Vibrational spectra of anhydrous chromium(III) chloride

The vibrational structure of anhydrous chromic chloride, CrCl₃, has been investigated at room temperature in the spectral range 240 – 500 cm^?1 by means of Raman and infrared absorption spectroscopy.     

Vibrational spectra of CsNCS single crystals

Single crystal i.r. and Raman spectra are reported for CsNCS. The main spectral features are in accord with the factor group predictions. Many of the polarisation sensitive multiphonon modes can be accounted for by k = 0 frequencies but for others k ≠ 0 critical points are implicated.     

Vibrational spectra and noncovalent interactions of carbohydrates molecules

The differences between the vibrational spectra of carbohydrates of the same chemical structure caused by the noncovalent intra- and intermolecular interactions have been systematized. In the general case, these differences show up as the following specific features of changes in the bond intensities: change in the intensity ratio of closely spaced bands (IR and Raman spectra); selective change (increase, decrease) in intensities of individual bands (IR and Raman spectra); change (increase, decrease) in intensities of practically all bands (IR and Raman spectra); appearance of strong bands in the region of low frequencies from 50 to 200 cm⁻¹ (Raman spectra); appearance of strong diffuse bands in the low-frequency range with a simultaneous great reduction in the other bands (practical disappearance of the majority of bands) (Raman Spectra). The causes of such a kind of changes in the band intensities in the vibrational spectra of carbohydrates are discussed.     

Vibrational spectra of several fluorine-substituted allene molecules

The i.r. spectra (200–4000 cm⁻¹) of gaseous and the Raman spectra (0–4000 cm⁻¹) of both gaseous and liquid H₂CCCF₂, HDCCCCF₂, D₂CCCF₂, F₂CCCF₂, H₂CCCHF, FHCCCHF and F₂CCCHF have been recorded and the data interpreted in detail. The normal modes for these molecules have been assigned on the basis of the i.r. band contours, the depolarization ratios, the observed isotopic shifts, and group frequency correlations. Normal coordinate calculations have been carried out for the 1,1-difluoroallene and tetrafluoroallene molecules. For the 1,1-difluoroallene molecule, extensive mixing was found between the symmetric CCC stretch and the symmetric CF₂ stretch whereas less extensive mixing was found for the CF₂ wag with the CCC out-of-plane bend, the CF₂ scissors with the CCC symmetric stretch, and the CCC in-plane bend with the CF₂ rock. For the tetrafluoroallene molecule, extensive mixing was found for the antisymmetric CCC stretch and the out-of-phase symmetric CF₂ stretch.     

Vibrational spectra of the ethylenediammonium ion in crystals

The IR and Raman spectra of three isotopic varieties of the ethylenediammonium ion in the tetrachlorocadmate(II) have been measured. It is shown that the ion assumes the centrosymmetric trans conformation in this salt. A complete vibrational assignment has been proposed for the observed fundamentals. Certain assignments for related compounds in the literature are modified. The spectra of the ion in the hexachloroplatinate(IV) and hexachlorostannate(IV) have also been obtained; compared to these two compounds hydrogen bonding in the tetrachlorocadmate is appreciable. Combination bands observed in the 2100-1800 cm^?1 region of the IR and Raman spectra of compounds containing the C-NH₃⁺ grouping are discussed and assignments are suggested for those observed in ethylenediammonium salts.     

Vibrational spectra of rare earth bromate nonahydrate crystals

Polarized Raman spectra of lanthanum and neodymium bromate nonahydrate crystals and IR spectra in a powdered sample have been observed from 30–4000 cm^?1. Observed Raman bands are experimentally classified into three active symmetry species. The vibrational assignment is made by comparing the spectra of these nonahydrates with those of the corresponding nonadeuterates and similar crystals containing various rare earth ions. From a comparison of the Br-O stretching and lattice vibrations, the binding character between the rare earth ions and ligands is discussed. This consideration is consistent with the result supposed from the lanthanide contraction.     

Vibrational spectra and H-bondings in anhydrous and monohydrate α-Zr phosphates

A new FTIR and FT-Raman investigation on α-zirconium phosphate (Zr(HPO₄)₂·H₂O) and its anhydrous form has been performed in order to obtain an affordable assignment of their vibrational spectra as well as to highlight the hydrogen bonding structure formed by the P-OH groups and the intercalated water molecules. To this end the spectral changes induced by both temperature and isotopic exchange were observed on several high-purity grade samples of different morphology especially prepared and well characterized by SEM, RX, DSC and TGA. In particular, it is also presented as a detailed discussion of the results obtained by FTIR-PAS for different sample morphology. The observed spectra have been analyzed and interpreted according to the α-Zr(HPO₄)₂·H₂O crystal structure and H-bond geometry. The obtained results allowed to clarify the mechanism of the α-Zr(HPO₄)₂·H₂O→α-Zr(HPO₄)₂ dehydration process as well as the H-bonding changes involved in the high temperature phase transition of anhydrous α-Zr(HPO₄)₂.     

Vibrational spectra of polyatomic molecules assisted by quantum thermal baths

We assess the performance of colored-noise thermostats to generate quantum mechanical initial conditions for molecular dynamics simulations, in the context of infrared spectra of large polyatomic molecules. Comparison with centroid molecular dynamics simulations taken as reference shows that the method is accurate in predicting line shifts and band widths in the ionic cluster (NaCl)(32) and in the naphthalene molecule. As illustrated on much larger polycyclic aromatic hydrocarbons, the method also allows fundamental spectra to be evaluated in the limit of T = 0, taking into account anharmonicities and vibrational delocalization.     

Phase transitions and thermodynamic properties of anhydrous caffeine

Caffeine has been found to display a low-temperatureβ- and a high-temperatureα-modification. By quantitative DTA the following data were determined: transformation temperature 141±2°; enthalpy of transition 4.03±0.1 kJ·mole^?1; enthalpy of fusion 21.6±0.5 kJ·mole^?1; molar heat capacity $$\begin{array}{*{20}c} {{\vartheta \mathord{\left/ {\vphantom {\vartheta {^\circ C}}} \right. \kern-\nulldelimiterspace} {^\circ C}}} & {100(\beta )} & {100(\alpha )} & {150(\alpha )} & {100(\alpha )} \\ {{{C^\circ _\mathfrak{p} } \mathord{\left/ {\vphantom {{C^\circ _\mathfrak{p} } {J \cdot K^{ - 1} \cdot mole^{ - 1} }}} \right. \kern-\nulldelimiterspace} {J \cdot K^{ - 1} \cdot mole^{ - 1} }}} & {271 \pm 9} & {287 \pm 10} & {309 \pm 11} & {338 \pm 10} \\ \end{array} $$ in good accord with drop-calorimetric data. For the constants of the equation log (p/Pa)=?A/T+B, static vapour pressure measurements on liquid and solidα-caffeine, and effusion measurements on solidβ-caffeine yielded: $$\begin{array}{*{20}c} {A = 3918 \pm 37; 5223 \pm 28; 5781 \pm 35K^{ - 1} } \\ {B = 11.143 \pm 0.072; 13.697 \pm 0.057; 15.031 \pm 0.113} \\ \end{array} $$ . The evaporation coefficient ofβ-caffeine is 0.17±0.03.     

Vibrational spectra of l-lysine monohydrochloride dihydrate and its two anhydrous forms

ATR FTIR and Raman spectra of l-Lys·HCl·2H₂O and two crystalline forms of anhydrous l-Lys·HCl are reported and discussed. Distinction with earlier published IR spectra of l-Lys·HCl·2H₂O registered by the KBr pellet method is explained by distortion of the spectra because of possible ionic exchange and partial decomposition.     

Thermochemical study of two anhydrous polymorphs of caffeine

The mean values of the standard massic energy of combustion of caffeine in phase I (or alpha) and in phase II (or beta) measured by static-bomb combustion calorimetry in oxygen, at T = 298.15 K, are Δ_cu^° (C₈H₁₀O₂N₄, I) = −(21823.27 ± 0.68) J · g⁻¹ and Δ_cu^° (C₈H₁₀O₂N₄, II) = −(21799.96 ± 1.08) J · g⁻¹, respectively.The standard (p^° = 0.1 MPa) molar enthalpy of formation in condensed phase for each form was derived from the corresponding standard molar enthalpies of combustion as, and .The difference between the standard enthalpy of formation of the two polymorphs in condensed phase was also evaluated by using reaction-solution calorimetry. The obtained result, 2.04 ± 0.25 kJ · mol⁻¹, is in agreement, within the uncertainty, with the difference between the molar enthalpies of formation obtained from combustion experiments (4.5 ± 3.2) kJ · mol⁻¹, which can be considered as an internal test for consistency of the results.A value for the standard enthalpy of formation of caffeine in the gaseous state was proposed: , estimated from the values of the standard enthalpies of formation of both crystalline forms obtained in this work, and the data on standard enthalpies of sublimation collected from the literature.     

Sorption of caffeine and theophylline on hypercrosslinked polystyrene

The sorption of caffeine and theophylline from water solutions on hypercrosslinked polystyrene MN-200 was studied. It is shown that sorption is described by the Langmuir equation. The conditions of the dynamic sorption concentration of caffeine and theophylline on a concentrating column and the conditions of its desorption were found. A procedure for spectrophotometric determination of caffeine and theophylline with a dynamic preconcentration is proposed.     

Solubilities of caffeine and theophylline in supercritical ammonia

The solubilities of caffeine and theophylline in supercritical ammonia at 140°C have been determined. Both compounds exhibited substantial solubility in the ammonia, with caffeine showing the greatest increase in solubility near the critical pressure where density increases most rapidly. However, theophylline showed an appreciable solubility at pressures substantially below the critical pressure. Possible chemical reactions of supercritical ammonia with these solutes at this temperature were found to be unimportant in the present study. The role of supercritical ammonia as a solvent for polar organic molecules is discussed.     

Room temperature phosphorimetry studies of caffeine and theophylline

Room temperature phosphorimetry (RTP) of a mixture of two methylxanthines is described. The similar spectral characteristics of caffeine and theophylline require use of a separation procedure prior to RTP analysis. A facile and efficient separation method is reported, and the efficiency of the separation, and the RTP characteristics of the methylxanthines are presented.     

Vibrational spectra of molecules in the Hartree–Fock dielectric screening approach

Traditionally, the calculation of the vibrational spectra of molecules involves at one point or another a numerical differentiation procedure. Such a method has some serious drawbacks both in efficiency and in accuracy. In this paper, an alternative method based on linear response theory is presented. The second derivative of the ground-state energy is expressed in terms of the electron density response matrix by means of perturbation theory. The unperturbed wave functions are obtained from the Hartree–Fock equation. First-order perturbation theory applied to this equation leads to the Hartree–Fock linear response. As an illustration of this method the vibrational frequency of a H₂ molecule is calculated. The result is 1.348 × 10¹⁴ Hz as compared to the experimental value of 1.319 × 10¹⁴ Hz. This method is also applicable in the calculation of the phonon dispersion curves of solids.