High pressure ESR and other complementary studies of copper trimers with the quartet spin ground state

The intriguing, earlier observed, temperature shift of the X-band ESR magnetic field Hres was studied for powder samples of linear Cu3EtBz·2MeOH and Cu3BuNaEs·2MeOH trimeric Cu(II) complexes. For Cu3BuNaEs·2MeOH, Hres changes dramatically at ∼100 K, where a crystallographic phase transition was found by X-ray diffraction. For Cu3EtBz·2MeOH, ESR measurements were performed under high pressure. It was observed that Hres decreases with increasing pressure but the linewidth increases. Far-infrared absorption measurements were performed as a function of temperature for Cu3EtBz·2MeOH and Cu3BuNaEs·2MeOH. No clear change of vibration frequencies with temperature was observed. These results agree with the earlier interpretation that the temperature shift of Hres is caused by the thermal averaging of ESR lines due to the short spin-lattice relaxation time