Homopolyatomic Bismuth Ions, Part 2 Electronic Excitations in Homopolyatomic Bismuth Cations: Spectroscopic Measurements in Molten Salts and an ab initio CI-Singles Study

Abstract: The electronic excitations of the low-valence bismuth cluster cations Bi 5 3 , Bi 8 2 , and Bi 9 5 have been studied with experimental and theoretical techniques. The UV-visible spectra of the bismuth ions were measured in acidic chloroaluminate melts (mixture of 1-methyl-3-benzyl imidazolium chloride and AlCl 3 ). The spectra of the Bi 5 3 and Bi 8 2 ions agree fairly well with previous reports, but also revealed additional low-energy absorptions. Ab initio methods were employed to assign the experimentally observed electronic transitions of these homopolyatomic bismuth cations. Structures were optimized at the RHF, MP2, and B3LYP levels of theory by using split-valence LANL2DZ basis sets that were augmented with one and two sets of pure d functions. The computed structures agree well with the results of neutron diffraction analyses of melts. Electronically excited states of the three clusters were treated by using the CI-Singles theory. The results of these calculations were used to explain the observed UV-visible spectra. The observed electronic excitations in the UV-visible range are all found to result from transitions involving the molecular orbitals formed by 6p-atomic-orbital overlap. This leads to the necessity of using basis sets that include d-type functions, which allow for an adequate description of the bonding that results from such p-orbital overlap. Spin-orbit coupling becomes increasingly important with increasing atomic number and its consideration is necessary when describing the electronic transitions in clusters of heavy atoms. The calculations show that singlet ± triplet transitions, which are made accessible by strong spin-orbit coupling, are responsible for some of the observed absorptions

Neutron Diffraction of Homopolyatomic Bismuth Ions in Liquid Bi₅(AlCl₄)₃ and ab initio Study of the Structure and Bonding of the Isolated Bi₅³⁺ Ion

Time-of-flight neutron diffraction measurements were carried out at 350 °C and 450 ± 5 °C for molten Bi5(AlCl4)3. Contributions from AlCl4- were estimated using molten LiAlCl4 data and yielded radial distribution functions RDFrem(r) that allowed for the determination of the D3h-Bi53+ structure:  d(Bia-Bie) = 3.2 Å and d(Bie-Bie) = 3.4 Å. The functions RDFrem(r) were simulated successfully by a model of intermediate range order similar to the environment in Bi5(AlCl4)3 crystals. The first sharp diffraction peak (FSDP) occurs at ca. 1.2 Å-1 and suggests a distance between Bi53+ and AlCl4- ions of about 5.5 Å in close agreement with values for the solid (5.6-6.2 Å). The free D3h-Bi53+ ion was studied with RHF, MP2, and QCISD(T) methods. Effective core potentials were used in conjunction with polarized split-valence LANL1DZ basis sets. At the QCISD(T)/LANL1DZ+PP level, distances of d(Bia-Bie) = 3.073 Å and d(Bie-Bie) = 3.331 Å were determined. The Bia-Bie distances consistently are shorter by about Δ(Bi-Bi) = 0.26 Å in the free ion, by ca. 0.3 Å in Bi5(AlCl4)3 crystals, and by ca. 0.25 Å in the liquid. Natural population analysis shows a larger charge on Bia (+0.74) than on Bie (+0.51). Natural electron configuration analyses show intact 6s-type Bi lone pairs. The lowest-lying cluster MOs are a1\u27 (radial), e\u27\u27 (mixed), a2\u27\u27 (∥), and e\u27(⊥), and they are illustrated via contour plots of partial electron density functions. The molecular graph of Bi53+ shows compelling evidence for strong bonding along all edges