STRONG HYDROGEN-BONDS IN CA(HSEO3)(2)CENTER-DOT-H2O - CRYSTAL-STRUCTURE AND TEMPERATURE-INDUCED FORMATION OF ABC BANDS

Ca(HSeO3)(2) . H2O is a model compound for the formation of AB and ABC multi band systems in the OH stretching mode region of oxo acids, aci d oxo salts or other compounds with strong hydrogen bonds. It was obta ined from aqueous solutions of Ca(HSeO3)(2) and has been investigated by means of single crystal X-ray diffraction measurements at ambient t emperature and temperature dependent powder X-ray diffraction and IR m easurements. From the single crystal X-ray measurements, the H-bond sc heme of Ca(HSeO3)(2) . H2O including a hexacyclic H-bond system was de termined. The temperature dependent IR and X-ray data mainly result in (i) a reversible phase transition between or at T-1 approximate to 20 0 and T-2 approximate to 305 K, (ii) a strong reversible contraction o f the hexacyclic H-bond system, and (iii) a correlated, reversible for mation of a gamma(OH) and a C band in the out-of-plane bending and OH stretching mode region of the SeOH group, respectively. These findings are discussed with respect to the AB/ABC band transition in the vibra tional spectra of Ca(HSeO3)(2) . H2O and the formation of ABC multi ba nd systems of the other compounds mentioned above. (C) 1997 Elsevier S cience B.V.