A high-resolution near infrared spectrum of the CrCl radical has been recorded in thermal emission with Fourier transform techniques in the interval from 6 900 cm−1 to 11 500 cm−1. Two dominating band systems have been found in this region; (1) The A 6Σ+→X 6Σ+ system, analyzed in the present work, consisting of the (0,0), (0,1), (1,0), (2,0), and (3,0) bands, the (0,0) band being located at around 9450 cm−1; (2) the B 6Π→X 6Σ+ system, with a band interpreted as (0,0), located between 8700 cm−1 and 9000 cm−1, overlapping the (0,1) band of the A 6Σ→X 6Σ+ system. A rotational analysis of the A 6Σ+→X 6Σ+ system has been carried out, and the following principal parameters (cm−1) have been derived: X 6Σ+: ωe=396.6621, Be=0.167 587 3, De=1.1835⋅10−7; A 6Σ+: ωe=379.39, Be=0.156 51. Local perturbations in the A 6Σ+ (v=0,1) levels have been attributed to interactions with the B 6Π (v=1,2) levels. In the present work, we have constrained the investigations on the B 6Π→X 6Σ+ system to the verification that the lower state is indeed X 6Σ+. This verification has been achieved through successful matchings of branches in that system to known combination differences of X 6Σ+. The assignment of the upper state as B 6Π is based on the characteristic appearance of the band. Ligand field and density functional calculations have been performed on CrF and CrCl. The results from these calculations indicate that the approximate structure of the B 6Π and 1 6Δ excited states of CrF and CrCl is Cr+[3d44s(6D)]X−, while for the X 6Σ+ ground state in both molecules it is Cr+[3d44s(6D)∼3d5(6S)]X−. A combined DFT/LFT treatment has been carried out on the Cr+ 3d5and 3d44s configurations of both molecules in order to calculate the location of the A 6Σ+ state and of a number of low-lying states in the quartet manifold.

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.