Resonance Raman spectroscopy

A Raman spectmm is normally excited using light of a frequency that is not absorbed by the sample. If we use light whose absorption probability is high, the effects of local heating by the laser beam can cause the sample 

Energy-level diagram illustrating the formation of a progression of Stokes lines in a resonance Raman spectrum. 

The resonance-enhanced method offers three major advantages over standard Raman spectroscopy  

The intensity is enhanced, enabling weakly scattering or very dilute samples to be used. 

The spectrum is greatly simplified, as only a few vibrations are enhanced. 

Resonance Raman (RR) spectroscopy, as with conventional Raman spectroscopy, is a spectroscopic technique helpful for studying characteristic vibrational, rotational, and other low-frequency modes of matter. However, this method differs from conventional Raman spectroscopy in the excitation wavelength, which is tuned to be near an electronic transition and the intensity of vibration associated with the transition increases tremendously and the remaining Raman signals are overwhelmed. As this approach can be used to study some certain vibrational modes without interruption of other signals, it is especially useful for determining the structure or components of complex samples, e.g. biomolecules. Moreover, the detection limits is about 10 times larger than conventional Raman spectroscopy and, hence, substances of much lower concentrations in complex systems can be studied. 

A comparison of standard Raman versus resonance Raman spectroscopic techniques. [Copyright Semrock, Inc., reproduced by permission from http //www.semrock.com/ultraviolet-uv-raman-spectroscopy.aspx (accessed December 21, 2013).] 

We can further simplify the equation by applying the Born-Oppenheimer approximation to separate the electronic from the vibrational wave functions, as shown in Equation (9.23), where f, v, and / are the vibrational levels of the final, intermediate, and initial states and Mp(Q) is the pure electronic transition moment. M(Q) is a function of the vibrational or nuclear coordinates and can be expanded as the Taylor series in Equation (9.24), where the summation occurs over all of the normal coordinates  

After separation, the polarizability in Equation (9.22) is the sum of four terms (i) the A-term, also known as the Condon approximation, is a result of the pure electronic transition moment and vibrational overlap integrals (ii) the B-term results from vibronic coupling of the resonant excited state with other excited states (iii) the C-term has to do with the vibronic coupling of the ground state with one or more excited states and (iv) the D-term results from vibronic coupling of the resonant excited state to other excited states coupling in both the electronic transition moments. 

The Franck-Condon factors, shown here for the process of absorption (blue line) followed by emission (green line). In a resonance Raman experiment, the LASER beam is tuned to be in resonance with an allowed electronic transition (indicated here by the blue arrow), which takes the electron from some initial state I to some intermediate state V. Unlike fluorescence, there is no vibrational relaxation of the excited state. The electron is scattered from the intermediate V state down to the i/ = 1 vibrational level of the ground electronic state, which we will call the final state F. The Franck-Condon overlap integrals involve the amount of overlap between the wave functions, which are shown on this diagram in brown. [ Mark M Samoza/CC-BY-SA 2.5/GFDL /Wimimedia Commons reproduced from http //en.wikipedia.org/wiki /Franck%E2%80%93Condon principle (accessed December 21,2013).] 

The Franck-Condon factors under different circumstances. 

Non-porphyrin FeIV=0 has been detected in bacterial cytochrome oxidases containing a chlorin cytochrome d at a slightly higher frequency (815 cm-1) than that seen in porphyrins [206]. In contrast, the non-haem ferryl model compound analysed by Leising et al. [130] has a significantly lower stretch for the FeIV=0 bond (666 cm-1). 

One advantage of Raman spectroscopy is that it is relatively easy to perform time-resolved spectra on a sub-millisecond time scale. Therefore it is possible to obtain spectra of a reaction intermediate that decays rapidly. For example, coupling resonance Raman to flash photolysis resulted in the detection of the much-hypothesised ferryl intermediate in cytochrome c oxidase [207]. 

Free radicals also have distinctive Raman stretching frequencies. For example, ribonucleotide reductase (Fig. 9) has a stretch at 1498 cm-1 that is not present in the hydroxyurea-treated radical-free protein [208], This is close to that observed [209] for deprotonated phenoxy radicals (1505 cm-1), and different [210] from that of the protonated radicals (1426 cm-1). Thus it was concluded that the tyrosine radical was the neutral deprotonated radical, a conclusion that is difficult to reach from the EPR spectra alone. 

FIGURE 6.5 Absorption spectrum (a) and CD spectrum (b) of the Fe4S4 cluster of a high-potential iron protein (HiPIP- from Chwmatiwn sp.). (From Cowan, 1997. Copyright 1997 with permission from John Wiley and Sons.) 

measurement of relative band intensities leads to a C-M-C bond angle, 6. This procedure can be generalized to other metal carbonyl systems for simple systems, rather good agreement with crystallographically determined angles is obtained. Unfortunately, few ligands other than CO prove amenable to such an analysis. 

In normal Raman spectroscopy a sample is placed in a (monochromatic) laser beam and the very weak scattered light of lower frequency is studied. In such a study the colour of the laser light is usually chosen to be away from any absorption band of the sample because such a choice reduces the risk that the focused laser beam will destroy the sample by heating it. In the resonance Raman effect the laser beam colour is deliberately chosen to coincide with an absorption band—an electronic transition—of the sample. Whilst this may lead to the destruction of the sample, for favourable cases it leads to Raman scattering which is much stronger than normal. This, in turn, means that the laser power can be reduced, improving the chances of sample survival. The spectra obtained from compounds showing such a resonance Raman effect are both simpler and more complicated than normal Raman spectra. They are simpler because, often, only totally symmetric vibrational modes are seen. The reason for this is that if the electronic 

6 Resonance Raman spectra, (a) Crystalline Til4 is the v(Ti-l) totally symmetric breathing mode, reproduced with permission from R. J. H. Clark and P. D. Mitchell, 

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The varying actual orientation of molecules adsorbed at an aqueous solution-CCU interface with decreasing A has been followed by resonance Raman spectroscopy using polarized light [130]. The effect of pressure has been studied for fatty alcohols at the water-hexane [131] and water-paraffin oil [132] interfaces. 

RRS Resonance Raman spectroscopy [212, 213] Incident light is of wave length corresponding to an absorption band Enhanced sensitivity... 

SERS. A phenomenon that certainly involves the adsorbent-adsorbate interaction is that of surface-enhanced resonance Raman spectroscopy, or SERS. The basic observation is that for pyridine adsorbed on surface-roughened silver, there is an amazing enhancement of the resonance Raman intensity (see Refs. 124—128). More recent work has involved other adsorbates and colloidal... 

Infrared and Raman spectroscopy each probe vibrational motion, but respond to a different manifestation of it. Infrared spectroscopy is sensitive to a change in the dipole moment as a function of the vibrational motion, whereas Raman spectroscopy probes the change in polarizability as the molecule undergoes vibrations. Resonance Raman spectroscopy also couples to excited electronic states, and can yield fiirtlier infomiation regarding the identity of the vibration. Raman and IR spectroscopy are often complementary, both in the type of systems tliat can be studied, as well as the infomiation obtained. 

One of the well known advantages of resonance Raman spectroscopy is that samples dissolved in water can be studied since water is transparent in the visible region. Furthennore, many molecules of biophysical interest assume their native state in water. For this reason, resonance Raman spectroscopy has been particularly strongly embraced in the biophysical connnunity. 

The advantages of resonance Raman spectroscopy have already been discussed in section BL2.2.3. For these reasons it is rapidly becoming the method of choice for studying large molecules in solution. Flere we will present one study that exemplifies its attributes. There are two complementary methods for studying proteins. 

Asher S A 1993 UV resonance Raman-spectroscopy for analytical, physical and biophysical chemistry 2 Anal. Chem. 

Bell S E J 1996 Time-resolved resonance Raman spectroscopy A/ a/ysf 121 R107-20... 

Biswas N and Umapathy S 1998 Resonance Raman spectroscopy and ultrafast chemical dynamics Curr. Sol. 74 328-40... 

Hoskins L C 1984 Resonance Raman-spectroscopy of beta-carotene and lycopene—a physical-chemistry experiment J. Chem. Educ. 61 460-2... 

Johnson B R, Kittrell C, Kelly P B and Kinsey J L 1996 Resonance Raman spectroscopy of dissociative polyatomic molecules J. Chem. Educ. 100 7743-64... 

Kincaid J R 1995 Structure and dynamics of transient species using time-resolved resonance Raman-spectroscopy Biochemical Spectroscopy Methods Enzymol. vol 246, ed K Sauer (San Diego, CA Academic) pp 460-501... 

Strommen D P and Nakamoto K 1977 Resonance Raman-spectroscopy J. Chem. Educ. 54 474-8... 

Plenary 3. Ronald E Hester et al, e-mail address reh York.ac.uk (SERS). Use of dioxane envelope to bring water insoluble cliromophores (chlorophylls) into contact with aqueous silver colloids for SERS enliancement. PSERRS— protected surface-enhanced resonance Raman spectroscopy . 

Qin L, Tripathi G N R and Schuler R H 1987 Radiolytic oxidation of 1,2,4-benzenetriol an application of time-resolved resonance Raman spectroscopy to kinetic studies of reaction intermediates J. Chem. Phys. 

Pollard W T, Dexhelmer S L, Wang Q, Peteanu L A, Shank C V and Mathles R A 1992 Theory of dynamic absorption spectroscopy of nonstatlonary states. 4. Application to 12 fs resonant Raman spectroscopy of bacterlorhodopsin J. Phys. Chem. 96 6147-58... 

Johnson A E and Myers ABA 1996 A comparison of time- and frequency-domain resonance Raman spectroscopy in triiodide J. Cham. Phys. 104 2497-507... 

Lesieur P, Vandevyver M, Ruaudel-Teixier A and Barraud A Orientational studies of Langmuir-Blodgett films of porphyrins with polarized resonant Raman spectroscopy Thin Soiid Fiims 159 315-22... 

Figure C3.1.11. Apparatus for pump-probe time-resolved resonance Raman spectroscopy. (From Varotsis C and Babcock G T 1993 K4ethods Enzymol. 226 409-31.)...

Friedman J M 1994 Time-resolved resonance Raman spectroscopy as probe of structure, dynamics, and reactivity in hemoglobin Methods Enzymol. 232 205-31... 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

P. R. Carey, ed.. Biochemical Applications of Raman and Resonance Raman Spectroscopies, Acedemic Press, Inc., New York, 1982. 

G Backes, Y Mino, TM Loehr, TE Meyer, MA Cusanovich, WV Sweeny, ET Adman, J Sand-ers-Loehr. The environment of Ee4S4 clusters in ferredoxms and high-potential iron proteins. New information from X-ray crystallography and resonance Raman spectroscopy. J Am Chem Soc 113 2055-2064, 1991. 

Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. 

The yellow disulfide radical anion and the briUiant blue trisulfide radical anion often occur together for what reason some authors of the older Hterature (prior to 1975) got mixed up with their identification. Today, both species are well known by their E8R, infrared, resonance Raman, UV-Vis, and photoelectron spectra, some of which have been recorded both in solutions and in solid matrices. In solution these radical species are formed by the ho-molytic dissociation of polysulfide dianions according to Eqs. (7) and (8). 8ince these dissociation reactions are of course endothermic the radical formation is promoted by heating as well as by dilution. Furthermore, solvents of lower polarity than that of water also favor the homolytic dissociation. However, in solutions at 20 °C the equilibria at Eqs. (7) and (8) are usually on the left side (excepting extremely dilute systems) and only the very high sensitivity of E8R, UV-Vis and resonance Raman spectroscopy made it possible to detect the radical anions in liquid and solid solutions see above. 

Heating of certain alkali halides with elemental sulfur also produces colored materials containing the anions 82 or 83 which replace the corresponding halide ions. For example, NaCl and KI crystals when heated in the presence of sulfur vapor incorporate di- and trisulfide monoanions [116-119] which can be detected, inter alia, by resonance Raman spectroscopy [120, 121] ... 

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