Trimeric Hydrogen Bond in Geometrically Frustrated Hydroxyl Cobalt Halogenides

The mid-infrared absorption spectra of geometrically frustrated hydroxyl cobalt halogenides Co2 （OH）3 CI and Co2 （OH）3Br are measured by FTIR spectrometers, and the stretching vibrational modes of hydroxyl groups are found to be 3549 cm-1 and 3524 cm-1 respectively. Through finding their true terminal O II group stretching vibration frequencies, we obtain 107cm-1 and 99cm-1 red shift caused by the corresponding O-H...Cl and 0 H...Br hydrogen bonds. Rarely reported trimeric hydrogen bonds （Co3≡O H）3….C1/Br are pointed out to demonstrate the relative weakness of this kind of hydrogen bond which may have a critical effect on the lattice symmetry and magnetic structures.