Vibronic Effects in Cu(II)-Doped Ba2Zn(HCO2)6·4H2O

The temperature-dependent electron paramagnetic resonance (EPR) spectrum of ∼1% Cu(II) ions doped into Ba2Zn(HCO2)6·4H2O was analyzed at the Q-band frequencies over the temperature range 100−350 K to obtain structural information about the local environment. It can be concluded that the host crystal imparts a large orthorhombic strain which mainly corresponds to a tetragonal compression imposed onto the CuIIO6 species. This results in a copper center which adopts an orthorhombically distorted elongated geometry with the elongated axis perpendicular to the direction of the tetragonal compression due to the host crystal. There are two possible axes of elongation, and these represent two conformers separated by ∼320 cm−1. The thermal population of the higher energy level averages the g values, giving the observed temperature-dependent EPR spectra. The averaging process is between vibronic levels that are localized at two different minima of a single ground-state potential energy surface. These vibronic levels correspond to vibrational levels having different electronic properties. The determination of the host lattice strain parameters from the Cu(II) EPR spectra means that the guest ion is used as a probe of the environment of the Zn(II) site. The structural data derived from the lattice strain parameters are correlated with those from the Ba2Zn(HCO2)6·4H2O crystal structure.