Double resonance study of the 17O quadrupole coupling in paraelectric and ferroelectric KH2PO4

The pure quadrupole resonance spectrum of ¹⁷O has been measured in a 10% enriched KH₂PO₄ sample at 190°K at at 77°K. In the paraelectric phase all oxygen sites are chemically equivalent with e²qQ/h = 5.16 MHz and η = 0.55, whereas there are two chemically non-equivalent ¹⁷O sites in the ferroelectric phase: (e²qQ/h)₁ = 5.96 MHz, η_t = 0.72 and (e²qQ/h)_II = 4.85 MHz, η_II = 0.18. The results can be interpreted in terms of a double minimum type O-H—O hydrogen bond potential, where the protons above T_c rapidly fluctuate between the two equilibrium sites. In contrast with the neutron scattering results in KD₂PO₄, the present data show no evidence for any anomalous oxygen ferroelectric mode displacements in KH₂PO₄, which would not condense out below T_c.     

Investigation of the phonon band gap effect on Raman-active optical phonons in SrMoO4 crystal

The phonon dispersions of SrMoO₄ crystal are calculated using the lattice dynamical calculations approach. Spontaneous Raman spectra in the SrMoO₄ were measured in the temperature range from 10 K to 295 K, and the temperature dependence of the linewidth of the B_g (95 cm⁻¹) and A_g (888 cm⁻¹) Raman modes was analyzed using the lattice dynamical perturbative approach. We found that different behaviors of these two modes in the case of temperature broadening could be attributed to the large energy band gap in the phonon spectrum resulting in different anharmonic interactions. The calculated temperature dependence of the linewidth of A_g (888 cm⁻¹) mode was well accounted for the experimental one by including both down-conversion by the cubic term and the dephasing by quartic term. The dephasing processes are increased only at high temperatures and the effect of dephasing is related to the size of a large phonon band gap.     

Raman intensities of the lattice modes of KH1−xDxF2 as the order parameter for phase transition

In this work, the Raman intensities of the librational mode of the bifluoride ion and the translational mode of the K⁺ ion are considered as the order parameters for the phase transition of KH_1−xD_xF₂ since group theoretical consideration shows that the Raman active translational and librational modes in the low temperature phase will be inactive in the high temperature phase if the latter is of symmetry group O or higher. The measured Raman intensities are correlated with the temperature from 163°C to 196°C to elucidate the exponent β which relates the Raman intensity to the temperature close to T_c through (T^c-T)^β for various deuteration compositions. The β value of KHF₂ is very close to that predicted by the Ising model of three-dimensional space with a one-dimensional order parameter, suggesting that the nearest-neighbour interaction of the translational modes can play a very important role for the phase transition. Experiments also show that the phase transition temperature is slightly isotope composition dependent.     

Complex formation of HCONH2 in CH3OH environment and investigation of linewidth changes of ν(CO) stretching and NH2 bending modes

The isotropic part of the Raman bands corresponding to NH₂ bending and ν(CO) stretching modes of formamide (HCONH₂) at ∼1593 and 1668 cm⁻¹, respectively, in neat HCONH₂ as well as in binary mixtures with methanol (CH₃OH) were reinvestigated. Variations of their linewidths exclusively with mole fractions of HCONH₂, in the range C = 0.1–0.9 were studied. The linewidth variation of the NH₂ bending mode shows a departure from the trend expected on the basis of concentration fluctuation model and this has been explained using a recently suggested empirical model by invoking the concept of microviscosities of the solute, HCONH₂ and the solvent, CH₃OH. The other peak at ∼1668 cm⁻¹ shows a peculiar variation of the linewidth with concentration having two minima at C = 0.8 and 0.4, which have been explained in terms of formation of hydrogen bonded complexes, NH₂HCO⋯HOCH₃, and NH₂HCO⋯(HOCH₃)₂ and the two phenomena, namely motional narrowing and diffusion dynamics being simultaneously operative. The equilibrium constants have been evaluated from the spectral data and their variation with total molar concentration has been presented.     

Local Raman tensors in adenosine triphosphoric acid

A polarized laser (488.0 nm) Raman spectroscopic measurement has been made on a single crystal of a disodium salt of adenosine triphosphoric acid (Na₂ATP·3H₂O) by the use of a Raman microscope. The crystal belongs to an orthorhombic system of the space group P2₁2₁2₁, and has dimensions of 10, 100 and 100 μm along the crystallographic axes a, b and c, respectively. For each Raman band in the 300–1800 cm⁻¹ range, the scattering intensity ratio I _bb/I _cc of the bb and cc polarization components has been determined. For a few bands, the relative intensities of the bc components were also estimated. To augment the data, the depolarization ratio of each Raman band of ATP has also been determined for its acidic (pH = 2.42) H₂O and D₂O solutions. From these experimental results, the shapes and orientations of the Raman scattering tensors which are considered to be localized in the adenine-H⁺ portion and in the phosphate portion of the molecule have been derived.     

Magnetic and spectroscopic investigation of partially reduced vanadium pentoxides. I. α-CuxV2O5

The electron paramagnetic resonance (EPR) and magnetic susceptibility of both powdered and crystalline α-Cu_xV₂O₅ near the compositional limit have been studied as a function of temperature. The EPR data indicate that the paramagnetic species are isolated V⁴⁺ centers in a nearly axially symmetric ligand field. It is demonstrated that reasonably precise EPR parameters can be derived from powdered spectra. The narrowness of the EPR line suggests that the spectra are motionally narrowed via thermally activated hopping of the paramagnetic electron of V⁴⁺. Expressions are derived for the temperature-dependent magnetic moments of the octahedral ²T_2g term under the combined perturbation of spin-orbit coupling and an axially symmetric ligand field, and the susceptibility data are interpreted in terms of this model. It is found that the sixfold degenerate ²T_2g level is split into three levels with the lowest and highest levels possessing a magnetic moment and the intermediate level having no moment. This model is also consistent with the g-tensor and can account for the temperature dependence of the EPR linewidth.     

Theoretical study on the structures,force field,and vibrational spectra of cyclooctatetraene and cyclooctatetraene-d8

The structures and force field of 1,3,5,7-cyclooctatetraene (COT) have been studied using ab initio theory at the SCF level with the 4-21G basis set. The quadratic force field of the D_2d structure obtained by systematic scaling of the ab initio force constants successfully reproduces the observed frequencies of COT and COT-d₈ with a mean deviation of less than 10 cm⁻¹ for non-CH stretching modes. On the basis of the calculated results, assignments of the fundamental vibrations are examined. The normal mode υ₅ is reassigned to a weak band at 758 cm⁻¹ in the Raman spectrum of COT and to a weak band at 591 cm⁻¹ in the Raman spectrum of COT-d₈. The calculations favor the assignment of υ₂₆ given by Lippincott et al. [J. Am. Chem. Soc. 73, 3370 (1951)] over the revised assignment of Perec [Spectrochim. Acta 47A, 799 (1991)]. The calculations also furnish reliable prediction for the inactive A₂ fundamentals of COT and COT-d₈. The fundamental frequencies and IR and Raman intensities of ¹³CC₇H₈, which constitutes about 9% of COT in natural abundance, are also calculated. Only ν₁₀ (calculated at 908 cm⁻¹) of the formal inactive A₂ modes has appreciable Raman intensity (0.23 Å⁴/amu). A spectral feature due to this fundametal is identified in the liquid Raman spectrum of Tabacik and Blaise [C. R. Acad. Sci. Ser. II 303, 539 (1986)] as a weak peak at 908 cm⁻¹.     

The Raman spectrum of tetraselenium tetranitride Se4N4

The Raman spectrum of solid, dry tetraselenium tetranitride, Se₄N₄, has been recorded in the region of 30–1000 cm⁻¹. The assignment has been made on the basis of slightly deformed D_2d symmetry, which is in agreement with the X-ray structure determination and the infrared spectrum.     

Conformational study of 1,2-dithiacyclononane

The temperature dependence of the Raman spectrum of 1,2-dithiacyclononane (1,2-DTCN) in the SS stretching region has been used to infer the existence of a conformational equilibrium with ΔH⁰ = 5.0 ± 0.8 kJ/mol. Molecular mechanics calculations predict a (2 2 5)-C₂ lowest energy conformation in equilibrium with a (2 3 4) structure. The fully decoupled ¹³C NMR spectrum at −80°C and the Raman spectra are consistent with this postulate. The temperature dependence of the ¹H NMR spectrum of 1,2-DTCN is characteristic of the ring inversion process. A crude lineshape analysis allows us to calculate ΔG⁰ = 49.0 ± 1.2 kJ/mol.     

Accurate measurement of the pressure broadening of the ν1 Raman line of CH4 in the 1–50 atm region by inverse Raman spectroscopy

The lineshape of the ν₁ Raman band of methane is measured as a function of pressure between 1 and 50 atm by inverse Raman spectroscopy using the 488 nm output of a cw Ar⁺ laser as a probe beam and the output of a pulsed dye laser as a Stokes pump beam. The linewidth is found to increase linearly in this pressure region, and the fwhm Δν (in cm^?1) can be expressed as Δν = 0.32 + 0.012p, p being the methane pressure in atm.     

Anisotropic exchange interaction in tetrabutylammonium bis(stilbene-1,2-dithiolato)nickelate(III), [NBu4][Ni(sdt)2]

The frequency-dependent EPR linewidth of [NBu₄][Ni(sdt)₂] has been found to derive a considerable contribution from anisotropic exchange broadening. From the linewidth data at X-band, values of 0.52 and 0.05 cm⁻¹ have been estimated for isotropic and anisotropic exchange coupling respectively, in this system.     

ESR characterisation of SO−3 and SO−4 radicals in X-irradiated kainite (KMgClSO4.3H2O)

The ESR spectra of the kainite (KMgClSO₄.3H₂O) crystal revealed an intense isotropic (g = 2.004) peak C attributed to the SO⁻₃ radical and two pairs of lines (A₁, A₂) and (B₁, B₂) bearing intensity ratio 5:3. The intensity and linewidth variation of peak C suggested that the signal contains an unresolved shf structure. The power saturation studies on SO⁻₃ indicate that its ESR line is homogeneously broadened and its line shape is Lorentzian. The spin—lattice and spin—spin relaxation times (T₁ and T₂) of SO⁻₃ have been estimated to be 0.44 s and 656 μs, respectively. The analysis of the anisotropic pairs of lines show that they constitute two sets A and B and are due to two chemically inequivalent SO⁻₄ radical species in the lattice. The ESR spectra of the polycrystalline samples recorded at 300 and 77 K confirm the isotropic behaviour of SO⁻₃ and chemical inequivalence of two types of SO⁻₄ radicals. The principal g-values of the SO⁻₄ radical were evaluated to be: g₁ = 2.007, g₂ = 2.011, g₃ = 2.014 for species A and g₁ = 2.008, g₂ = 2.012, g₃ = 2.015 for species B. The low microsymmetry of the SO²⁻₄ ion in the lattice seems to promote the radiation damage.     

Effect of methylation on wavenumber shift of ring breathing mode of pyrimidine in the solution of H2O and D2O by employing Raman difference spectroscopic technique and DFT approach

The relative structural and dynamic properties of hydrogen-bonding between Pyrimidine (Pmd) + H₂O and Pmd + D₂O, and 4-Methylpyrimidine (Mpmd) + H₂O and Mpmd + D₂O are investigated experimentally by linear Raman spectroscopy using Raman difference spectroscopic (RDS) technique. The focus has been given to the ring breathing mode (ν₁). The effect of methylation on the Pmd ring has been studied in terms of wavenumber shift (Δν), peak-position and linewidth variation with mole fraction of the solvent. The wavenumber shift has been calculated by assuming the Voigt profile of the Raman band. In order to explain our experimental results, we have optimized single Pmd and 4Mpmd molecules and their various complexes with H₂O and D₂O in the stoichiometric ratio of 1:1, 1:2, 1:3 and 1:4 by employing DFT/B3LYP functional with 6-311+G(d,p) basis set using Gaussian software. There is a good correspondence between experimental and theoretical results. Our result reveals that with RDS technique, Δν of a band up to 1/100th of the FWHM can be measured precisely.     

Vibrational spectrum and conformation of cyclopropyldichloroborane

The infrared (3500-40 cm^?1) and Raman spectra (3200-0 cm^?1) have been recorded for cyclopropyldichloroborane in both the gaseous and solid states. Additionally, the Raman spectrum of the liquid was recorded and qualitative depolarization values obtained. Only one conformation has been found in all three physical states and, on the basis of the polarized nature of the Raman band assigned as the BCl₂ antisymmetric stretch, this conformer has been identified as being the bisected structure with C_s molecular symmetry. A complete vibrational assignment is proposed based on Raman depolarization data, infrared gas phase band contours, and group frequencies. These results are compared with the corresponding data in other organoboranes.     

Microviscosity effects on the vibrational relaxation rates in molecular liquids

The isotropic Raman band of the CO stretching mode of the N,N-dimethylformamide (DMF) molecule has been studied as a function of solvents' hydrodynamic properties. The effect of solvent viscosity on linewidth (Γ_iso) has been studied in detail, particularly using the theory of microviscosity. Modifications have been made in the ƒ(ϱ, η, n) parameter which relates the vibrational relaxation rate with viscosity, density and dispersion energy on the basis of microviscosity.     

Crystal and molecular structure, hydrogen bonding and electrostatic interactions of bis(pyridine betaine) perchlorate

Bis(pyridine betaine) perchlorate, (PB)₂H·ClO₄, has been characterized by X-ray diffraction, B3LYP and MP2 calculations, FTIR, Raman and NMR spectroscopy and DSC. The complex crystallizes in space group C2/c. A pair of PB molecules is bridged by a short symmetric O·H·O hydrogen bond of the length 2.456(5) Å. Three conformers (I-III), with respect to the N⁺?O distances, of (PB)₂H cation have been analyzed at the DFT and MP2 level of theory and compared with the C(1)?O distances in the (PhCH₂COO)₂H anion. The calculated O?O distances for the analyzed cations are slightly shorter than that in the crystal. The type of hydrogen bond depends on the cation conformation and the method of calculations. The FTIR spectra of (PB)₂H·ClO₄ and (PBd₅)₂H·ClO₄ (bis(pyridine-d₅ betaine) perchlorate) show a broad and intense absorption in the 1500-400 cm⁻¹ range, which is similar to the type A acid salts of carboxylic acids and other 2:1 betaine complexes with mineral acids. In the Raman and second FTIR-derivative spectra (d²) the broad absorption is absent. A linear unit-slope correlation between d² negative band and Raman band is found. This confirms that derivative spectrometry can be used to estimate frequencies of the narrow bands covered by the broad absorption due to the O·H·O stretching and bending vibrations.     

Protonic conduction in NH4H2PO4

The conductivity of ammonium dihydrogen phosphate has been measured as a function of temperature and dopant concentration. A previously disputed break in the log conductivity vs reciprocal temperature plots has been observed. The activation energy is in agreement with previous work on NH₄H₂PO₄. In addition, the conductivity vs concentration of NH₄HSO₄ plot is linear, permitting the calculation of the L defect mobility and indicating that the proton is the conducting species. It is concluded that the mechanism of conduction is the same as previously proposed for KH₂PO₄ and KH₂AsO₄.     

The spectroscopic characterization of the sulphate mineral ettringite from Kuruman manganese deposits,South Africa

The mineral ettringite has been studied using a number of techniques, including XRD, SEM with EDX, thermogravimetry and vibrational spectroscopy. The mineral proved to be composed of 53% of ettringite and 47% of thaumasite in a solid solution. Thermogravimetry shows a mass loss of 46.2% up to 1000 °C. Raman spectroscopy identifies multiple sulphate symmetric stretching modes in line with the three sulphate crystallographically different sites. Raman spectroscopy also identifies a band at 1072 cm⁻¹ attributed to a carbonate symmetric stretching mode, confirming the presence of thaumasite. The observation of multiple bands in the ν₄ spectral region between 700 and 550 cm⁻¹ offers evidence for the reduction in symmetry of the sulphate anion from T_d to C_2v or even lower symmetry. The Raman band at 3629 cm⁻¹ is assigned to the OH unit stretching vibration and the broad feature at around 3487 cm⁻¹ to water stretching bands. Vibrational spectroscopy enables an assessment of the molecular structure of natural ettringite to be made.     

Infrared and Raman study of alunite—jarosite compounds

Infrared and Raman spectra of alunite and jarosite compounds, AR₃(SO₄)₂(OH)₆, where R is either Fe³⁺ or Al³⁺ and A is a monovalent cation, have been obtained between 4000 and 40 cm⁻¹. The observed number and position of the i.r. and Raman bands have been analysed under the centrosymmetric D⁵_3d space group. In addition, some relationships between the crystallochemical characteristics of these compounds and their vibrational properties are presented.     

Investigation of anharmonicity in rotational phonon mode for BaWO4 crystal

Spontaneous Raman spectra in the BaWO₄ were measured in the temperature range from 4 K to 280 K, and the temperature dependence of the linewidth of the A_g (191 cm⁻¹) Raman mode was analyzed using the lattice dynamical perturbative approach and one-phonon density of states (PDOS). The linewidth slope for the 191 cm⁻¹ peak for an external mode is 7.2 times larger than that for the 926 cm⁻¹ peak for a breathing mode. The different behaviors of these two modes in the case of temperature broadening could be attributed to the large energy band gap in the one-phonon density of states (PDOS) resulting in different anharmonic interactions. The origin may be that the ratio of up-conversion TDOS to down-conversion TDOS for Eg mode (191 cm⁻¹) is more than that for A_g (926 cm⁻¹). The peak of the Eg mode (191 cm⁻¹) is attributed to the coupling mode both a rotation of the Barium and an out-of-phase rotation of the oxygen in x–y plane as a librational mode.