High frequency EPR of a copper(II) trimer: experiment time scale effects in EPR spectroscopy

High-frequency (190 GHz) EPR spectra are reported, that were recorded on an exchange coupled, trimeric copper cluster, octachlorodiadeniniumtricopper(II). The lines in the rhombic spectra shift as a function of temperature. This can be traced to the presence of two close lying spin doublet states, with different g tensors, that interconvert quite rapidly. Therefore, the lines observed in the spectrum have the Boltzmann weighted average of the two g values. The interesting observation is that signals due to the excited spin doublet are observed. This means that the interconversion rate is of the same order of magnitude as the Electron Paramagnetic Resonance experiment. Previous experiments at X-band frequency (9 GHz) could only ascertain that the interconversion rate was faster than that frequency.