Investigations of ligand field effects in the spectra of trisdibenzoylmethide, trisbenzoylacetonate, and trisacetylacctonatc chelates of trivalent ytterbium and thulium are reported. Chelate absorption and luminescence spectra corresponding to intra-4f electronic transitions exhibit line splittings similar in magnitude to the crystal field splitting observed in the spectra of inorganic complexes of the rare earth ions.
Absorption spectra of carbon tetrachloride solutions of all the chelates studied were measured in the visible and near infrared regions. Each ytterbium chelate exhibits an absorption band at 9800A, consisting of three well-resolved peaks. Thulium intra-4f absorption bands appear at 4700A, 6800A, 8000A, 12000A, and 17000A. All absorption spectra were measured at room temperature.
Luminescence spectra of microcrystalline powder samples of the trisdibenzoylmethide and trisbenzoylacetonate chelates of ytterbium exhibit rare earth ion emission near 100000A. For thulium trisbenzoylacetonate dihydrate, intra-4f emission spectra were observed at 4800A, 6500A, and 7900A. Luminescence of rare earth ions from the other compounds studied either could not be excited by the excitation process used or simply exhibited spectral intensities too weak for convenient observation. All luminescence measurements were carried out at the temperature of liquid nitrogen.
The quantum theory of atomic structure (with particular reference to rare earth ions) is discussed, beginning with a review of the variational techniques of calculation for free atoms and ions (in the central field approximation) and including a presentation of prescriptions for the use of the theory of group representations as an aid in the calculations. As an
example of the free-ion techniques, a detailed calculation of the energy levels of trivalent thulium is carried out for the case of intermediate spin-orbit coupling. The splitting of the free-ion energy levels of rare earths in chelates is discussed from the point of view of each of two theories. In the electrostatic crystal field theory, it is assumed that term splitting arises from electrostatic interactions between the 4f-electrons and the surrounding ligands, treated as point charges. On the other hand, the contact potential model of ligand field theory hypothesizes that the splitting is due to very weak covalent bonding involving the ligand orbitals and the rare earth ion 4f-orbitals. The derivation of this covalent bonding theory is presented here in greater generality than has previously been reported.
Comparison of the experimental energy levels of chelated Yb3+ and Tm3+
with predictions of the two theories leads to much better agreement between theory and experiment for the electrostatic crystal field calculation than is obtained using the contact potential. The crystal field calculation for chelates uses a single geometrical parameter θ and a ligand charge parameter Q. By using the calculations, the observed optical spectra, and X-ray crystal studies for other compounds, it has been possible to fix the values for those parameters and thus to obtain a reasonable estimate of the molecular geometry, which appears to be that of octahedron distorted by shortening a three-fold symmetry axis.
Preliminary calculations of coordination energies for rare earth chelates, based on the electrostatic model, indicate that crystal field splitting in these complexes probably will produce only very small effects on their thermodynamic properties.
Level of Degree
Department of Chemistry and Chemical Biology
First Committee Member (Chair)
Glenn A. Crosby
Second Committee Member
Third Committee Member
Guido H. Daub
Fourth Committee Member
Perkins, Walter George. "Crystal Field Splitting in Rare Earth Chelates." (1964). https://digitalrepository.unm.edu/chem_etds/168