Librational modes of the water molecules in barium and strontium halide monohydrates, MX2 · 1H2O (M = Ba, Sr; X = Cl, Br, I)

Collective librations of water molecules in the crystal lattice of rubidium bromide: experiment and simulation

Terahertz spectroscopy of RbBr reveals four prominent absorption lines at room temperature and a further 15 lines at 10 K. Via density-functional-theory (DFT) numerical modelling using the PBE0 hybrid GGA functional, all the absorptions are identified as correlated librations of water molecules in the RbBr lattice. Each libration mode is a combination of rocking, wagging and twisting motions of the water molecules. The number of libration lines and numerical modelling show that the C2v symmetry of water in RbBr is broken. Our modelling shows that the distribution of libration amplitudes and phases for different water molecules in the RbBr unit cell varies greatly between the different modes. All librational lines red-shift with increasing temperature. The rate of change for most lines is in the range 60-90 MHz K(-1) (or (2-3) × 10(-3) cm(-1) K(-1)). Two lines shift more rapidly with temperature, at rates of 240 and 300 MHz K(-1) (or (8 and 10) × 10(-3) cm(-1) K(-1)), respectively. Furthermore, the temperature dependence of the linewidth distinguishes two groups of lines. For one group, with weak linear temperature dependence of linewidth, cubic anharmonic terms in the RbBr crystal field are significant. This group is mainly associated with fully symmetric correlated librations. For the second group, with strong non-linear temperature dependence of the linewidth, quartic anharmonic terms in the RbBr crystal field are significant. However, the distribution of libration amplitudes, as well as the type of libration modes, influence the temperature dependence of the red shift, the linewidth, and the intensity, as well. Our combined experimental and theoretical investigation confirms the necessity of obtaining low-temperature data to observe all the calculated modes; moreover, the richness of detail in the temperature dependence of the data invites further modelling spanning a range of temperatures.

Polarised IR and Raman spectra of non-centrosymmetric Na3Li(SeO4)2.6H2O crystal--a new Raman laser material

Polarised IR and Raman spectra of Na3Li(SeO4)2.6H2O single crystal have been recorded. Discussion of the results has been based on the factor group approach for the trigonal R3c (C3v6) space group with Z = 2. The obtained results for the spontaneous Raman scattering have been used in the discussion of the stimulated Raman spectra of the material studied--a new Raman laser crystal.

The equilibrium structures, vibrational spectra, NLO and directional properties of transition dipole moments of diguanidinium arsenate monohydrate and diguanidinium phosphate monohydrate. The theoretical DFT calculations

For diguanidinium arsenate monohydrate and diguanidinium phosphate monohydrate the energies were minimized and the theoretical vibrational frequencies and potential energy distribution (PED) were calculated by density functional method. The 6-31++G(d,p) basis set was used. The assignment of the bands has been made on the basis of the calculated PED. For calculated equilibrium geometries two methods for determination of direction of transition dipole moments (TDM) were used. The oriented gas model was used for calculation of "static" TDM and from the other side the "dynamic" approximation of TDM by analysis of changes in internal coordination during characteristic vibrations was performed. The restricted Hartree-Fock (RHF) methods were used for calculation of the hyperpolarizability for both investigated molecules. The theoretical results are compared with experimental value of beta.

Polarized vibrational studies of bisguanidinium hydrogenphosphate monohydrate

Detailed vibrational studies (FTIR and Raman on powder samples, polarized FTIR microscope on a small single crystal, polarized FTIR using Bruker reflection unit on a single crystal and polarized Raman) have been carried out. Vibrational spectra are discussed in relation to the crystal structure published previously. In this crystal a network of hydrogen bonds link water molecules, guanidinium cations and hydrogenphosphate ions. The 13 different hydrogen bonds in G2HP crystal structure are detected. On the basis of detailed vibrational studies the detailed assignment of observed bands was made. Calorimetric (DSC) studies have been performed, but no phase transition was found in the temperature range 100-350 K.

Structural and polarized vibrational studies of pentacaesium trihydrogentetraselenate monohydrate

Polarized IR and Raman spectra of single crystal were measured at room and at low temperature. The polarized IR spectra were measured by transmission and specular reflection method. The IR-microscope was used for vibrational measurement of small monocrystals. Assignment of the bands is proposed on the basis of oriented gas model approximation. The polarized vibrational spectra are discussed in relation to the crystal structure and possible proton conductivity and phase transition.

Metal-water interactions and hydrogen bonding in dittmarite-type compounds M'M''PO4.H2O (M'=K+, NH4+; M''=Mn2+, Co2+, Ni2+). Correlations of IR spectroscopic and structural data

The vibrational behavior of the uncoupled nu(OD) modes and of the water librations in dittmarite-type compounds M'M''PO4.H2O (M'=K+, NH4+; M''=Mn2+, Co2+, Ni2+) is analyzed in terms of the influence of two types of metal-water interactions (M+-H2O and M2+-H2O), the hydrogen bonding and the repulsion potential of the lattices. The M+-H2O interaction is found to be the main factor, which influences nuOD. The strong K+-H2O interaction weakens in a higher degree the intramolecular O-H bonds than the corresponding M2+-H2O interactions (M2+=Mn, Co, Ni). As a result nuOD is shifted to lower wavenumbers in the potassium series than in ammonium one, irrespective of the synergetic effect of M2+, the hydrogen bond lengths and the repulsion potential of the lattices. The analysis of the spectroscopic data evidences for the dominating influence of the M2+-H2O interaction on the wagging mode. The blue shift of nuwag strictly follows the increasing synergetic effect of M2+, i.e. nuM'/Mn<nuM'/Co<nuM'/Ni, in all cases, irrespective of the strength of the M+-H2O interactions, the hydrogen bond lengths and the higher repulsion potential of the K-lattices. The rocking mode is insensitive to the replacement of the M2+ ions. Some relations between the spectroscopic and structural data are given.

Diguanidinium hydrogenarsenate monohydrate and its deuterated analogue vibrational, DSC and X-ray investigations

The crystal structure of diguanidinium hydrogenarsenate monohydrate has been found to belong to the P42(1)/c space group of the tetragonal system, with Z = 8, a = 17.114(2) A, c = 7.3500(10) A. In this complex, a network of hydrogen bonds links water molecules and hydrogenarsenate ions. The hydrogenarsenate ions form hydrogen-bonded chains along the crystallographic c-axis. Detailed vibrational studies have been carried out (FTIR and FT-Raman on powder samples, polarized FTIR microscope on a small single crystal at room temperature). The vibrational spectra are discussed in relation to the crystal structure. Calorimetric (DSC) studies have been performed, but no phase transition was found in the temperature range 100-350 K.

Infrared spectroscopic study of mixed copper-cobalt and copper-nickel formate dihydrates (cation distribution in mixed crystals)

The infrared (IR) spectra of Co(HCOO)2 x 2H2O, Ni(HCOO)2 x 2H2O and Cu2Co(Ni)1-x (HCOO)2 x 2H2O mixed crystals (0 < x < or = 0.5) have been recorded and the internal modes of the formate groups and the water molecules are discussed. The analysis of the spectra of the mixed crystals reveals that when copper ions replace cobalt and nickel ions in Co(HCOO) x 2H2O and Ni(HCOO)2 x 2H2O, the Cu2+ ions are localized at the two available positions. However, the occupancy degree of the Me(1) and Me(2) sites by the different cations needs X-ray diffraction (XRD) studies of the single crystals. The new crystal phase Co0.17Cu0.83(HCOO)2 x 2H2O obtained from the Co(HCOO)2 x 2H2O-Cu(HCOO)2 x 2H2O-H2O system at 50 degrees C crystallizes in the monoclinic system with lattice parameters: a = 12.329(4); b = 7.241(2); c = 8.707(5) A and beta = 103.13(3) degrees (SG probably P2/c). The number of the bands corresponding to the uncoupled OD vibrations and the water librations shows that probably more than two water molecules are expected to exist in the structure. Furthermore, it is assumed that the water molecules bonded to the copper ions form stronger hydrogen bonds (stronger Cu-OH2 interaction) than those bonded to the cobalt ions.