Trimeric Hydrogen Bond in Geometrically Frustrated Hydroxyl Cobalt Halogenides

The mid-infrared absorption spectra of geometrically frustrated hydroxyl cobalt halogenides Co₂(OH)₃Cl 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–H group stretching vibration frequencies, we obtain 107 cm⁻¹ and 99 cm⁻¹ red shift caused by the corresponding O–H⋅⋅⋅Cl and O–H⋅⋅⋅Br hydrogen bonds. Rarely reported trimeric hydrogen bonds (Co₃≡O–H)₃⋅⋅⋅Cl/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.