Dirk Reinen

Effects of vibronic coupling on the EPR spectra of copper(II) doped K2ZnF4

Abstract The single crystal g-values of ≈≈ 1% Cu2+ doped into tetragonal K2ZnF4 measured over a temperature range between 4 and 295 K are reported. The results are interpreted in terms of a predominantly dz2 ground state wavefunction for the CuF4−6 guest species, with a small admixture of dx2-y2 caused by vibronic coupling. To a first approximation both the magnitudes and temperature dependence of the g-values may be described using a model directly analogous to that conventionally used to represent the temperature dependence of the intensity of parity-forbidden electronic transitions. The generality of this model has been investigated by carrying out numerical calculations of the vibronic wavefunctions and energy levels, and values of the ligand-field and warping parameters of the “Mexican-hat” potential surface of the copper(II) guest complex have been derived. The results reported for several other systems where vibronic coupling had been thought to influence EPR parameters are also discussed using the present model.

Fluxionality and stereochemistry of 5-coordinate Cu2+ complexes. The potential energy surface and spectroscopic implications

Abstract Cu2+ ions in five-coordination (Cl−1, NH3, NCS−, etc.) generally stabilize an elongated square pyramid, which is slightly preferred to a compressed trigonal bipyramid. This stereochemical behaviour can be understood by considering the vibronic interaction between the A′1 ground state and the first excited E′ state via the e′ vibrations in D3h symmetry (pseudo-Jahn-Teller coupling), in combination with an E′ ⊗ e′ interaction (Jahn-Teller coupling in the excited state), leading into the lower C2v (C4v) and Cs symmetries. The adiabatic ground state potential surface is calculated in a semi-empirical model on the basis of available spectroscopic data. The minima at the points, which characterize the elongated C2v (C4v) geometry, are rather flat and can be shifted to any other point of the potential surface by steric ligand and/or geometric packing influences. The CuCl53− square pyramids in [Co (NH3)6] CuCl5 undergo a pseudorotation to (dynamically averaged) trigonal bipyramids at 280 K, with the Cl− ligands along the threefold axis remaining fixed in space. In contrast the nuclear displacements along the e′ coordinates occur in an unrestricted manner above 285 K in case of the Cu(NH3)52+ square pyramids in [Cu(NH3)5]Br2 (“Berry rotation”). The wavefunctions, g-tensor components and d-d transition energies have been derived as functions of the e′ distortion coordinates and were explicitly calculated for the CuCl53− model case.

SECOND-SPHERE LIGAND FIELD EFFECTS ON OXYGEN LIGATOR ATOMS AND EXPERIMENTAL EVIDENCE : THE TRANSITION METAL-OXYGEN BOND IN OXIDIC SOLIDS

Abstract The properties of the M–O bond in oxidic solids, where M is a low-valent 3d transition metal ion such as Ni II , Cu II , Co II or Cr III , are very variable depending on the structure and constitution of the respective compound, and are studied by optical and EPR spectroscopy. Specifically, high-valent cations in the cationic coordination of the oxygen ligator atoms beside M determine the bonding behaviour of oxygen towards the transition metal ions. Applying a newly developed extension of the angular overlap model (AOM) to fit the d–d spectra and the EPR g and hyperfine tensor components (Cu 2+ ), the bonding parameters e σ and e π for a great variety of structures and solids are evaluated. In these calculations a 2s–2p hybridisation on oxygen is anticipated, where the number and the geometrical arrangement of the high-valent cations (Nb V , Sb V ; W VI , Te VI , etc.) determine the kind of hybridisation (sp, sp 2 , sp 3 ) and the directional properties of the hybrid orbitals. A wide range of AOM parameters— e σ ranging from ∼3 to ∼5×10 3 cm −1 in the case of Ni 2+ —is obtained, the most striking changes being observed, if a d 0 configurated cation (Nb V , W VI ) is substituted by a cation with d 10 configuration (Sb V , Te VI ). Particularly interesting are anisotropies in the π -interactions in the case of sp 2 hybridised oxygen atoms, which lead to large symmetry splittings of the d–d transitions. The e σ and e π energies, which constitute the ligand field parameter Δ, turn out to be rather complex bonding parameters. They can be classified according to the local symmetry of the M–O bond in the respective crystal structure and correspond to the σ - and π -overlap capacities resulting from the modeling of the oxygen bonding properties by strongly contrapolarising cations in the oxygen coordination sphere. Additional information, specifically about the degree of covalence within the transition metal–oxygen bond, comes from the nephelauxetic ratio β and the mixing coefficient α of Cu II in the ground state MO—both parameters deduced from the experiment. The obtained results can be nicely interpreted by defining effective electronegativities of oxygen towards the transition metal. Thus, high-valent and small cations as S VI and P V are strongly contrapolarising and generate highly ionic oxygen atoms comparable to the fluoride ions. The collected data are finally used to define oxygen ligand fields with the property to stablise the higher oxidation states of the late 3d metal ions (Cu III , Ni III , Ni IV ).

The low lying electronic states of O−3

The energies of the three lowest lying excited states of the ozonide anion (1 2B2, 1 2A1, 1 2A2) at the optimized geometry of the X 2B1 ground state are theoretically predicted at the MRCI‐SD level of theory using large atomic natural basis sets. The calculated vertical excitation energy Tv=2.85 eV for the 1 2A2←X 2B1 transition, which has a large transition moment, is in good agreement with the experimental results for the isolated O3− anion in host matrices and solution between 2.69–2.81 eV. The state symmetry forbidden transition 1 2B2←X 2B1 is calculated with Tv=2.26 eV. The excitation energy for the 1 2A1←X 2B1 transition is theoretically predicted with Tv=2.28 eV and a very low transition moment. The theoretical results are also discussed in comparison with recently observed low lying absorption bands of O3−.

Crystal structure of bis(2,2′-bipyridyl)monochlorocopper(II) hexafluorophosphate monohydrate at 298 K and the electron spin resonance spectra of some bis(2,2′-bipyridyl)copper(II) complexes to 4.2 K

The crystal structure of the title compound [Cu(bipy)2Cl][PF6]·H2O (1)(bipy = 2,2′-bipyridyl) has been determined by X-ray diffraction, using diffractometer data collection. The structure was solved by the heavy-atom method and successive Fourier synthesis. Compound (1) crystallises in the monoclinic space group P21/n with a= 21.403(5), b= 12.235(3), c= 8.599(2)A, β= 92.68(2)°, and Z= 4. The CuN4Cl chromophore involves a near-regular trigonal bipyramidal stereochemistry, but with a significant distortion of the three in-plane angles from 120°(115.7, 123.8, and 120.5°). The poly-crystalline and single-crystal e.s.r. spectrum of (1) and a number of other 2,2′-bipyridyl complexes with [Cu(bipy)2X]n+ cations (X = Cl or I, n= 1; X = NH3, n= 2) are reported between 300 and 4 K. The molecular g tensors are consistent with a near-regular trigonal bipyramidal or square pyramidal distorted trigonal bipyramidal stereochemistry. As the e.s.r. spectra do not change significantly with temperature a dynamic behaviour in the sense of the pseudo-rotation of the Berry–Twist mechanism is not considered to operate.

Ligand field analysis of Mn5+ in tetra-oxo coordination☆

Abstract Polarized single crystal absorption and powder reflection spectra of Mn5+ (d2) in tetrahedral oxo-coordination of host compounds with the spodiosite and apatite structure from literature are analysed using the angular overlap model (AOM). A small number of model parameters such as the AOM parameters eσ and eπ, the interelectronic repulsion parameters B and C as well as geometric distortion angles 2θ were used to fit the band positions. Covalency is pronounced, however, enforcing a treatment with varying B values for different strong field configurations e2, e1t21, t22− the C/B ratios being kept at the free ion value of 4.25. Deduced best-fit parameter values are Δ ≈ 10650 cm−1 (eσ ≈ 12000 cm−1, eσ/eπ = 4), Bee ≈ 530 cm−1, Bet ≈ 420 cm−1, Btt ≈ 340 cm−1, showing that the central field covalency dominates over the symmetry restricted covalency. The estimated angular distortion parameters 2θ and the Δ values indicate that the geometry of the tetrahedral sites is significantly modified when Mn5+ is incorporated into positions of host ions with a smaller ionic radius (P5++).

Phase transitions in CuZrF6 and CrZrF6: A Mössbauer and EPR study of local and cooperative Jahn-Teller distortions

Abstract CuZrF6 and CrZrF6 undergo phase transitions between 100 and 450K, which are induced by crystal packing effects and changes from dynamical to static Jahn-Teller distortions of the Cu(Cr)F6 octahedra. We analyzed in particular the transitions of the Jahn-Teller type using 57Fe2+, doped into the Cu2+(Cr2+) sites, as a Mossbauer probe. The quadrupole splitting is large in the region of static distortions and essentially reflects the distortion symmetry of the host Cu(Cr)F6 polyhedra, while it vanishes in the case of a dynamical Jahn-Teller effect. Ligand field, EPR, and magnetic data are given in addition and are discussed with respect to the structures of the host compounds and the cooperative Jahn-Teller order of the tetragonally elongated Cu(Cr)F6 polyhedra in the low-temperature phases.

Die Untersuchung des Phasenübergangs von FeZrF6 mit der Mössbauer-Spektroskopie

Abstract FeZrF 6 exhibits a phase transition from the trigonal LiSbF 6 structure type to the cubic high-temperature modification with an ordered ReO 3 lattice at 212.3°K. From the temperature dependence of the quadrupole splitting in the temperature range 212°K ⩽ T ⩽ 4.1°K an axial splitting parameter | Δ 1 | = 68 cm −1 could be deduced, which is valid for T −1 (210°K) with increasing temperature. There is structural evidence supporting this interpretation and identifying the axial ligand field component as being of trigonal symmetry. Ligand field and EPR spectroscopic results, however, prove the existence of an additional dynamic Jahn-Teller coupling of tetragonal symmetry, which is obviously not seen by Mossbauer spectroscopy.

The Optical-Spectrum of Ba2zn[cu]f6

Abstract The low temperature polarized absorption and magnetic circular dichroism spectra of ≈3% copper (II) doped into the tetragonally compressed sites of Ba 2 ZnF 6 are reported. The interpretation of the optical spectrum requires that the CuF 6 4− octahedra are tetragonally compressed. This implies that the host site strain dominates with respect to the higher order Jahn-Teller coupling. Considerable vibrational fine structure was observed on sharp electronic origins for the spin-orbit components of the 2 A 1g ( z 2 ) → 2 E g ( xz, yz ) transition. It has been found that cubic anisotropy in the orbital reduction parameters is necessary to interpret the spin-orbit splitting of the 2 E g ( xz, yz ) excited state. The magnitude of the splitting has been used to derive the tetragonal components of the ligand field at the equilibrium geometry of the excited state, and this has been compared with values obtained for other similar systems.

The modulation of Jahn-Teller coupling by elastic and binding strain perturbations-a novel view on an old phenomenon and examples from solid-state chemistry.

Cations in 6-coordination with orbitally degenerate E(g) ground states, such as Cu(2+) and low-spin Co(2+), play an important role in coordination chemistry-in particular, in modern complex biochemistry. The stereochemistry and the binding properties within the basic polyhedra are the subject of pronounced modifications due to vibronic coupling in such cases, but may be also significantly influenced by what is usually called an imposed strain. The latter effect makes allowance for the general observation that the host sites into which the Jahn-Teller unstable centers are substituted are seldom of O(h) symmetry and built from six equal ligands. Hence, the finally observed molecular and binding structure of the pseudo-octahedral complex is the result of the combined action of vibronic coupling and strain. The closer analysis of host-site strain effects demands to distinguish between elastic strain components, which modify the force constant of the vibronically active (here, ε(g)) vibration, and binding strain perturbations, which take account of possibly present ligands with different binding properties. A symmetry-met semiempirical strain model on such a basis is presented and a corresponding formulation within the vibronic coupling formalism is given, on the molecular level. Well-established model examples of Cu(2+) in octahedral fluoride coordination in various host solids, where a great variety of experimental results is available, are given. The derived parameters allow a detailed characterization of the structural and energy qualities of the Jahn-Teller centers, and might help to steer these properties in cases where synthesis strategies are needed. The proposed strain concept is more complex than that of Ham [F. S. Ham, Electron Paramagnetic Resonance; Plenum Press: New York, 1972; F. S. Ham, Phys. Rev. 1965, A138, 1727]; the advantage is that it is directly tied to the structure and energy of the Jahn-Teller complex in focus, although more data (experimental and possibly computed) are needed in such a model.