Adsorption study of 4‐MBA on TiO2 nanoparticles by surface‐enhanced Raman spectroscopy

In this paper, the adsorption of 4‐mercaptobenzoic acid (4‐MBA) on TiO₂ nanoparticles was studied mostly by surface‐enhanced Raman spectroscopy (SERS) and UV‐vis spectroscopy, at different pH values as well as under different temperatures and concentrations. The results show that the 4‐MBA molecules are bonded to the TiO₂ surface both through the sulfur atoms and COO⁻ groups at neutral or alkaline pH, but only through the sulfur atom at acidic pH. Furthermore, the 4‐MBA molecules possess high adsorptive stability on TiO₂ at a comparatively high temperature (150 °C). Concentration‐dependent SERS experiments show that the saturation concentration for 4‐MBA adsorbed on TiO₂ is about 10⁻³ M in natural case (pH = 6). Copyright © 2009 John Wiley & Sons, Ltd.     

Ultraviolet resonance Raman spectroscopy of a β‐sheet peptide: a model for membrane protein folding

The partitioning of a hydrophobic hexapeptide, N‐acetyl‐tryptophan‐pentaleucine (AcWL5), into self‐associated β‐sheets within a vesicle membrane was studied as a model for integral membrane protein folding and insertion via vibrational and electronic spectroscopy. Ultraviolet resonance Raman spectroscopy allows selective examination of the structures of amino acid side chains and the peptide backbone and provides information about local environment and molecular conformation. The secondary structure of AcWL5 within a vesicle membrane was investigated using 207.5‐nm excitation and found to consist of β‐sheets, in agreement with previous studies. The β‐sheet peptide shows enhanced Raman scattering cross‐sections for all amide modes as well as extensive hydrogen‐bonding networks. Tryptophan vibrational structure was probed using 230‐nm excitation. Increases in Raman cross‐sections of tryptophan modes W1, W3, W7, W10, W16, W17, and W18 of membrane‐incorporated AcWL5 are primarily attributed to greater resonance enhancement with the B_b electronic transition. The W17 mode, however, undergoes a much greater enhancement than is expected for a simple resonance effect, and this observation is discussed in terms of hydrogen bonding of the indole ring in a hydrophobic environment. The observed tryptophan mode frequencies and intensities overall support a hydrophobic environment for the indole ring within a vesicle, and these results have implications for the location of tryptophan in membrane protein systems. Copyright © 2009 John Wiley & Sons, Ltd.     

In‐situ Raman spectroscopy study of Ru/TiO2 catalyst in the selective methanation of CO

Raman spectroscopic technique has been used to characterize a Ru/TiO₂ catalyst and to follow in situ their structural changes during the CO selective methanation reaction (S‐MET). For a better comprehension of the catalytic mechanism, the in‐situ Raman study of the catalysts activation (reduction) process, the isolated CO and CO₂ methanation reactions and the effect of the composition of the reactive stream (H₂O and CO₂ presence) have been carried out. Raman spectroscopy evidences that the catalyst is composed by islands of TiO₂–RuO₂ solid solutions, constituting Ru–TiO₂ interphases in the form of Ru_xTi_{1 − x}O₂ rutile type solid solutions. The activation procedure with H₂ at 300 °C promotes the reduction of the RuO₂–TiO₂ islands generating Ru^o–Ti³⁺ centers. The spectroscopic changes are in agreement with the strong increase in chemical reactivity as increasing the carbonaceous intermediates observed. The selective methanation of CO proceeds after their adsorption on these Ru^o–Ti³⁺ active centers and subsequent C―O dissociation throughout the formation of CH_x/C_nH_x/C_nH_xO/CH_x―CO species. These intermediates are transformed into CH₄ by a combination of hydrogenation reactions. The formation of carbonaceous species during the methanation of CO and CO₂ suggests that the CO presence is required to promote the CO₂ methanation. Similar carbonaceous species are detected when the selective CO methanation is carried out with water in the stream. However, the activation of the catalysts occurs at much lower temperatures, and the carbon oxidation is favored by the oxidative effect of water. Copyright © 2015 John Wiley & Sons, Ltd.     

Detailed analysis of the charge transfer complex N,N‐dimethylaniline–SO2 by Raman spectroscopy and density functional theory calculations

Although the amine sulfur dioxide chemistry was well characterized in the past both experimentally and theoretically, no systematic Raman spectroscopic study describes the interaction between N,N‐dimethylaniline (DMA) and sulfur dioxide (SO₂). The formation of a deep red oil by the reaction of SO₂ with DMA is an evidence of the charge transfer (CT) nature of the DMA–SO₂ interaction. The DMA–SO₂ normal Raman spectrum shows the appearance of two intense bands at 1110 and 1151 cm⁻¹, which are enhanced when resonance is approached. These bands are assigned to ν_s(SO₂) and ν(ϕ N) vibrational modes, respectively, confirming the interaction between SO₂ and the amine via the nitrogen atom. The dimethyl group steric effect favors the interaction of SO₂ with the ring π electrons, which gives rise to a π–π* low‐energy CT electronic transition, as confirmed by time‐dependent density functional theory (TDDFT) calculations. In addition, the calculated Raman DMA–SO₂ spectrum at the B3LYP/6‐311 + + g(3df,3pd) level shows good agreement with the experimental results (vibrational wavenumbers and relative intensities), allowing a complete assignment of the vibrational modes. A better understanding of the intermolecular interactions in this model system can be extremely useful in designing new materials to absorb, detect, or even quantify SO₂. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopy of sol–gel derived titanium oxide thin films

Raman spectra of TiO₂ films prepared via the sol–gel process were studied by UV and visible Raman spectroscopy. The evolution of the phases of TiO₂ films during annealing was investigated, and the relative intensities of the Raman bands excited with 325 nm were found to be distinct from those of the bands excited with 514 nm. The transmittance and FTIR spectra of the films annealed at different temperatures were characterized. The crystallization process of the powders and thin films treated by different annealing methods were also studied with Raman spectroscopy. The results show that the change in the relative intensities is caused by the resonance Raman effect. The anatase to rutile transition of the powder occurs at 700 °C, while that of the thin film occurs at 800 °C. The analysis of Raman band shape (peak position and full width at half‐maximum) after conventional furnace annealing and rapid thermal annealing indicates the influence of the non‐stoichiometry and phonon confinement effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Adsorption of 2‐amino‐5‐cyanopyridine on a gold surface as probed by surface‐enhanced Raman spectroscopy

The adsorption of 2‐amino‐5‐cyanopyridine (2‐ACP) was investigated in solution at different pH values by i n situ surface‐enhanced Raman scattering (SERS) spectroscopy combined with the electrochemical method. The assignments of the main bands were first performed on the basis of the spectral features of similar compounds and with the help of density functional theory calculations. The results revealed that the adsorption and the interfacial structure of 2‐ACP on the Au electrode depended on the applied potential and the pH values of the solution. In the natural solution, 2‐ACP was adsorbed on the surface with a vertical orientation by the CN group from − 0.4 to − 1.0 V, whereas in the − 0.4 to 0.8 V range, the N atom of the pyridine ring was bound to the surface. A transition region for the reorientation of the two adsorption modes was observed from − 0.8 to − 0.4 V. A flat configuration was preferred at an extremely negative potential. A similar surface adsorption behavior was observed in the alkali environment, while the Stark effect slope decreased because of the adsorption of OH⁻. Due to the protonation of N atom in the acidic solution, the potential region for the coexistence of two configurations ranged from − 0.4 to 0.2 V. Additionally, a similar adsorption configuration was proposed on the Au colloids at various pH values. The results revealed that the adsorption behavior became more complex on colloidal surfaces than that on a rigid electrode surface. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental assessment of the phonon confinement in TiO2 anatase nanocrystallites by Raman spectroscopy

TiO₂ aerogels prepared by sol‐gel method and followed by supercritical drying have been annealed at temperatures ranging between 400 and 550 °C. The obtained TiO₂ anatase crystallites with the mean size between 6.4 and 13.9 nm, as obtained from transmission electron microscopy measurements, have been further investigated by Raman spectroscopy. It was found that the peak position and full width at half maximum (FWHM) of the TiO₂ anatase Raman bands located around 144, 398, and 638 cm⁻¹ are influenced by crystallite dimension. These spectral changes can be assigned to the combined action of several nanosize effects such as phonon confinement, phonon coupling, strain, and stoichiometry defects. Surprisingly, the best discrimination of the FWHM change with the nanocrystallite mean size was achieved for the 638 cm⁻¹ band, whereas the best discrimination for the peak position was found for the 398 cm⁻¹ band. The critical size values obtained from the peak position and FWHM evaluation were between 12.7 and 13.1 nm. Taking into consideration that only the phonon confinement and inhomogeneous strain can induce an asymmetric broadening of the Raman signal, the bands asymmetry was evaluated, and the critical size values of the nanocrystallites were determined to be between 10 and 11 nm. For a symmetric size distribution of TiO₂ anatase crystallites with dimensions between 6.4 and 13.9 nm, the obtained result indicates that the phonon confinement contribution to the overall size effects is more than 75%. No evidence about the influence of the phonon coupling and vacancies on the Raman features was observed. The comparison of the data derived from the experimental analysis with those obtained by applying the theoretical phonon confinement model indicates the necessity of developing an improved phonon confinement model. The asymmetry approach can be applied for a great variety of nanostructures, as a measure of the confinement effect. Copyright © 2011 John Wiley & Sons, Ltd.     

The excited state dynamics study of di‐2‐pyridylketone in the A‐band and B‐band absorptions by using resonance Raman spectroscopy,IR and UV–visible spectroscopy

Excitation wavelengths of 282.4, 273.9 (A band), 252.7, 239.5 and 228.7 nm (B band) resonance Raman spectra were acquired for di‐2‐pyridylketone, and density functional calculations were carried out to help in the elucidation of the photo relaxation dynamics of A‐band and B‐band electronic transitions. The resonance Raman spectra show that the intensity pattern of the A band presents great difference from that of the B band, which indicate that the short‐time A‐band (S₀→S₄) photo relaxation dynamics have substantial difference from that of B band (S₀→S₁₀) . The overall picture of short‐time dynamics and the vibronic coupling mechanisms are interpreted using Albrecht's theory. Copyright © 2012 John Wiley & Sons, Ltd.     

Excited‐state structural dynamics and vibronic coupling of 1,3‐dithiole‐2‐thione—resonance Raman spectroscopy and density functional theory calculation study

1,3‐Dithiole‐2‐thione (DTT) was synthesized and characterized using NMR, FT‐Raman, FT‐IR, UV spectroscopies. Resonance Raman spectra (RRs) were obtained with 341.5, 354.7 and 368.9 nm excitation wavelengths and density functional calculations were done to elucidate the electronic transitions and the RRs of DTT in cyclohexane solution. The RRs indicate that the Franck‐Condon region photodynamics is predominantly along the CS stretch+ H‐CC‐H scissor υ₄, accompanied by the H‐CC‐H scissor υ₃, S‐C‐S symmetric stretch υ₆, CC stretch υ₂, and overtone of the non‐totally symmetric SC‐S2 out‐of‐plane deformation 2υ₁₁. The excited‐state dynamics and the force constant of CS stretch calculated by the RRs were discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

UV‐resonance Raman micro‐spectroscopy to assess residual chromophores in cellulosic pulps

Ultraviolet‐resonance Raman (UV‐RR) micro‐spectroscopy is an appropriate and sensitive tool to assess the chromophore structures in bleached cellulosic pulps used for papermaking. The particular selectivity in detection and identification of chromophores in pulps is achieved by acquiring the UV‐RR spectra in the solid state with laser excitation at 325 nm. This wavelength corresponds to absorption of poly‐unsaturated chromophore structures in partially bleached/fully bleached pulps, and linearly correlated with the signal at ca 1600 cm⁻¹ in the UV‐RR spectra. The characteristic vibrations from particular pulp chromophore structures have been assigned from experiments with model compounds, thus allowing the establishment of a UV‐RR database. Among the components of bleached pulp, the xylan–lignin complex was suggested to be an important source of chromophores. The monitoring of pulp bleaching by UV‐RR allowed us to suggest that it is the formation of new polysaccharide‐derived chromophores upon bleaching that hinders development of further brightness and is co‐responsible for the brightness reversion of fully bleached pulps. Copyright © 2010 John Wiley & Sons, Ltd.     

Darling–Dennison resonance of thiourea adsorbed on the silver electrode revealed by surface enhanced Raman spectroscopy

We report surface enhanced Raman spectroscopy (SERS) of Darling–Dennison resonance of thiourea on Ag electrode excited at 514.5 nm laser excitation. Darling–Dennison resonance indicates that two degenerate Raman modes interact with each other and their degenerate first‐order overtone modes obtain energy and appear in Raman spectra. Our study showed that the ratio of intensity of the Darling–Dennison resonance is up to 0.24 of its fundamental Raman intensity, when the applied electrode voltage is at –0.4 V versus the saturated calomel electrode. This phenomenon was also observed on the Ag island film surface at ambient condition. These observations demonstrated strong evidence for Darling–Dennison resonance band in SERS. The implications of these observations are also discussed. Copyright © 2013 John Wiley & Sons, Ltd.     

Resonance Raman spectroscopic and density functional theory investigation of the excited state structural dynamics of 2‐mercapto‐1‐methylimidazole

The resonance Raman spectroscopy in conjunction with the density functional theory calculations were used to study the excited state structural dynamics of 2‐mercapto‐1‐methylimidazole (MMI). The experimental UV absorption bands were assigned according to the time‐dependent density functional calculations. The vibrational assignments were done for the A‐band resonance Raman spectra of MMI in water and acetonitrile on the basis of the Fourier transform infrared (FT‐IR) and FT‐Raman measurements in solid, in water and in acetonitrile and the corresponding B3LYP/6‐311+G(d, p) computations. The dynamic structures of MMI were obtained by analysis of the resonance Raman intensity pattern and normal mode analysis. The differences in the dynamic structures of MMI and thiourea were revealed and explained. The structural dynamic of MMI was found to be similar to that of 2‐thiopyrimidone in terms of major reaction coordinates and thus favored the intra‐molecular proton transfer reaction. Copyright © 2012 John Wiley & Sons, Ltd.     

UV resonance Raman spectroscopy probes the localization of tryptophan‐containing antimicrobial peptides in lipid vesicles

In this work we employed UV resonance Raman spectroscopy with 229 nm excitation to study two tryptophan‐containing antimicrobial peptides with a broad‐spectrum activity against Gram‐positive and Gram‐negative bacteria: lactoferricin B (LfB, RRWQWRMKKLG) and pEM‐2 (KKWRWWLKALAKK). UV resonance Raman spectra of both peptides are dominated by tryptophan bands. Raman spectra of LfB and pEM‐2 in D₂O and 2,2,2‐trifluoro ethanol (TFE) have been measured and used to identify the hydrogen‐bond strength marker bands W6 and W17. The tryptophan doublet, W7, at 1340 and 1360 cm⁻¹ was used to detect an increase in the hydrophobicity of Trp environment in TFE. The spectra of LfB in complex with model cell membranes composed of zwitterionic dipalmitoylglycero‐phosphocholine (DPPC) or anionic dipalmitoyglycero‐phosphoglycerol (DPPG) lipid vesicles revealed a more hydrophobic Trp environment in DPPG, suggesting stronger interactions between the cationic peptide and anionic model cell membrane. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental and theoretical studies of Raman spectroscopy on 4‐mercaptopyridine aqueous solution and 4‐mercaptopyridine/Ag complex system

Raman scattering and surface‐enhanced Raman scattering (SERS) have been used to study the behavior of 4‐mercaptopyridine (4‐Mpy) dissolved in water and adsorbed on silver mirrors. In order to gain the actual structure and the theoretical modes of the 4‐Mpy dissolved in water and adsorbed on the surface of silver mirror, ab initio calculation at the Hartree–Fock (HF) level and density functional theory (DFT) at Beck's three‐parameter Lee‐Yang‐Parr (B3LYP) level were performed to calculate the vibrational modes and wavenumbers. 4‐Mpy/2H₂O and 4‐Mpy/Ag complex systems were optimized, and then the corresponding Raman spectra were calculated and analyzed. Compared with the experimental results, the calculated results of 4‐Mpy and 4‐Mpy/2H₂O complex systems obtained from DFT method were more accurate. Among the results calculated with HF method, the one with three Ag atoms was economical, which took less computer time but gave equivalent results to those with more noumber of Ag atoms. Copyright © 2007 John Wiley & Sons, Ltd.     

Evidences of the evolution from solid solution to surface segregation in Ni‐doped SnO2 nanoparticles using Raman spectroscopy

Ni‐doped SnO₂ nanoparticles, promising for gas‐sensing applications, have been synthesized by a polymer precursor method. X‐ray diffraction (XRD) and transmission electron microscopy (TEM) data analyses indicate the exclusive formation of nanosized particles with rutile‐type phase (tetragonal SnO₂) for Ni contents below 10 mol%. The mean crystallite size shows a progressive reduction with the Ni content. Room‐temperature Raman spectra of Ni‐doped SnO₂ nanoparticles show the presence of Raman active modes and modes activated by size effects. From the evolution of the A_1g mode with the Ni content, a solubility limit at ∼2 mol% was estimated. Below that content, Raman results are consistent with the occurrence of solid solution (ss) and surface segregation (seg.) of Ni ions. Above ∼2 mol% Ni, the redshift of A_1g mode suggests that the surface segregation of Ni ions takes place. Disorder‐activated bands were determined and their integrated intensity evolution with the Ni content suggest that the solid‐solution regime favors the increase of disorder; meanwhile, that disorder becomes weaker as the Ni content is increased. Copyright © 2010 John Wiley & Sons, Ltd.     

Polarized Raman scattering in helimagnetic β‐MnO2

A rutile β‐MnO₂ film was grown on MgO substrate using plasma‐assisted molecular beam epitaxy (PAMBE) monitored by reflection high‐energy electron diffraction (RHEED). Polarized Raman spectra at various temperatures were obtained to investigate the influence of the helimagnetic structure on the vibrational modes of β‐MnO₂. A red shift of E_g modes indicates a gradual formation of spin angles between neighboring Mn⁴⁺ ions. The intensities of the E_g and A_1g modes with y‐polarized incidence increase remarkably below the Néel temperature. A new view as vibrational mode projection (VMP) indicates the interactions between the magnetic component of incident light and the helimagnetic structure. Copyright © 2008 John Wiley & Sons, Ltd.     

Out‐of‐plane deformations of the heme group in different ferrocytochrome c proteins probed by resonance Raman spectroscopy

We measured the low‐wavenumber polarized resonance Raman spectra of horse heart (hhc), chicken (chc) and yeastC102T (yc) ferrocytochromes c with Soret excitation. We examined the out‐of‐plane (oop) deformations of the heme groups by virtue of relative intensities and depolarization ratios of a variety of oop and in‐plane (ip) Raman active bands. Analysis of relative Raman intensities shows differences in deviation from planarity of the heme groups of yeast, horse heart and chicken cytochromes c. The heme groups in cytochrome c proteins have been shown by normal coordinate static deformation (NSD) analysis from crystal structures to exhibit a dominant ruffling (B_1u) deformation. As a consequence the B_1u modes, γ₁₀ − γ₁₂, become resonance Raman active. We used normalized Raman intensity ratios and depolarization ratios of oop Raman active modes, whose intensities are attributable to specific nonplanar deformations, to estimate and compare their Franck‐Condon‐type and Jahn‐Teller‐type coupling magnitudes for horse heart, chicken and yeast ferrocytochrome c at neutral pH. These coupling magnitudes allow for a quantitative comparison of oop deformations between individual heme groups. Chicken ferrocytochrome was found to have the largest ruffling deformation of the three investigated proteins, followed by horse heart and yeast cytochrome c. The heme group of the former is slightly more ruffled than the corresponding active site of the latter, while saddling in both proteins is substantially larger than in chicken ferrocytochrome c. The Raman data are sensitive enough to allow a comparison of lesser deformations. Doming, which is a kinetic coordinate in many heme proteins, is largest in chicken and smallest in yeast cytochrome c. Waving is largest in yeast, followed by horse heart and chicken cytochrome c. Propellering deformations could be compared for chicken and horse heart cytochrome c and were found to be substantially larger in the latter. A comparison with heme deformations obtained from X‐ray structures (for horse heart and yeast cytochrome c) and from molecular dynamics simulations (MDS) (performed for all three proteins) yields some agreement with the main ruffling and saddling deformations derived from the crystal structures, whereas the heme conformations produced by MDS seem to account better for smaller deformations like doming and propellering. The present study demonstrates the usefulness of resonance Raman spectroscopy for the analysis of nonplanar deformations in heme proteins. Copyright © 2008 John Wiley & Sons, Ltd.     

Absorption and resonance Raman spectroscopy of the 1Bu state of trans‐1,3,5‐hexatriene including vibronic coupling

The vibronic coupling between the first excited S₁ (2¹A_g) and the second excited S₂ (1¹B_u) singlet electronic states in spectroscopy of trans‐1,3,5‐hexatriene molecule is investigated on the basis of a model consisting of two electronic states coupled by two vibrational modes. Employing a perturbation theory that treats the intramolecular couplings in a perturbative manner, the absorption and resonance Raman cross sections and excitation profiles of this molecule are calculated using the time‐correlation function formalism. The non‐Condon corrections are included in evaluation of cross sections. The multidimensional time‐domain integrals that arise in these calculations have been evaluated for the case in which S₀ (1¹A_g) S₂ (1¹B_u) electronic transition takes place between displaced and distorted harmonic potential energy surfaces. The calculated spectra are in good agreement with the experimental ones. Copyright © 2011 John Wiley & Sons, Ltd.     

Enhanced Raman scattering based on fabry‐perot like resonance in a metal‐cladding waveguide

A liquid‐core metal‐cladding waveguide structure of millimeter scale is designed to enhance Raman signal via the excitation of Fabry–Perot‐like resonance. Theoretically, an oscillating field distribution covering the whole guiding layer is generated by the multireflection at the two metal interfaces. The large detection area covers the whole sample chamber because of the oscillating nature of excited high order modes with concentrated intensity. By adding metal nanoparticles, the Fabry–Perot‐like resonance can be combined with local surface plasmons resonance to further enhance the light‐matter interaction with the target molecules, which is also confirmed by the experimental results. Copyright © 2015 John Wiley & Sons, Ltd.     

Time‐resolved resonance Raman and density functional theory study of the photochemistry of 4‐benzoylpyridine in acetonitrile and 2‐propanol

A nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopic investigation of the intermolecular hydrogen‐abstraction reaction of the triplet state of 4‐benzoylpyridine (4‐BPy) in 2‐propanol solvent is reported. The TR³ results reveal a rapid hydrogen abstraction (<10 ns) by the 4‐BPy triplet state (nπ*) with the 2‐propanol solvent, leading to formation of a 4‐BPy ketyl radical and an associated dimethyl ketyl radical partner from the solvent. The recombination of these two radical species occurs with a time constant about 200 ns to produce a para‐N‐LAT (light absorbing transient). The structure, major spectral features, and identification of the ketyl radical and the para‐N‐LAT coupling complex have been determined and confirmed by comparison of the TR³ results with results from density functional theory (DFT) calculations. A reaction pathway for the photolysis of 4‐BPy in 2‐propanol deduced from the TR³ results is also presented. The electron‐withdrawing effect of the heterocyclic nitrogen for 4‐BPy on the triplet state makes it have a significantly higher chemical reactivity for the hydrogen abstraction with 2‐propanol compared to the previously reported corresponding benzophenone triplet reaction under similar reaction conditions. In addition, the 4‐BPy ketyl radical reacts with the dimethyl ketyl radical to attach at the para‐N atom position of the pyridine ring to form a cross‐coupling product such as 2‐[4‐(hydroxy‐phenyl‐methylene)‐4h‐pyridin‐1‐yl]‐propan‐2‐ol instead of attacking at the para‐C atom position as was observed for the corresponding benzophenone reaction reported in an earlier study. Copyright © 2008 John Wiley & Sons, Ltd.