Gitterschwingungsspektren. XIX—infrarot‐ und Ramanspektren von BaBr₂·2H₂O und BaBr₂·2D₂O

Abstract

Low temperature infrared and raman spectra of polycrystalline BaBr2.2H20 at different degrees of deuteration are studied in the frequency range from 4000 to 30cm-'. The single peaks observed for the bending fundamentals of isotopically dilute H 2 0 and D20 show that the molecules of the water of crystallization are equivalent, as required by Bang's structure determination. The singlets observed for the three fundamentals of isotopically dilute HDO show that there is a symmetric force field in the water molecules which is not predicted from the structure determination. In BaBr2.2H20 weak hydrogen bonds to the bromine ions are present which are largely bent. From the shift of the stretching modes relative to the frequencies of free water the hydrogen bond entbalpy was found to be 20 kJ. The temperature shifts of all the stretching modes are positive, which is not in agreement with Falk's results on bent hydrogen bridges. The internal modes and the twisting libration of the water molecules show an unusual large correlation splitting due to intermolecular coupling of the vibrations of the two water molecules in the 2Br-.2H20 structure unit.Bariumbromid-Dihydrat kristallisiert in der monoklinen Raumgruppe C2/c mit vier Formeleinheiten in der Elementarzelle. '.' Die Kristallwassermolekule des BaBr2.2H20 sind kristallographisch aquivalent. Herausragende Baueinheiten sind Gruppen aus 2Br--Ionen und 2H2O-Molekiilen (vgl. Abb. 1). Die Wasserstoff atome der H20-Molekiile bilden schwache Wasserstoffbrucken zu den Br--Ionen der Baueinheit aus. Die OBr-Bindungsabstande, 325 und 331 pm, unterscheiden sich relativ stark; der BrOBr-Winkel ist mit 79,2" erheblich kleiner als der Bindungswinkel der Wassermolekule.Schwingungsspektroskopische Untersuchungen, iiber die im folgenden berichtet wird, zeigten jedoch, dal3 sich die Kristallwassermolekule (Lagesymmetrie CI) in einem Gitterpotential der Symmetrie CZv befinden. Dieser Befund und eine ungewohnlich grol3e intermolekulare Kopplung der Schwingungen der Kristallwassermolekule veranlal3ten uns, das IR-und Ramanspektrum des Bariumbromid-Dihydrats, die bisher nur sehr unvollstandig untersucht ~u r d e n ,~-~ naher zu analysieren. ERGEBNISSEDie Abzahlung der optischen Schwingungen ( k = 0) des BaBr2.2H20 zeigt Tabelle 1. Ramanaktiv slnd die Schwingungen der Rassen A , und B,, IR-aktiv die Schwingungen des Typs A , und B,. Die drei inneren Schwingungen (uasym, vsym,und 6 ) sowie die drei Librationen (& R,, R,] der Kristallwassermolekule treten in t XVIII. Mitt.: H. Haeuseler und H. D. Lutz, J. Solid State Chem.

Crystal chemistry of tetraradial species. Part 2. Crystal structures of Et4NI, Ph4PBr•H2O, and Ph4PBr•2H2O

“…Br distances (Table 7), 0.04(1) A. The v(0D) values are practically the sama as those in SrBr2.H20 (51), and the v(0H) frequencies in the unde Iterated sample are similar to those in SrBr2.H20 and BaBr?.H20 at ro2m temperature (51), and about 40 cm-I higher than in BaBr !.2H20 (52).…”

Local pseudosymmetry of the water molecule in BaBr2 · 2H2O, a neutron diffraction study

“…Based on the structural data obtained in this work, water-water interactions within the actual tetrahedral unit can easily be understood: the respective distances are short and, hence, the orientation of the water molecules allows strong dipolar interactions. This leads to strong intermolecular vibrational coupling which is proved by the observed Davydov splittings of the internal vibrations of the water molecules (Lutz, Pobitschka, Frischemeier and Becker, 1978).…”

“…Two possible orientations for the water molecule have been discussed in the literature: an almost tetrahedral [Brz(HzO)z]Z-unit (Bang, 1961;McGrath, 1965) and additionally an approximately square planar [Brz(HzO)z]Z-arrangement (Lutz, Engelen and Freiburg, 1980). Detailed infrared and Raman spectroscopic studies (Lutz, Pobitschka, Frischemeier and Becker, 1978) showed that only one OH (OD) stretching frequency is observed for isotopically dilute samples indicating that the two OH bonds are energetically equivalent, although the respective OW'" Br distances from X-ray data are significantly different for both possible arrangements. Furthermore, the unusually large Davydov splittings of the HzO stretching and bending modes (which have not very often been observed for crystalline hydrates) led to the conclusion that significant intermolecular water-water interactions must be present.…”

Local pseudosymmetry of the water molecule in BaBr2 · 2H2O, a neutron diffraction study

“…This leads to strong intermolecular vibrational coupling which is proved by the observed Davydov splittings of the internal vibrations of the water molecules (Lutz, Pobitschka, Frischemeier and Becker, 1978).…”

“…Two possible orientations for the water molecule have been discussed in the literature: an almost tetrahedral [Br 2 (H 2 0) 2 ] 2 " unit (Bang, 1961;McGrath, 1965) and additionally an approximately square planar [Br 2 (H 2 0) 2 ] 2_ arrangement (Lutz, Engelen and Freiburg, 1980). Detailed infrared and Raman spectroscopic studies (Lutz, Pobitschka, Frischemeier and Becker, 1978) showed that only one OH (OD) stretching frequency is observed for isotopically dilute samples indicating that the two OH bonds are energetically equivalent, although the respective O w -Br distances from X-ray data are significantly different for both possible arrangements. Furthermore, the unusually large Davydov splittings of the H 2 0 stretching and bending modes (which have not very often been observed for crystalline hydrates) led to the conclusion that significant intermolecular water-water interactions must be present.…”