Jahn-Teller Research of Copper(II) Complexes

We are looking mainly at six-coordinated Cu²⁺ complexes which have a distorted octahedral geometry, the result of the so-called Jahn-Teller Effect. Cu²⁺ is a d⁹ electron system with an unpaired electron that makes it an ideal candidate for EPR spectroscopy (EPR = Electron Paramagnetic Resonance). Because we don't have the facilities to make the measurements at UHH, they are done in Australia in collaboration with Prof. Michael A. Hitchman, University of Tasmania, and Prof. Mark J. Riley, The University of Queensland. Most of the copper complexes show a dynamic behavior with temperature-dependent bond lengths and g-values. To investigate the causes of this phenomenon, X-ray and EPR data are collected at many different temperatures.

We are currently investigating the so-called Tutton Salts, A₂Cu(SO₄)₂•6H₂O (A= K, Rb, Cs, NH₄, or ND₄).

Cu(H₂O)₆²⁺ ion in a Tutton salt.

The copper is coordinated by six water molecules. The intermediate and longest Cu-O bond lengths of the Jahn-Teller distorted octahedral Cu(H₂O)₆²⁺ ion progressively converge as the temperature is raised.

Temperature dependence of the Cu-O bond lengths in (ND₄)₂[Cu(D₂O)₆](SO₄)₂. Note the convergence of the Cu(1)-O(7) and Cu(1)-O(8) bond lengths at higher temperatures.

The g-values derived from the EPR spectrum of the compound exhibit similar behavior. To explain this, we use the computer program ExE (written by Dr. Mark Riley) which calculates the wave functions of a Jahn-Teller system by coupling the d-orbitals of the electronic E_g ground state with the Jahn-Teller active e_g vibration. It also includes the effects of strain parameters that take into account crystallographic packing effects. The resulting vibronic wave functions can be divided in two parts:

The electronic part of the five lowest wave functions is shown here next to the potential surface showing energy levels.

The second part is the vibrational part of the wave functions. The five lowest levels are shown in the figure below.

The electronic wave functions are used to calculate the g-values that are measured with EPR spectroscopy, while the vibronic part of the wave functions gives information about the geometries in different energy levels.

A simple Boltzmann summation at each temperature adds up the g-values and bond lengths, respectively, to reproduce the temperature dependent experimental values. The beauty of this model is that one set of parameters defines the global wave function of the system and allows it to fit two physically completely different observables, bond lengths and g-values.

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