Electronic and Molecular Structures of the CeB₆ Monomer

Abstract

The electronic and molecular structure of the CeB 6 molecular unit has been probed by anion PE spectroscopy and DFT calculations to gain insight into structural and electronic relaxation on edge and corner sites of this ionic material. While boron in bulk lanthanide hexaboride materials assumes octahedral B 6 3− units, the monomer assumes a less compact structure to delocalize the charge. Two competitive molecular structures were identified for the anion and neutral species, which include a boat-like structure and a planar or near-planar teardrop structure. Ce adopts different orbital occupancies in the two isomers; the boat-like structure has a 4f superconfiguration while the teardrop favors a 4f 6s occupancy. The B 6 ligand in these structures carries a charge of −4 and −3, respectively. The teardrop structure, which was calculated to be isoenergetic with the boat structure, was most consistent with the experimental spectrum. B 6 -local orbitals crowd the energy window between the Ce 4f and 6s (HOMO) orbitals. A low-lying transition from the B-based orbitals is observed slightly less than 1 eV above the ground state. The results suggest that edge and corner conductivity involves stabilized, highly diffuse 6s orbitals or bands rather than the bulk-favored 5d band. High-spin and open-shell low-spin states were calculated to be very close in energy for both the anion and neutral, a characteristic that reflects how decoupled the 4f electron is from the B 6 2p-and Ce 6s-based molecular orbitals.

Photoelectron Spectra of Gd2O2− and Non-Monotonic Photon-Energy Dependent Variations in Populations of Close-Lying Neutral States

“…Transitions in manifold I in the 3.495 eV PE spectrum: Based on previous work, [21][22][23][24][25][26][27][28][29] homo-and heteronuclear lanthanide oxide cluster anions with average metal oxidation state less than +3 exhibit photodetachment transitions around e − BE  1 eV. The transitions can be described as creating a hole in a 6s-based molecular orbital.…”

∆SCF Dyson Orbitals and Pole Strengths from Natural Ionization Orbitals

“…The NIO model provides a compact orbital representation of ionization processes by using the difference of one-particle density matrices and provides a means for interpreting and characterizing electronic detachment processes, including distinguishing between one-electron transitions and shake-up/shake-off transitions. 20,21,[23][24][25] In the case of electron detachment, the difference density matrix, ∆ P , can be calculated for a vertical detachment according to…”

“…Work from our lab in collaboration with experimental colleagues considering such experiments, for example, has led to detailed understanding of a number of transition metal and inner-transition metal containing systems. [19][20][21][22][23][24][25] Particularly because electron number changes in electron detachment and electron attachment, the quality of electronic structure theory studies of such processes can be quite sensitive to electron correlation treatment. [26][27][28] Early approaches specifically designed to address this concern implemented wave function models beyond the Hartree-Fock (HF) self-consistent field (SCF) model.…”

“…As our own work has shown, access to these additional handles is critical for discriminating between spectral candidates and making definitive spectral assignments. [19][20][21][22][23][24] A particular advantage of EPT models in electron detachment/attachment studies has been their direct relationship to the Dyson orbital framework. Dyson orbitals, also referred to as generalized overlap amplitudes, are defined as the overlap between n el and n el = (n el −1) wave functions.…”

“…Indeed, over the last few years we have employed NIO analysis in studies of complicated photoelectron spectra of transition metal and lanthanide containing clusters. 20,21,[23][24][25]83,84 In this report, we develop a formal relationship between the NIO model and the Dyson orbital theory. Importantly, this work establishes a clear and concise connection between the 71,85 More recently, our lab proposed an orbital description for electron detachment processes studied with ΔSCF approaches based on a natural orbital transformation of the difference density from initial and final states, which we named the NIO model.…”