Vibrational spectra and structures of urazole and 4-methylurazole: DFT calculations of the normal modes in aqueous solution and in the solid state,and the influence of hydrogen bonding

Solid-state IR and Raman as well as aqueous solution state Raman spectra are reported for urazole, 4-methylurazole and their deuterated derivatives. DFT calculations, at the B3-LYP/cc-pVTZ level, established that the structures and vibrational spectra of the molecules can be interpreted using models with hydrogen-bonded water molecules, in conjunction with the polarizable continuum solvation method. The vibrational spectra were computed at the optimised molecular geometry in each case, enabling normal coordinate analysis, which yielded satisfactory agreement with the experimental IR and Raman data. Computed potential energy distributions of the normal modes provided detailed vibrational assignments. Solid-state pseudopotential-plane-wave DFT calculations, using the PW91 functional were also carried out, reflecting the importance of intermolecular hydrogen bonding in the solid state.     

Vibrational spectra of alpha-amino acids in the zwitterionic state in aqueous solution and the solid state: DFT calculations and the influence of hydrogen bonding

The zwitterionic forms of the two simplest alpha-amino acids, glycine and l-alanine, in aqueous solution and the solid state have been modeled by DFT calculations. Calculations of the structures in the solid state, using PW91 or PBE functionals, are in good agreement with the reported crystal structures, and the vibrational spectra computed at the optimized geometries provide a good fit to the observed IR and Raman spectra in the solid state. DFT calculations of the structures and vibrational spectra of the zwitterions in aqueous solution at the B3-LYP/cc-pVDZ level were found to require both explicit and implicit solvation models. Explicit solvation was modeled by inclusion of five hydrogen-bonded water molecules attached to each of the five possible hydrogen-bonding sites in the zwitterion and the integration equation formalism polarizable continuum model (IEF-PCM) was employed, providing a satisfactory fit to observed IR and Raman spectra. Band assignments are reported in terms of potential-energy distributions, which differ in some respects to those previously reported for glycine and l-alanine.     

A combined theoretical and experimental study of the structure and vibrational absorption, vibrational circular dichroism, Raman and Raman optical activity spectra of the L-histidine zwitterion

A combined theoretical and experimental study of the vibrational absorption (VA)/IR, vibrational circular dichroism (VCD), Raman and Raman optical activity (ROA) spectra of l-histidine in aqueous solution has been undertaken to answer the questions (i) what are the species present and (ii) which conformers of the species are present under various experimental conditions. The VA spectra of l-histidine have been measured in aqueous solution and the spectral bands which can be used to identify both species (cation, zwitterion, anion) and conformer of the species have been identified and subsequently used to identify the species (zwitterion) and conformer (gauche minus minus, gauche minus plus for the side chain dihedral angles) present in solution at pH 7.6. The VCD spectral intensities have been used subsequently in combination with further theoretical studies to confirm the conclusions that have been arrived at by only analyzing the VA/IR spectra. Finally a comparison of measured Raman and ROA spectra of l-histidine with Raman and ROA spectral simulations for the conformers and species derived from the combined VA/IR and VCD experimental and theoretical work is presented as a validation of the conclusions arrived at from VA/IR and VCD spectroscopy. The combination of VA/IR and VCD with Raman and ROA is clearly superior and both sets of experiments should be performed.     

The cytidinium—cytidine complex: infrared and Raman spectroscopic studies

Using IR and Raman spectra, it is shown that the sytidinium cation hydrogen bonds to cytidine to form a stable 1:1 complex, in both aqueous solution (pH ~ 3.3) and as a solid. The spectra indicate that the proton is located asymmetrically in the NH?N bond of the complex, on the vibrational time scale, in both solution and the solid. The perdeuterated systems were also examined; their spectra support these conclusions.     

水合铵硼氧酸盐及其饱和溶液的FTIR和Raman光谱研究

研究了NH₄B₅O₈·4H₂O和(NH₄)₂B₈O₁₃·6H₂O及其饱和溶液于20℃的FTIR和Raman光谱,对振动频率进行了归属.根据振动光谱特征,预测(NH₄)₂B₈O₁₃·6H₂O中所含基本结构单元为[B₇O ₁₁(OH)·B(OH)₃]^2-.首次将Raman光谱中516cm^-1处的强散射峰归属为这一多聚硼氧配阴离子的对称脉冲振动峰,并对以上2种铵硼氧酸盐饱和溶液中硼氧配阴离子的存在形式{B(OH)₃,[B₃O₃(OH)₄]^-和[B₅O₆(OH)₄]^-}和相互作用机理进行了探讨.     

Prediction among different spectroscopic properties for aqueous systems

Spectroscopic properties obtained by NMR and vibrational spectra both reflect the microscopic environment of solutions, and the local composition (LC) theory can be used to study environmental effects on spectroscopic properties. Based on the LC model, the relationship between NMR and vibrational spectra, including infrared (IR) spectroscopy and Raman, were investigated. For the aqueous systems-water+N, N-dimethylformamide, water+acetone, water+methanol, and water+ethanol, we performed prediction between concentration-dependent peak positions of IR and Raman, as well as between concentration-dependent vibrational properties and 1H NMR chemical shifts. The results showed that reliable prediction could be achieved with the help of the LC model. This suggests that 1H NMR chemical shifts and vibrational spectroscopic properties may tell us the same story about the local environment encountered in solution.     

Molecular structure and vibrational spectra of phenolphthalein and its dianion

Infrared (IR) and Raman spectra of phenolphthalein (PP) and its dianion form (sodium and potassium salts) were studied both in the solid state and in aqueous solution. Band assignments were carried out on the basis of the isotope shifts of the ring deuterated and 13C-substituted derivatives. Spectral analyses reveal that the PP dianion exists as mixtures of the benzenoid form (colorless) and the quinonoid form (colored) in the solid state and in aqueous solution, while the neutral PP solely takes the gamma-lactone form. This work provides the first vibrational spectroscopic evidence for the coexistence of the two species in the PP dianions.     

Vibrational analysis of chlorinated methylphosphonic acids and their anions: Part III. CCl3PO3H2

The IR and Raman spectra of CCl₃PO₃H₂and its anions are reported. Included are the spectra of aqueous solutions of the anions and solid state IR spectra of the acid and of the Na and K salts. The results of the normal coordinate analysis for the dibasic anion are given.     

How delocalized is N,N,N',N'-tetraphenylphenylenediamine radical cation? An experimental and theoretical study on the electronic and molecular structure

The electronic and molecular structure of N,N,N',N'-tetraphenylphenylenediamine radical cation 1(+) is in focus of this study. Resonance Raman experiments showed that at least eight vibrational modes are strongly coupled to the optical charge resonance band which is seen in the NIR. With the help of a DFT-based vibrational analysis, these eight modes were assigned to symmetric vibrations. The contribution of these symmetric modes to the total vibrational reorganization energy is dominant. These findings are in agreement with the conclusions from a simple two-state two-mode Marcus-Hush analysis which yields a tiny electron-transfer barrier. The excellent agreement of the X-ray crystal structure analysis and the DFT computed molecular structure of 1(+) on one hand as well as the solvent and solid-state IR spectra and the DFT-calculated IR active vibrations on the other hand prove 1(+) adopts a symmetrical delocalized Robin-Day class III structure both in the solid state and in solution.     

Crystal structure and vibrational spectra of piperazinium bis(4-hydroxybenzenesulphonate) molecular-ionic crystal

The piperazinium bis(4-hydroxybenzenesulphonate) crystallizes from water solution at room temperature in P2(1)/c space group of monoclinic system. The crystals are built up of doubly protonated piperazinium cations and ionized 4-hydroxybenzenesulphonate anions that interact through weak hydrogen bonds of O-H...O and N-H...O type. Mutual orientation of anions is determined by non-conventional hydrogen bonds of C-Hcdots, three dots, centeredpi type. Room temperature powder FT IR and FT Raman measurements were carried out. The vibrational spectra are in full agreement with the structure obtained from X-ray crystallography. The big single crystals of the title salt can be grown.     

Vibrational spectroscopy of cyclohexylaminoglutethimide in low-temperature matrices, solutions and the solid state

The vibrational spectra of cyclohexylaminoglutethimide (CHAG) molecules were studied in low-temperature Ar and N₂ matrices and in CCl₄, CHCl₃, CS₂ and CH₃CN solutions (at different concentrations). It was found that increasing the solute concentration in CCl₄, CS₂ and CHCl₃ solutions leads to the formation of dimers. On the other hand, in CH₃CN solution only the solute–solvent complexes occur.

The molecular structure and theoretical IR and Raman spectra of CHAG were predicted with the use of ab initio RHF/6-31G^** and density functional B3LYP/6-31G^** quantum mechanical methods. A comparative analysis of theoretical and experimental spectra of monomeric (isolated) species has led to the assignment of most of the absorption bands in terms of CHAG normal modes.

The IR and Raman spectra of crystalline samples (solid film and KBr pellet) were recorded as well. A reliable assignment of the IR and Raman spectra of crystalline CHAG was obtained with the help of theoretically simulated spectra and the results of our earlier investigations of aminoglutethimide and glutethimide vibrational spectra.     

Hydrogen bonding in 3-azetidinol—3. Ab initio and experimental spectra of the hydrogen-bonded species

The IR and Raman spectra in the range 4000-10 cm⁻ of 3-azetidinol and the O,N-dideuterated derivative have been recorded in the solid state and in aqueous solution. The interpretation of the vibrational spectra has been based upon ab initio calculations in the STO-3G approximation. A model structure for the calculations has been adopted in which 3-azetidinol binds two molecules of water and two molecules of ammonia. The results obtained for the aqueous solution are in accordance with the occurrence of 3-azetidinol as a hydrated, stacked, intermolecularly hydrogen-bonded chain.     

Infrared reflection-absorption spectra of C2H4 and C2H6 on Cu: effect of surface roughness

Infrared reflection absorption spectroscopy (IRRAS) of the highly symmetric molecules C2H4 and C2H6 adsorbed as mono- and multilayers onto copper films is studied in relation to the type of metal-film roughness. Spectra of C2H4 show Raman lines on cold-deposited Cu films but not on Cu deposited at room temperature. For C2H6, the IR spectra from both types of metal films are similar; the surface infrared selection rule holds and no Raman bands are observed. The Raman lines that appear in the IR spectra already at low exposures are attributed to species adsorbed at special defect sites, identical to the so-called active sites in surface enhanced Raman scattering (SERS). The IR excitation mechanism by transient electron transfer to the adsorbate pi* state can deliver a discrete vibrational band of a Raman-active vibration only under certain circumstances, for example, for adsorbates at the "SERS-active sites". C2H6 at these sites cannot deliver Raman bands in IRRAS, because it has no pi* state. We also discuss IRRAS measurements on Cu(111) and Cu(110) single crystals, where Raman bands of C2H4 have been observed.     

Infrared and Raman Spectroscopy of Methylcyanodiacetylene (CH3C5N)

A spectroscopic study combining IR absorption and Raman scattering is presented for methylcyanodiacetylene (CH₃C₅N). Gas‐phase, cryogenic matrix‐isolated, and pure solid‐phase substance was analyzed. Out of 16 normal vibrational modes, 14 were directly observed. The analysis of the spectra was assisted by quantum chemical calculations of vibrational frequencies, IR absorption intensities, and Raman scattering activities at density functional theory and ab initio levels. Previous assignments of gas‐phase IR absorption bands were revisited and extended.     

The pH dependent Raman spectroscopic study of caffeine

First of all the surface enhanced Raman spectroscopy (SERS) and normal Raman spectra of caffeine aqueous solution were obtained at different pH values. In order to obtain the detailed vibrational assignments of the Raman spectroscopy, the geometry of caffeine molecule was optimized by density functional theory (DFT) calculation. By comparing the SERS of caffeine with its normal spectra at different pH values; it is concluded that pH value can dramatically affect the SERS of caffeine, but barely affect the normal Raman spectrum of caffeine aqueous solution. It can essentially affect the reorientation of caffeine molecule to the Ag colloid surface, but cannot impact the vibration of functional groups and chemical bonds in caffeine molecule.     

Effects of intermolecular vibrational coupling and liquid dynamics on the polarized Raman and two-dimensional infrared spectral profiles of liquid N,N-dimethylformamide analyzed with a time-domain computational method

A time-domain method for calculating polarized Raman and two-dimensional infrared (2D-IR) spectra that includes the effects of both the diagonal frequency modulations (of individual molecules in the system) and the off-diagonal (intermolecular) vibrational coupling is presented and applied to the case of the amide I band of liquid N,N-dimethylformamide. It is shown that the effect of the resonant off-diagonal vibrational coupling and the resulting delocalization of vibrational modes is clearly seen as the noncoincidence effect in the polarized Raman spectrum and some spectral features (especially as asymmetric intensity patterns) in the 2D-IR spectra. The type of 2D-IR spectra (concerning the polarization condition) most appropriate for observing this effect is discussed. On the basis of the agreement between the observed and calculated band profiles of the polarized Raman spectrum, the time dependence of the transient IR absorption anisotropy is also calculated. The method of evaluating the extent of delocalization of vibrational modes that is relevant to the features of these optical signals in the time and frequency domains is discussed. The nature of the molecular motions (concerning the liquid dynamics) that are effective on the diagonal frequency modulations is also examined.     

The infrared and Raman spectra of ethylammonium hexafluorosilicate [C2H5NH3]2SiF6

The X-rays powder diffraction pattern of [C2H5NH3]2SiF6 was obtained and indexed on the basis of a hexagonal unit cell. The vibrational spectra (IR and Raman) of this compound were recorded and discussed in relation to the above-mentioned crystal structure. The vibrational spectra of the cations indicate that they are disordered and hydrogen bonded to the anions. On the opposite, the Raman spectrum of the anions could be interpreted in terms of ordered groups. The combination bands observed in the 2300-1800 cm(-1) spectral region in the IR spectrum indicate that this compound may contain C-NH3 groups.     

Vibrational spectra and IR dichroism of poly(tetramethyl-p-silphenylene-siloxane) and its chain structure

Raman spectra of poly(tetramethyl-p-silphenylene-siloxane) were investigated in solution with depolarization ratios and in the solid state. Polarized infrared (IR) spectra were studied on oriented polymer samples. Assignments of the vibrational bands are given and IR dichroic and Raman data are in agreement with tetragonal P4₁ space group symmetry. These data provide information on the arrangement of the Si? C₆H₄? Si groups in the unit cell. On the basis of vibrational spectra the dynamic interaction between adjacent benzene rings is negligibly small and the rings preserve their local D_2h Pseudosymmetry.     

UV-vis and Raman spectroelectrochemical investigation of the redox behavior of poly(5-cyanoindole) in acidic aqueous solutions

Spectroelectrochemical properties of conducting poly(5-cyanoindole) films deposited on indium tin oxide (ITO) and platinum electrodes are investigated using UV-vis and resonant Raman spectroscopies. The transitions from undoped to semi-conducting state of P5CN require the partial oxidation of the polymer to create radical-cations by insertion of charge-neutralizing anions into the polymer. In order to obtain detailed structural information from the vibrational spectra, it is necessary to know the vibrational modes of oxidation-sensitive bands. Vibrational assignments were made on the basis of the results obtained on polyindole and P5CN in acetonitrile solution. The drastic changes in optical absorption and Raman spectra observed at various stage of oxidation were explained by the conversions between at least three different structures. On the basis of the Raman spectra, we have identified the vibrational modes associated with neutral and polaronic segments. The perturbation associated with the coexistence of these polaronic segments has been described as a quinoid structure growing on the expense of the benzoid one. The results obtained indicate that the molecular properties of the conducting polymers at various stages of an oxidation are better revealed by in-situ Raman spectra than by ex-situ studies.     

Vibrational spectroscopic analysis of cytosine monohydrate and its copper(II) complex

Single crystals of cytosine monohydrate and its copper(II) complex were grown by slow evaporation in an aqueous solution at room temperature. The solubility studies were carried out at different temperatures in deionized water. Cytosine monohydrate and the copper(II) complex of cytosine were characterized by recording IR, Raman and UV spectra. The various vibrational modes of the crystals have been classified using factor group and site group analysis. Vibrational assignments were proposed for both the systems based on the spectral investigations.