Temperature Dependence of the Crystal Structure and EPR Spectrum of Bis(1,3,5-Trihydroxycyclohexane)copper(II) Tosylate. A Unified Interpretation Using a Model of Dynamic Vibronic Coupling.

The crystal structure of bis(1,3,5-trihydroxycyclohexane)copper(II) tosylate is reported at temperatures of 293, 233, 188, 163, and 93 K, as are the structures of the Zn(II) and Ni(II) analogues at room temperature for comparison. The isomorphous compounds are triclinic, space group P&onemacr;, with one formula unit in the unit cell. The unit cell parameters of the Cu compound at 293 K are a = 6.456(5) Å, b = 9.505(3) Å, c = 12.544(3) Å, alpha = 76.57(2) degrees, beta = 87.48(4) degrees, gamma = 76.65(4) degrees. The centrosymmetric ZnO(6) and NiO(6) octahedra are tetragonally compressed with a slight orthorhombic distortion. The Cu(2+) polyhedra exhibit similar geometries, but with considerably larger deviations from a regular octahedron. Two of the three independent Cu-O bond lengths and two of the g-values change significantly as a function of temperature. A model of dynamic vibronic coupling is presented which explains both the EPR and structural data. Vibronic wave functions associated with a Jahn-Teller potential energy surface modified by an orthorhombic lattice "strain" are given. The temperature dependence of the structures is calculated from the nuclear parts and that of the g-values from the electronic parts of the wave functions. The temperature dependence of the structures and g-values is also interpreted using a simpler model involving an equilibrium between two forms of the complex which differ solely in their orientation in the crystal lattice, and the results of the two approaches are compared.