Time resolved absorption and resonance Raman spectroscopic studies of the oxidation of benzidine in aqueous solution

We report a time resolved resonance Raman study of transient radicals produced in the pulse radiolytic oxidation of benzidine in aqueous solution. The intense and structured transient absorption in the 400–470 nm region, observed at microsecond times in the acidic medium, is attributed to the benzidine radical cation. The Raman spectrum, observed by excitation in resonance with this absorption, exhibits eight prominent bands which are assigned to planar phenyl vibrations. The ring breathing mode (v₁) at 844 cm^-1 is most highly resonance enhanced, indicating an overall expansion of the ring CC bonds in the excited state. The interring CC bond, with partial double bond character, is characterized by an intense (v₁₃) Raman band at 1335 cm^-1. The frequency of the in-phase v_7a CN stretching vibration is 1540 cm^-1. These frequencies and the presence of weak bands attributable to non-planar phenyl vibrations indicate the radical to be slightly non-planar. The pK_a for the proton loss from the radical cation is 10.87, four units higher than for the aniline radical cation. At high pH the observed transient has a broad and structureless absorption at ∽ 380 nm. It is identified from its resonance Raman features as the 4(4′aminophenyl)anilino radical formed by proton loss from the radical cation. The interring CC bond is characterized by a Raman band at 1292 cm^-1, indicating it to be a single bond. The structure of this neutral radical is highly nonplanar, with little conjugation between the two ring systems so that electronic excitation is primarily confined to the anilino moiety. The acidic and basic forms of the radical react rapidly in second order processes to produce products which absorb strongly at, respectively, 360 and 410 nm.     

Near-infrared excitation of Raman scattering by chromophoric proteins

Fourier-transformed Raman spectra of bacteriorhodopsin, the photosynthetic reaction center, and myoglobin in aqueous solution excited at 1064 nm are presented. These proteins are representative of three important classes of chromophoric proteins. The observed vibrational modes are assigned and discussed based on the known resonance Raman spectra of these proteins. In each case, chromophore vibrations dominate the Raman scattering, with little or no contribution from other protein vibrations. However, the limitations encountered in resonance Raman studies of chromophoric proteins due to sample fluorescence or sample photolability are circumvented. The relative intensities in the bacteriorhodopsin Raman spectrum excited at 1064 nm are nearly identical to the relative intensities previously observed by resonance excitation. The Raman spectrum of the reaction center of the photosynthetic bacterium Rhodobacter sphaeroides excited at 1064 nm contains contributions from both bacteriochlorophyll and bacteriopheophytin pigments, with possible preresonance enhancement of bacteriochlorophyll modes. The 1064-nm-excited Raman spectrum of myoglobin displays several marker bands that have been characterized previously in resonance Raman investigations with excitation in both the Soret and Q-band regions.     

IR,Raman, and Surface‐enhanced Raman Spectroscopic Study on Triruthenium Dipyridylamide Diruthenium Nickel Dipyridylamide Family: Metal‐metal Bonding and Structures

We report the infrared, Raman, and surface‐enhanced Raman scattering (SERS) spectra of triruthenium dipyridylamido complexes and of diruthenium mixed nickel metal‐string complexes. From the results of analysis on the vibrational modes, we assigned their vibrational frequencies and structures. The infrared band at 323–326 cm^?1 is assigned to the Ru₃ asymmetric stretching mode for [Ru₃(dpa)₄Cl₂]^0–2+. In these complexes we observed no Raman band corresponding to the Ru₃ symmetric stretching mode although this mode is expected to have substantial Raman intensity. There is no frequency shift in the Ru₃ asymmetric stretching modes for the complexes with varied oxidational states. No splitting in Raman spectra for the pyridyl breathing line indicates similar bonding environment for both pyridyls in dpa^– , thus a delocalized structure in the [Ru₃]^6–8+ unit is proposed. For Ru₃(dpa)₄(CN)₂ complex series, we assign the infrared band at 302 cm^?1 to the Ru₃ asymmetric stretching mode and the weak Raman line at 285 cm^?1 to the Ru₃ symmetric stretching. Coordination to the strong axial ligand CN^– weakens the Ru‐Ru bonding. For the diruthenium nickel complex [Ru₂Ni(dpa)₄Cl₂]^0–1+, the diruthenium stretching mode ν_Ru‐Ru is assigned to the intense band at 327 and 333 cm^?1 in the Raman spectra for the neutral and oxidized forms, respectively. This implies a strong Ru‐Ru metal‐metal bonding.     

Raman spectra of stilbene negative ions in tetrahydrofuran solution

Raman spectra of stilbene mono- and di-negative ions in tetrahydrofuran solution were obtained by Ar⁺ laser exciting lines. A considerable frequency shift was observed for several vibrations in the successive steps: stilbene → (stilbene)^? → (stilbene)^2?. The observed shift is discussed in a simple VB scheme, particularly in comparison with the results on anthracene negative ions. The resonance Raman effect was striking for both ions with intensity maxima at the exciting wavelengths close to the absorption maxima.     

Micro-Raman Spectroscopy of Nanostructured Silver Sulfide

Nanostructured silver sulfide powder with an average particle size of about 45 nm, an acanthite α-Ag₂S monoclinic structure (space group P2₁/c), and nonstoichiometric composition Ag_1.93S has been synthesized by the chemical deposition method. The silver sulfide nanopowder has been studied by Raman spectroscopy. According to the Raman scattering data, heating the nanopowder with high-power laser radiation in air leads to photoinduced decomposition of the Ag_1.93S nanopowder to give silver metal. The Raman spectrum of the silver sulfide nanopowder shows a series of bands in the low-frequency range from 90 to 260 cm^–1 associated with vibrations of silver atoms, Ag–S bonds, and symmetric Ag–S–Ag longitudinal modes. Raman spectroscopy confirmed an acanthite monoclinic structure of synthesized silver sulfide nanopowder.     

Raman spectra of anthracene negative ions in tetrahydrofuran solution

Raman spectra of anthracene mono- and di-negative ions in tetrahydrofuran solution were obtained by HeNe and Ar⁺ laser exciting lines. In addition to some frequency shifts from neutral anthracene, remarkable enhancement of intensities by the resonance Raman effect was observed for many aromatic ring vibrations.     

Vibrational spectra and normal coordinate analysis of p-cresol and its deuterated analogs

I.r. and Raman spectra of p-cresol and its seven deuterated analogs were investigated in dilute solutions of hydrophobic solvents. Assignments of the observed i.r. and Raman bands were made on the basis of isotopic frequency shifts, Raman polarization properties, i.r. intensifies and normal coordinate calculations. The calculated normal frequencies are in good agreement with the experimental ones: the average error below 1700 cm⁻¹ is 3.8 cm⁻¹ for 164 in-plane vibrations and 3.3 cm⁻¹ for 59 out-of-plane vibrations. The calculated vibrational modes may be useful in analysing the vibrational spectra of tyrosine. It is suggested that several doublets due to Fermi resonance and a trio of Raman bands in the 1260-1160 cm⁻¹ region are potential probes for the micro-environments of tyrosine side chains in proteins.     

1064-nm-excited Fourier transform Raman studies of bacteriochlorophyll-a in solid films and in a blue-green mutant of Rhodobacter sphaeroides

1064-nm-excited Fourier transform Raman spectra of bacteriochlorophyll-a (BChl) in various solid films and in chromatophores from a blue-green mutant of Rhodobacter sphaeroides have been obtained. The observed Raman spectra are free from high fluorescence backgrounds and sample degradation. The observed intensities seem to be enhanced because of a pre-resonant effect between the exciting radiation at 1064 nm and the Q_y absorption at 770–870 nm of BChl. The spectral features are substantially different from the Soret and Q_x resonance Raman spectra extensively investigated so far; several bands in the wavenumber region lower than 1200 cm⁻¹ are particularly enhanced in the Q_y pre-resonance Raman spectra. Bands due to both the C₂O and C₉O stretches appear at 1700–1620 cm⁻¹, providing structural information on these carbonyl groups. In the CC stretching region (1620–1490 cm⁻¹), the correlation between band positions and the co-ordination number of central magnesium, which was previously found in the Soret-excited Raman spectra, is preserved in the Q_y, pre-resonance Raman spectra as well. The relative intensities of strong bands in the 1200–1000 cm⁻¹ region appear to be useful for characterizing the BChl state. By using these advantages of the Q_y, pre-resonance Raman spectra, molecular interactions and arrangements of BChl in hydrated films and in the B870 light-harvesting complex of R. sphaeroides are discussed.     

Infrared, optical absorption, and EPR spectroscopic studies on natural gypsum

The natural gypsum has been investigated by infrared, Raman, X-ray diffraction, optical absorption and electron paramagnetic resonance spectroscopy. The fundamental stretching and bending vibrations observed in the infrared region for SO₄²⁻ and H₂O are compared with the near-infrared overtones and combinations of these vibrations. MIR and Raman spectral features are attributed to sulfate fundamentals and lattice vibrations of H₂O, SO₄²⁻. The charge transfer and ligand field transition bands were observed near 490, 630, and 800–900 nm and were compared to those of iron oxides. The optical absorption spectrum indicates the presence of ferric and ferrous ions in the mineral. The site symmetry of Fe(III) in the sample is tetragonally distorted. EPR results indicate the presence of the ferric ion in a tetragonally distorted state.     

Vibrational and electronic studies on interactions of Cu (II) with protic amides: Structural aspects of biological importance

Raman, IR and UV–Vis–NIR experiments of formamide (FA), N-methylformamide (NMF) and their solutions with copper perchlorate at different compositions were carried out. The downshift of the ν_CO mode and the upshift of the νCN vibration have been observed for both amides and suggest that an ionic structure is stabilized by Cu (II). The quantitative Raman study at the νCN region reveals that six FA molecules are coordinated to Cu (II) while four NMF molecules are around the metal ion. The data are complemented by information at the region characteristic of the metal–ligand vibrations, which evidences coordination through the O atom. The spectral changes observed at the ν_CN region have been then combined to the electronic data and show that [Cu(FA)₆]²⁺ and [Cu(NMF)₄]²⁺ are described as distorted octahedral and square planar complexes.     

Scaled quantum mechanical force fields and vibrational spectra of solid state nucleic acid constituents V: thymine and uracil

The scale factors of the ab initio SCF STO-3G and MINI-1, and semiempirical PM3 harmonic force fields were determined by fitting to the Raman and IR spectra of polycrystalline uracil and thymine. Both in-plane and out-of-plane vibrational modes have been interpreted. The transferability of the scale factors between uracil and thymine and the performance of different computational methods were discussed. The Fermi resonance of the overtones of the out-of-plane deformation vibrations of oxygens with their stretching modes have been proposed as an explanation for the band splitting observed in the 1600–1800 cm⁻¹ region of uracil.     

Inter- or intramolecular electron transfer between triruthenium clusters: we’ll cross that bridge when we come to it

The purpose of this review is to examine the fundamental differences between intermolecular self-exchange vs. intramolecular ET in mixed-valence complexes based on similar triruthenium structural units. The role of orbital overlap between ancillary ligands of the electron donor and acceptor are considered in self-exchange reactions which are found to be strongly adiabatic and again in bridged mixed-valence systems. The method of infrared (IR) reflectance spectroelectrochemistry for the determination of extremely fast (10¹¹–10¹³ s^?1) ET rate constants is reviewed as a tool to provide quantitative information about the time scales of localization and delocalization. The role of internal vibrations of the bridging ligand in strongly delocalized mixed-valence ions is investigated by resonance Raman and IR spectroscopies. The role of solvent dipolar relaxation times in determining the rates of ultrafast intramolecular ET reactions is reviewed in the context of inorganic mixed-valence chemistry. Finally, the concept of Robin–Day Class II/III “borderline” complexes is considered, and a concise definition of the localized to delocalized transition is provided in terms of the relative contributions of external solvent and internal complex ion vibrational modes to ET.     

Intermolecular interactions and geometry of non-rigid molecules in weak crystalline complexes: Vibrational spectra of 4,4′-dinitrobiphenyl complexes with biphenyl,biphenyl derivatives and p-terphenyl

Raman spectroscopy is used to study the complexes of 4,4′-dinitrobiphenyl with biphenyl, 4-hydroxybiphenyl, 4-bromobiphenyl and p-terphenyl, which crystallize in a highly unusual geometry. Their phonon spectra at 125 K and 18 K are compared and the effect of isotopic substitution of biphenyl on the phonon spectra of its complex is examined. Internal vibrations of the components in the crystalline complex are compared with those observed in the pure crystals of the components. The results from both phonon and intramolecular vibration studies show that these complexes form in fixed stoichiometries, are governed by geometrical factors, and are stabilized primarily by van der Waals interaction, although other kinds of interactions may provide additional stabilization. The 4,4′-dinitrobiphenyl molecule as well as biphenyl and p-terphenyl are centrosymmetric and remain so when the complexes are cooled from room temperature to 18 K. For biphenyl complex, this conclusion is supported by the observed IR spectra which show mutual exclusion between IR-active and Raman active vibrations. Crystal splitting is observed on the 410 cm^?1 vibration of 4,4′-dinitrobiphenyl. This splitting is attributed to the presence of more than one 4,4′-dinitrobiphenyl molecules in the complex unit.     

Different level of fluorescence in Raman spectra of montmorillonites

The paper presents the study of selected montmorillonite standards by Raman spectroscopy and microscopy supported by elemental analysis, X-ray powder diffraction analysis and thermal analysis. Dispersive Raman spectroscopy with excitation lasers of 532 nm and 780 nm, dispersive Raman microscopy with excitation laser of 532 nm and 100× magnifying lens, and Fourier Transform-Raman spectroscopy with excitation laser of 1064 nm were used for the analysis of four montmorillonites (Kunipia-F, SWy-2, STx-1b and SAz-2). These mineral standards differed mainly in the type of interlayer cation and substitution of octahedral aluminium by magnesium or iron. A comparison of measured Raman spectra of montmorillonite with regard to their level of fluorescence and the presence of characteristic spectral bands was carried out. Almost all measured spectra of montmorillonites were significantly affected by fluorescence and only one sample was influenced by fluorescence slightly or not at all. In the spectra of tested montmorillonites, several characteristic Raman bands were found. The most intensive band at 96 cm⁻¹ belongs to deformation vibrations of interlayer cations. The band at 200 cm⁻¹ corresponds to deformation vibrations of the AlO₆ octahedron and at 710 cm⁻¹ can be assigned to deformation vibrations of the SiO₄ tetrahedron. The band at 3620 cm⁻¹ corresponds to the stretching vibration of structural OH groups in montmorillonites.     

Electrodeposition of nanostructured diamond-like films by oxidation of lithium acetylide

Diamond-like carbon (DLC) films have been deposited by anodic oxidation of 4 M solution of lithium acetylide in dimethylsulfoxide on the surface of stainless steel or nickel electrode at room temperature and moderate anodic current densities (0.2–2.0 mA/cm²) in the range of electrode potentials 0.3–2.5 V (vs. sat. Ag|AgCl reference electrode). Electrodeposited DLC coatings represented complete and optically transparent films of a thickness 50–100 nm having dark island inclusions with a diameter 0.8–5.0 μm. The concentration and average size of these particles increased with the prolongation of deposition time. Micro-Raman spectra obtained by the focusing of laser beam onto these dark inclusions are characterized by a broad peak centered at 1500 cm⁻¹ and weak peak at 1200 cm⁻¹. With a defocused laser beam, there appear two well-distinguished peaks on the integrated Raman spectra – at 1530 and 1130 cm⁻¹. Analysis of Raman spectra with the use of a Breit–Wigner–Fano lineshape and spectrum deconvolution indicates that the electrodeposited films consist of diamond-like nanostructured carbon with a high content (70–80%) of sp³ phase.     

A Raman and resonance Raman study of polybromide anions and a study of the temperature dependence of the Raman-active phonons of tetrabutylaminonium tribromide

Raman and resonance spectroscopy are shown to be capable of characterizing the tribromide and pentabromide anions and interstitial dibromine. The anions have Raman-active phonons which display a broad maximum in their excitation profiles at 600 nm, whereas interstitial bromine has a maximum in its excitation profile shifted towards the blue.Large shifts in band wavenumber are observed for the symmetric and asymmetric bromine stretching vibrations in tribromide salts with different cations. The bromine stretch force constants for the tribromide anions, for dibromine and for the recently reported decabromide dianion, have been shown to obey the relationship, k=0.01r^−6.7.The Raman-active phonons of tetrabutylammonium tribromide have been studied as a function of temperature, from 10 to 300 K. No first order phase changes were observed and remarkably small coefficients (∂ν/∂T)_P, were measured for both the internal and the external modes.     

Raman spectra of biphenyl negative ion in tetrahydrofuran solution

Raman spectra of the mono-negative ion of biphenyl in tetrahydrofuran solution were obtained by Ar⁺, He–Ne and Kr⁺ laser exciting lines. The observed frequency shift from the neutral molecule and intensity enhancement by resonance Raman effect is discussed on the basis of MO considerations.     

Observation of photodissociation of S2Cl2 and resonance Raman and fluorescence spectra of S2Cl under 514.5 nm radiation

The laser Raman spectrum of S₂Cl₂ varies with the sample temperature and/or the laser power. The Raman signals of S₂Cl₂ decreases as the sample molecules within the laser beam are dissociated by absorbing 514.5 nm photons. Above 540 K and 2 W of laser power, new resonance Raman and fluorescence bands appear. These bands were all assigned to S₂Cl. The fluorescence bands could be classified into two transition systems. Only one of them had the ground electronic state X̃ as its lower state. For the other, the low lying first excited state à was suspected. The fundamental frequencies suggested for the three vibrational modes were 664, 196 and 450 cm⁻¹ for the X̃ state and 630, 249 and 554 cm⁻¹ for the à state respectively.     

The resonance raman effect of TiBr4 and TiL4

Raman spectra of TiBr₄ and Til₄ in solutions were obtained using the excitation lines of a HeNe and Ar⁺ laser. The spectra of TiBr₄ showed a pre-resonance effect with selective enhancement of the stretching vibrations for the shorter wavelength of excitation. A typical resonance Raman effect was observed for TiI₄ by the 5145 and 4880 Å excitation lines, which lie close to the maximum of the first absorption band, giving an intense overtone progression of the totally symmetric mode ν₁.     

Comparison of the Electronic and Vibrational Optical Activity of a Europium(III) Complex

The geometry and the electronic structure of chiral lanthanide(III) complexes are traditionally probed by electronic methods, such as circularly polarised luminescence (CPL) and electronic circular dichroism (ECD) spectroscopy. The vibrational phenomena are much weaker. In the present study, however, significant enhancements of vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectral intensities were observed during the formation of a chiral bipyridine–Eu^III complex. The ten‐fold enhancement of the vibrational absorption and VCD intensities was explained by a charge‐transfer process and the dominant effect of the nitrate ion on the spectra. A much larger enhancement of the ROA and Raman intensities and a hundred‐fold increase of the circular intensity difference (CID) ratio were explained by the resonance of the λ=532 nm laser light with the ⁷F₀ → ⁵D₀ transitions. This phenomenon is combined with a chirality transfer, and mixing of the Raman and luminescence effects involving low‐energy ⁷F states of europium. The results thus indicate that the vibrational optical activity (VOA) may be a very sensitive tool for chirality detection and probing of the electronic structure of Eu^III and other coordination compounds.