanthanum monoxide, LaO

“…The variation of the permanent electric dipole moment amongst the four low-lying states is primarily a reflection of the effect of variation in the polarizability of the singly occupied orbital. The most extensive theoretical prediction for the ground and excited state properties of LaO was made by Schamps et al 1 in which they compared electronic state distributions and orbital compositions predicted by ligand field theory ͑LFT͒ with those from an ab initio multiconfiguration selfconsistent-field-multireference configuration interaction ͑MCSCF-MRCI͒ calculation. Interestingly, the LFT, which considers the electrostatic relaxation of the valence electrons when the La ϩ2 and O Ϫ2 point charges are placed at the equilibrium internuclear distances, predicted the vertical excitation energies for the AЈ 2 ⌬ r , A 2 ⌸, and B 2 ⌺ ϩ states somewhat more accurately than the MCSCF-MRCI prediction.…”