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 H res was studied for powder samples of linear Cu 3 EtBz·2MeOH and Cu 3 BuNaEs·2MeOH trimeric Cu(II) complexes. For Cu 3 BuNaEs·2MeOH, H res changes dramatically at ∼100 K, where a crystallographic phase transition was found by X-ray diffraction. For Cu 3 EtBz·2MeOH, ESR measurements were performed under high pressure. It was observed that H res decreases with increasing pressure but the linewidth increases. Far-infrared absorption measurements were performed as a function of temperature for Cu 3 EtBz·2MeOH and Cu 3 BuNaEs·2MeOH. No clear change of vibration frequencies with temperature was observed. These results agree with the earlier interpretation that the temperature shift of H res is caused by the thermal averaging of ESR lines due to the short spin-lattice relaxation time.