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Abstract: The low-temperature single-crystal polarized absorption and the luminescence spectra of Cs2[CrCl2(H2O)4]Cl3 are theoretically analyzed in order to determine the electronic structure of the trans-[CrCl2(H2O)4]+ complex. This study, based on the Racah theory, leads to a good agreement between the theoretical and experimental energy levels. The electronic-exited state 4T2g(Oh) is split into 4Eg and 4B2g components due to the lowering of the symmetry from Oh to D4h. The polarized absorption spectrum and the theoretical energies show that the 4Eg(D4h) state is lower in energy than the 4B2g(D4h) state. The resolved vibronic structure in the luminescence spectrum reveals a progression in a non-totally symmetric b1g mode, a manifestation of a Jahn–Teller effect in the emitting state 4T2g(Oh). Both Jahn–Teller and spin–orbit coupling in the orbitally degenerated 4Eg(D4h) state are necessary to account for the spectroscopic observations. [Copyright &y& Elsevier]

Abstract: The polarized absorption spectra of trans -NiCl2(H2O)4 complex were measured by Bussière et al. [Coord. Chem. Rev. 219–221 (2001) 509–543] at low-temperature. Using the experimental spectroscopic data, semiempirical calculations of the crystal-field levels of trans-NiCl2(H2O)4 chromophore are carried out, based on the Racah theory. We used idealized D4h point group symmetry to analyse the observed crystalline-field splitting of this chromophore. As a result, Racah and crystal-field parameters have been reliably obtained. A good agreement between the theoretical and experimental energy levels of trans-NiCl2(H2O)4 complex has been obtained. The region of 3T1g/1Eg(Oh) bands is of great interest and it is useful to use the tetragonal symmetry to understand the features of this spectral region. [Copyright &y& Elsevier]

The alkylammonium halogenocobalate families are subjected to diverse studies according to their wide field application. In this work, the [(C 3 H 7) 4 N] 2 CoBr 4 compound was synthesized using a slow evaporation solution growth method at room temperature and characterized by X-ray powder diffraction (XRPD) and UV-VIS-IR absorption spectrometry. The X-ray powder diffraction (XRPD) data confirmed the formation of a single-phase with monoclinic system (C2/c space group). The optical absorption and reflectance spectra of the named compound were measured at the range [240 nm–1900 nm] at room temperature. Using three methods, we have determined the direct optical band gap estimated at 3.8 eV and the Urbach energy. Different models were used to calculate the refractive index, which was found to be in the range of 2.12–2.58. This spectrum consists also of electronic transition d-d of Co2+ (3 d7) ions. This work examines a detailed crystal-field analysis of the UV–Vis–NIR lines associated to the transitions between the Stark levels of Co2+, occupying a T d site symmetry in [CoBr 4 ]2- (organic part). The theoretical study is based on the Racah tensor algebra methods. This analysis permits us to determine the electronic structure of the cobalt ion Co2+ based on the crystal field theory. As a result, Racah and crystal-field parameters have been reliably obtained. A good agreement between the theoretical and the experimental energy levels are obtained. This theoretical study confirms the T d site symmetry of Co2+ in [CoBr 4 ]2- and leads to a satisfactory correlation between experimental and calculated energy levels. In addition to internal transitions, the absorption spectrum shows vibrational peaks witness of anharmonic behavior. • Absorption and reflectance spectra were measured. • Optical band gap, Urbach energy, extinction coefficient and refractive index were determined. [ABSTRACT FROM AUTHOR]