Microstructured hydroxyl environments and Raman spectroscopy in selected basic transition-metal halides

Raman vibrational spectra of the selected basic (hydroxyl OH and deuteroxyl OD) transition-metal halides, geometrically frustrated material series \alpha -, \beta -, \gamma -Cu_2(OH)_3Cl, \alpha -Cu_2(OH)_3Br, \beta -Ni_2(OH)_3Cl, \beta -Co_2(OH)_3Cl, \beta -Co_2(OH)_3Br, \gamma -Cu_2(OD)_3Cl, and \beta -Co_2(OD)_3Cl are measured at room temperature and analysed to investigate the relationship between the microstructured OH environments and their respective Raman spectra. Among these selected samples, the last two are used to determine the OH stretching vibration region (3600 cm^-1--3300 cm^-1) and OH bending vibration region (1000 cm^-1--600 cm^-1) of OH systems in the spectra. Through the comparative analysis of the distances d(metal--O), d(O--halogen), and d(OH), the strong metal--O interaction and trimeric hydrogen bond (C_3v, C_s or C_1 symmetry) are found in every material, but both determine simultaneously an ultimate d(OH), and therefore an OH stretching vibration frequency. According to the approximately linear relationship between the OH stretching vibration frequency and d(OH), some unavailable d(OH) are guessed and some doubtful d(OH) are suggested to be corrected. In addition, it is demonstrated in brief that the OH bending vibration frequency is also of importance to check the more detailed crystal microstructure relating to the OH group.