Resonance Raman Cross Sections Research Articles

Using resonance Raman cross-section data to estimate the spin-state populations of cytochromes P450

The cytochromes P450 (CYPs) are heme proteins responsible for the oxidation of xenobiotics and pharmaceuticals and the biosynthesis of essential steroid products. In all cases, substrate binding initiates the enzymatic cycle, converting ferric low spin (LS) to high-spin (HS), with the efficiency of the conversion varying widely for different substrates, so documentation of this conversion for a given substrate is an important objective. Resonance Raman (rR) spectroscopy can effectively yield distinctive frequencies for the ν3 "spin state marker" bands. Here, employing a reference cytochrome P450 (CYP101), the intensities of the ν3 modes (ILS) and (IHS) relative to an internal standard (sodium sulfate) yield relative populations for the two spin states; i.e., a value of 1.24 was determined for the ratio of the relative cross sections for the ν3 modes. Use of this value was then shown to permit a reliable calculation of relative populations of the two spin states from rR spectra of several other Cytochromes P450. The importance of this work is that, using this information, it is now possible to conveniently document by rR the spin state population without conducting separate experiments requiring different analytical methods, instrumentation and additional sample.

Ultraviolet Resonance Raman Spectroscopy of Explosives in Solution and the Solid State

Resonance Raman cross sections of common explosives have been measured by use of excitation wavelengths in the deep-UV from 229 to 262 nm. These measurements were performed both in solution and in the native solid state for comparison. While measurements of UV Raman cross sections in solution with an internal standard are straightforward and commonly found in the literature, measurements on the solid phase are rare. This is due to the difficulty in preparing a solid sample in which the molecules of the internal standard and absorbing analyte/explosive experience the same laser intensity. This requires producing solid samples that are mixtures of strongly absorbing explosives and an internal standard transparent at the UV wavelengths used. For the solid-state measurements, it is necessary to use nanostructured mixtures of the explosive and the internal standard in order to avoid this bias due to the strong UV absorption of the explosive. In this study we used a facile spray-drying technique where the analyte of interest was codeposited with the nonresonant standard onto an aluminum-coated microscope slide. The generated resonance enhancement profiles and quantitative UV-vis absorption spectra were then used to plot the relative Raman return as a function of excitation wavelength and particle size.

Resonance Raman Intensity Analysis of ClNO2 Dissolved in Methanol

Halogens such as chlorine are converted from halides, including ClNO(2), to reactive radicals by UV solar radiation. These radicals can affect ozone production and destruction in the stratosphere. Recently, it became clear that halogen radicals can also play a significant role in the chemistry of the troposphere. The photochemistry of ClNO(2) has been the subject of several studies in the gas and solid state that demonstrated a clear phase-dependent reactivity. Here, we report our initial studies of nitryl chloride in solution. Resonance Raman (RR) spectra of ClNO(2) dissolved in methanol after excitation within the 1(1)A(1)-2(1)A(1) absorption band (D band) in the region 200-240 nm are presented. RR intensity along the NO symmetric stretch coordinate (v(1)) at 1291 cm(-1) is observed at all excitation wavelengths, whereas limited intensity corresponding to the transition of the N-Cl symmetric stretch (v(3)) was only observed at 199.8 nm, whereas no intensity corresponding to the O-N-O symmetric bend (v(2)) was observed. Depolarization ratios and absolute resonance Raman cross sections for v(1) were obtained at several excitation wavelengths spanning the D band. Depolarization ratios were found to deviate significantly from 1/3, consistent with more than a single dipole-allowed electronic transition contributing to the scattering. RR intensity analysis (RRIA) reveals that two closely spaced excited electronic states contribute to the scattering, which are dissociative along the Cl-N coordinate. In this study the role the solvent environment plays in ClNO(2) state energetics and excited structural evolution along fundamental coordinates is discussed.

Resonance Raman Spectra of TNT and RDX Using Vibronic Theory, Excited-State Gradient, and Complex Polarizability Approximations

Geometries, UV absorption bands, and resonance Raman (RR) cross sections of TNT and RDX are investigated using density functional theory (DFT) in conjunction with the Coulomb attenuated B3LYP exchange-correlation functional. The absorption and RR spectra are determined with use of vibronic (VB) theory, excited-state gradient, and complex polarizability (CPP) approximations. We examined low-energy isomers (two for TNT and four for RDX) whose energies differ by less than 1 kcal/mol, such that they would appreciably be populated at room temperature. The two TNT isomers differ by an internal rotation of the methyl group, while the four conformers of RDX differ by the arrangements of the nitro group relative to the ring. Our theoretical optical properties of the TNT and RDX isomers are in excellent agreement with experimental and recent CCSD-EOM results, respectively. For the two TNT isomers, the ultraviolet RR (UVRR) spectra are similar and in good agreement with recently measured experimental results. Additionally, the UVRR spectra computed using the excited-state and CPP approaches compare favorably with the VB theory results. On the other hand, the RR spectra of the RDX conformers differ from one another, reflecting the importance of the positioning of the NO2 groups with respect to the ring. In the gas phase or in solution, RDX would give a spectrum associated with a conformationally averaged structure. It is encouraging that the computed spectra of the conformers show similarities to recent measured RDX spectra in acetonitrile solution, and reproduce the 10-fold decrease in the absolute Raman cross sections of RDX compared to TNT for the observed 229 nm excitation. We show that in TNT and RDX vibrational bands that couple to NO2 or the ring are particularly resonance enhanced. Finally, the computed RDX spectra of the conformers present a benchmark for understanding the RR spectra of the solid-phase polymorphs of RDX.

Direct Measurement of Resonance Raman Spectra and Cross Sections by a Polarization Difference Technique

Resonant Raman (RR) spectroscopy, despite its many promising applications in analytical chemistry and biology, remains an experimental challenge (compared to standard Raman) primarily because of the presence of large fluorescence backgrounds overwhelming the RR signals. The observation of RR spectra of fluorophores therefore requires the use of specialized, picosecond-time-resolved setups. Here, we present and demonstrate a method, based on polarization-difference, by which RR spectra and cross sections can be measured using the most standard Raman setup with continuous wave excitation and CCD-based detection. The method is applied to the dyes Nile Blue and rhodamine 6G under resonant excitation. This work should open a new era in RR spectroscopy, where RR spectra can be routinely measured and studied with conventional Raman systems.

Resonance Raman enhancement of pyridine on Ag clusters

Calculation of the dimensionless excited state displacements (ESD) using density functional theory (DFT) was used to estimate the resonance Raman cross sections of pyridine on Ag clusters using the time-correlator formalism. The model systems studied are supermolecules consisting of Ag clusters with bound pyridine, Pyr(Ag)n (n=2, 4, 8, 14 and 20). It was found that structural changes in pyridine observed in the ESD for all five Pyr(Ag)n supermolecular clusters were essentially equivalent, and similar to those observed for the strongly allowed 168nm transition of free pyridine. Enhancements of ∼103–106, were calculated for all Pyr(Ag)n, which is comparable to the overall surface enhanced Raman spectroscopy (SERS) effect in studies of pyridine on Ag, suggesting that resonance Raman may be a significant contributor to the effect.

Resonance Raman Intensity Analysis of the A Band Short-Time Photochemical Dynamics of 4,5-Ethylenedithio-1,3-dithiole-2-thione

Resonance Raman spectra (RRs) for 4,5-ethylenedithio-1,3-dithiole-2-thione (EDDT) were obtained with 397.9 and 416 nm excitation wavelengths, and density functional calculations were performed to elucidate the electronic transitions and the RRs of EDDT in chloroform solvent. The RRs indicate that the Franck-Condon region photodynamics have multidimensional character with nuclear motion predominantly along the C(4)═C(5) stretch and the C(4)═C(5) twist out-of-plane. Resonance Raman cross-sections of A-band absorption have been obtained for the vibrational modes of EDDT with its excitation frequencies spanning the 408 nm. Resonance Raman intensity analysis of the resulting RRs excitation profiles and absorption spectrum using a time-dependent wave packet formalism yields mode-specific nuclear displacement and vibrational reorganizational energies. The intensity analysis results for EDDT were compared to previously reported results for dimethyl 1,3-dithiole-2-thione-4,5-dicarboxylate (DDTD), which shows that the additional six-member heterocycle of EDDT strongly affects the reorganizational energy and energy participation. The authors briefly discuss the differences and similarities of the spectra in terms of molecular symmetry and electron density.

Resonance Raman and density functional study of the excited state structural dynamics of 3-amino-2-cyclohexen-1-one in water and acetonitrile solvents

FT-Raman and/or FT-IR spectra of 3-amino-2-cyclohexen-1-one (ACyO) in solid state and/or in solvents of water and acetonitrile were obtained. Density functional theory calculations were done to help elucidate the vibrational band assignments. The A-band resonance Raman spectra of ACyO were acquired in water and acetonitrile solvents to examine the excited state structural dynamics and the state-mixing or curve-crossing tuned by solvents. A preliminary resonance Raman intensity analysis using the time-dependent wave-packet theory and simple model was done for ACyO in acetonitrile solvent. Resonance Raman spectroscopic probing of the excited state curve-crossing or state-mixing was proposed.

Templated Photonic Crystal Fabrication of Stoichiometrically Complex Nanoparticles for Resonance Raman Solid Cross Section Determinations

Development of methods to fabricate nanoparticles is of great interest for many applications. In this paper, we developed a facile method to fabricate complex stoichiometrically defined nanoparticles by ultilizing the defined volume interstices of close-packed photonic crystals. Fabrication of small defined size nanoparticles enables measurements of resonance Raman cross sections of solid materials. We successfully ultilized this method to fabricate mixed NaNO3/Na2SO4 nanoparticles with a defined stoichiometry on the surface of the photonic crystal spherical particles. We used these stoichiometrically defined NaNO3/Na2SO4 nanoparticles to determine the solid UV resonance Raman cross section of the NO3¯ ν1 symmetric stretching band (229 nm excitation wavelength) by monitoring the Raman spectrum of Na2SO4 as an internal standard. These are the first resonance Raman cross section measurements of solids that avoid the biasing of self-absorption. These NaNO3/Na2SO4 nanoparticles appear to show a more facile photolysis than is observed for normal solid NaNO3 samples. This templated photonic crystal fabrication of complex nanoparticle method will be important for many applications.

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

AbstractThe vibronic coupling between the first excited S1 (21Ag) and the second excited S2 (11Bu) 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 (11Ag)S2 (11Bu) 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.

UV Resonance Raman Study of TTR(105−115) Structural Evolution as a Function of Temperature

UV resonance Raman spectroscopy was used to probe the temperature dependence of the conformation of TTR(105-115) in solution. Resonance Raman spectra with excitation at 239.5 nm, show an increase in the absolute resonance Raman cross section of Tyr with an increase in temperature. This trend is associated with an increase in the hydrophobicity of the Tyr local environment, suggesting a conformational change at 28 °C. Excitation at ~200 nm is known to enhance scattering due to amide vibrations and provides insights as to the secondary structure of a peptide or protein. UVRR spectra at this excitation suggest that in solution the peptide assumes a disordered conformation with frequent formation of β-turns. Explicit-solvent replica-exchange MD simulations of the isolated peptide in the region 15 to 37 °C suggest that the dominant conformation assumed by the peptide corresponds to a coil with β-turns in the central and C-terminal region. In line with the experiments, an increase in temperature induces structural order in the peptide, reflected by an increase in the probability for the formation of β-turns and hydrophobic side-chain contacts, mainly in the 8-11 moiety, and to a lesser extent in the 4-7 moiety.

Ab Initio Calculation of Resonance Raman Cross Sections Based on Excited State Geometry Optimization

A method for the calculation of resonance Raman cross sections is presented on the basis of calculation of structural differences between optimized ground and excited state geometries using density functional theory. A vibrational frequency calculation of the molecule is employed to obtain normal coordinate displacements for the modes of vibration. The excited state displacement relative to the ground state can be calculated in the normal coordinate basis by means of a linear transformation from a Cartesian basis to a normal coordinate one. The displacements in normal coordinates are then scaled by root-mean-square displacement of zero point motion to calculate dimensionless displacements for use in the two-time-correlator formalism for the calculation of resonance Raman spectra at an arbitrary temperature. The method is valid for Franck-Condon active modes within the harmonic approximation. The method was validated by calculation of resonance Raman cross sections and absorption spectra for chlorine dioxide, nitrate ion, trans-stilbene, 1,3,5-cycloheptatriene, and the aromatic amino acids. This method permits significant gains in the efficiency of calculating resonance Raman cross sections from first principles and, consequently, permits extension to large systems (>50 atoms).

Quantifying Resonant Raman Cross Sections with SERS

We propose a method based on surface-enhanced Raman scattering (SERS) to estimate the resonance Raman cross sections of dyes. The latter are notoriously difficult (or impossible) to obtain by normal (spontaneous) constant wave Raman spectroscopy when the fluorescence quantum yield of the molecules is good and the overwhelming effect of fluorescence masks the Raman spectrum. We propose here to use the fluorescence quenching occurring in SERS conditions to overcome simply this problem. The principles of the method are described and its limitations discussed in detail. The method is demonstrated by estimating the resonance Raman differential cross sections for Rhodamine 6G for seven different excitation wavelengths across the visible range.

Excited-State Geometry Method for Calculation of the Absolute Resonance Raman Cross Sections of the Aromatic Amino Acids

The time correlator formalism was used to calculate the absolute resonance Raman cross sections for the aromatic amino acids based on density functional theory calculations of the ground-state potential energy surfaces combined with projection along normal mode eigenvectors in the excited state. The geometric difference between the minima of the ground and excited states along each normal mode was calculated to provide inputs for the time correlator in the linear approximation. The calculated dimensionless nuclear displacements, Delta(i), provide the electron-phonon coupling constants, S(i) = Delta(i)(2)/2, for the corresponding Raman active mode of frequency omega(t). The method is generally applicable to molecules that are Franck-Condon active. As an example we have chosen to calculate the absolute resonance Raman cross sections of models of the aromatic amino acids phenylalanine, tyrosine, and tryptophan. We discuss the role played by substituents on the aromatic ring that decrease vibronic activity to a level that permits application of the time correlator. While the method may have limitations for molecules of high symmetry, the current study of excited-state displacements and electronic structure indicates that the L(a),(b) states are Franck-Condon active in the aromatic molecules studied.

Calculation of absorption and resonance Raman cross sections of ClO2 in the gas‐phase and in solution: including Duschinsky effects

AbstractThe time‐correlation function formalism has been used to calculate resonance Raman cross sections, excitation profiles, and electronic absorption spectra of the OClO molecule in the gas‐phase and in different solvents like cyclohexane, chloroform, and water. The multidimensional time domain integrals that arise in these calculations have been evaluated for the case in which an X2B1Ã2A1 electronic transition takes place between displaced‐distorted‐rotated harmonic potential energy surfaces. Ab initio calculations have been performed to provide the spectroscopic constants required for the evaluation of these integrals. The calculated absorption spectra and resonance Raman cross sections have been compared with the experimental results. Copyright © 2009 John Wiley & Sons, Ltd.

Comparative analysis of the resonant Raman cross sections of pure samples and oxides

AbstractMonochromatic synchrotron radiation was used to study the Raman effect on pure samples of Mn, Fe, and Cu and on oxides such as Mn2O3, Fe2O3, CuO and Cu2O. Energy scannings were carried out for different ranges of energies near the absorption edge of the target element. Theoretical models for the energy distribution of the Raman scattered photons, convoluted with the instrument function, were used to determine the resonant Raman scattering (RRS) cross sections as a function of the incident energy. The results indicate that RRS cross sections for pure samples are similar to those for oxides for the same energy difference from the respective edge, showing that the Raman scattering takes place in the same way in both kinds of samples. Copyright © 2008 John Wiley & Sons, Ltd.

Study of resonance Raman cross section of aqueous β -carotene at low concentrations

Applying Teflon-AF waveguide cell, we determined the absolute resonance Raman cross sections (RCS) for C=C and C-C stretching modes of β-carotene in aqueous solution at low concentrations (10-6–10-10 M) by measuring Raman intensity. It was unexpected that the RCS increased rapidly with the decrease of concentration at extremely low concentration range (10-8–10-10 M), and it reached the value of 1.22×10-21 cm2 molecule-1Sr-1 at 2.5×10-10 M larger than general RCS (10-30 cm2 molecule-1Sr-1) for 109 times. Additionally, we give the tentative theory exploration for the experimental results that the large RCSs at low concentrations attributed to both resonance Raman effect and dipole-induced dipole interactions between solute and solvent molecules.

Electron-phonon interaction and its influence on reconstruction of single-walled carbon nanotube radial breathing mode spectra

The relative electron-phonon interaction matrix elements (Mph) for 28 (n,m) semiconducting single walled carbon nanotubes (SWNTs) species were extracted from the resonance Raman cross sections of individually dispersed high-pressure CO SWNTs. The observed Mph pattern was fitted according to nanotube family [i.e., (2n+m)=const] and modality [i.e., mod(n−m,3)=1, or 2] using an empirical equation based on trigonal warping effects. The predicted Mph trends enabled significant improvement to the reconstruction of the radial breathing mode spectra, so that a better match was obtained to the experimental spectra for both dispersed and aggregated SWNTs. The latter provides an initial indication that the observed Mph trends are preserved during aggregation.

Strain-Induced Interference Effects on the Resonance Raman Cross Section of Carbon Nanotubes

In this Letter, we report the effects of strain on the electronic properties of single-wall carbon nanotubes. When we normalize the electronic transition energies to the corresponding values obtained for unstrained tubes, we obtain that, regardless of the tube diameter, all the data collapse onto universal curves following an n - m = constant family pattern. In the case of metallic tubes, quantum interference effects on the Raman cross section are predicted for strained tubes when the energies of the lower and the upper components have nearly the same values. Experimental evidence for the strain-induced Raman cross section changes is observed in single nanotube spectroscopy.

Effects of distortion–rotation of potential energy surfaces on absorption and resonance raman cross sections of trans‐stilbene molecule

AbstractThe time–correlation function formalism has been used to calculate resonance Raman cross sections and electronic absorption spectra of the trans‐stilbene molecule. The multidimensional time domain integrals that arise in these calculations have been evaluated for the cases in which electronic transitions take place between displaced, displaced–distorted, and displaced–distorted–rotated harmonic potential energy surfaces. The effects of distortions and rotations of potential energy surfaces on absorption, and resonance Raman cross sections are investigated quantitatively. The calculated absorption spectra and resonance Raman cross sections have been compared with the experimental results. Finally, resonance Raman excitation profiles have been simulated for five vibrational modes of trans‐stilbene, assuming the displaced–distorted–rotated harmonic oscillator model. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005