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Molecular Workbench Practical and Raman Spectroscopy, Part II: Application of In this second installment of a two-part series we present polarized Raman spectra and discuss the association of the symmetry species of the normal vibrational mode and the of Raman . We discuss those aspects of and Raman polar- ization rules that can be applied with normal Raman instrumentation. Materials include liquids, single , and polycrystalline compounds.

David Tuschel

he excitation in Raman spectroscopy is usually of Raman sampling and sample grain size relative to that linearly polarized monochromatic light from a of the incident beam. T laser. The Raman scattered light can be polarized parallel or perpendicular with respect to the incident Polarized Raman Spectroscopy of laser polarization depending on the symmetry species of Recall that in the first part of this series we discussed the the vibrational modes. Normally, Raman spectroscopy is normal vibrational modes of water. We found that there performed without a polarization analyzer so that both were three Raman active modes, two stretching and one polarizations of the Raman scattered light are collected to bending. Furthermore, we found that the bending mode maximize the signal. This is frequently a sensible choice. (ν2) and one of the stretching modes (ν1) are totally sym- However, there are instances for which the use of a Raman metric (A1) and therefore polarized, whereas the antisym- polarization analyzer can be helpful in both band assign- metric stretch (ν3, B2) is depolarized. The polarized Raman ment and the characterization of molecular or solid-state spectra of water shown in Figure 1 includes the depictions structure. of the normal vibrational modes that we presented in part In part I (1), we summarized and presented the most sa- I (1). Spectra are shown in the parallel and perpendicular lient and beneficial aspects of group theory when applied polarized configurations and all three of the normal vi- to vibrational spectroscopy in general and Raman spec- brational modes can be assigned based on their energies troscopy in particular. Here, we apply that knowledge to and polarization responses. There are several things to Raman spectra obtained from liquids, single crystals, and notice in these spectra. First, you see that the energy of the polycrystalline compounds. The treatment of polycrystal- bending mode, which appears in the so-called fingerprint line compounds is a cautionary tale about the importance region of the vibrational spectrum, is substantially less 16 Spectroscopy 29(3) March 2014 www.spectroscopyonline.com

the angle bending mode. As a general rule, the symmetric stretches of bonds 10,000 tend to yield stronger Raman bands. ν1 (A1) νS Parallel polarized It is often said that Raman spec- Perpendicular polarized troscopy is the vibrational spectros- copy of choice for the analysis of ana- lytes in aqueous solutions. The Raman 5000 spectrum of water demonstrates why that statement has considerable ν3 (B2) νas merit. The angle bending mode at 1635 cm-1 is weak and produces little ν2 (A1) δs Intensity (counts) background or band interference in the fingerprint region. The presence 0 500 1000 1500 2000 2500 3000 3500 of the weak angle bending band leaves the entire fingerprint region without Raman shift (cm-1) any substantial background, and this is the basis for the claim that Raman Figure 1: Polarized Raman spectra of H O. 2 spectroscopy can readily be applied to the analysis of aqueous solutions. The stretching modes beyond 3000 cm-1 15,000 Parallel polarized are significantly more intense and 459 broader. The substantial width of the Perpendicular polarized stretching modes of water can be at- A1 tributed to H bonding. The entire OH 10,000 stretching from two allowed bands 762 covers the region from approximately 2900 cm-1 to 3700 cm-1! The Raman 314 Combination 5000 218 mode 789 spectrum of water is a remarkable E T2 T2 + A1 T1 demonstration of the distribution of Intensity (counts ) vibrational energy states created by H bonding in the liquid state. 0 200 300 400 500 600 700 800 Raman shift (cm-1) Carbon Tetrachloride, Depolarization Ratios, and Isotopic “Splitting” Figure 2: Polarized Raman spectra of CCl4. Carbon tetrachloride, CCl4, is some- times referred to as the Raman spectroscopist’s favorite for 15,000 teaching purposes. The molecular ν1b 459.5 Parallel polarized structure of CCl4 is tetrahedral and ν 462.3 Perpendicular polarized 1a belongs to the point group Td. The set ν 456.3 1c of Raman active vibrational modes is 10,000 35 ν1a (A1): C CI4 given by ν (A ): C 37CI 35CI 1b 1 3 ν 453.3 (A ): C 37CI 35CI 1d  ν1c 1 2 2 vib = A1 + E + T1 + T2. [1] 37 35 ν1e 450 5000 ν1d (A1): C CI3 CI 37 Intensity (counts) ν (A ): C CI 1e 1 4 The four (1 A1, 1 E, 1 T1, and 1 T2) normal modes of vibration are as-

signed to the bands in the CCl4 spec- 0 trum shown in Figure 2. A fifth band 400 420 440 460 480 500 -1 Raman shift (cm-1) at 762 cm is assigned the combina- tion mode T2 + A1. This band is an example of combination mode Raman Figure 3: A Symmetric stretching band of CCl . The band is split in accordance with the isotopic 1 4 scattering that can sometimes appear abundance of Cl. in conjunction with the first order than that of the stretching modes. either the symmetric or antisymmet- normal modes, the so-called funda- Second, note how much more intense ric stretching modes are relative to mental vibrational modes. The topic 18 Spectroscopy 29(3) March 2014 www.spectroscopyonline.com of second order and combination mode and group theory is beyond the scope of this work; perhaps we will address this in another installment. From the equa- 1003 Parallel polarized All polarized raman bands tion above, we should expect to see one Raman band that Perpendicular polarized 20,000 arise from totally symmetric A’ is polarized, the totally symmetric A1 species. The band vibrational modes -1 at 459 cm is easily identified as the A1 totally symmetric 786 stretch because the signal nearly vanishes with the Raman 10,000 analyzer configured perpendicular to the incident laser 1030 polarization. 521 1210 1605 216 The remaining E, T1, and T2 bands should be depolar- 0 ized and we can see that the ratio of perpendicular to par- 200 400 600 800 1000 1200 1400 1600 Raman shift (cm-1) allel Raman scattering strength conforms to the expected value of 0.75. The low energy bands at 218 and 314 cm-1 Figure 4: Polarized Raman spectra of toluene. are attributed to bending modes of E and T2 symmetry, respectively. That leaves the band at 789 cm-1 assigned to the fourth fundamental mode of species T1. You may be asking yourself: On what basis did we assign the 789 cm-1 -1 2943 band rather than the 762 cm band to the T1 mode? The Parallel polarized -1 10,000 justification is that the addition of 314 and 459 cm equals Perpendicular polarized 2253 773 cm-1, which is close to and just a little higher in energy than the 762 cm-1 band. Combination and second order 5000 bands normally appear at energies slightly less than the 919 values expected from addition or multiplication of the fun- 2293 Intensity (counts) 379 damental modes. Therefore, we assign the 762 cm-1 band 1374 -1 0 to the T2 + A1 combination mode and the 789 cm band to 500 1000 1500 2000 2500 3000 3500 Raman shift (cm-1) the fourth fundamental mode of species T1. Now, let’s look a little more carefully at the A1 symmet- ric stretch at 459 cm-1, which appears to split into several Figure 5: Polarized Raman spectra of acetonitrile. bands. An expanded plot of the A1 band is shown in Figure 3. The spectral resolution of this measurement is such that the molecule provides the tools to help us make those deci- one can resolve five adjacent bands designated ν1a through sions as well as understand and interpret spectra. Beyond ν1e. The fine structure that you see in this band is attrib- Raman activity alone, the Raman scattering strengths of uted to what is referred to as isotopic splitting. However, particular vibrational modes and the relative intensities this is not actually splitting in the familiar sense of the that we observe in a spectrum are just as important to us. word because no degeneracy has been lifted to separate Is there any way that group theory can help us understand energy levels. Rather, the fine structure is because of the that aspect of Raman spectra based on signal strength? naturally occurring relative abundances of isotopic Cl, Well, not directly. However, it happens that the totally which is approximately 25% 37Cl and 75% 35Cl (2). symmetric vibrational modes are associated with the larg- 35 37 An interesting observation is that only C Cl4 and C Cl4 est changes in polarizability with respect to the vibrational 37 35 (ν1a and ν1e) retain true Td symmetry, whereas C Cl Cl3 modulation of the bond, the Raman polarizability. Fur- 37 35 37 35 and C Cl3 Cl (ν1b and ν1d) belong to C3v and C Cl2 Cl2 thermore, the greater the change in vibrationally modu- (ν1c) belongs to C2v. Therefore, the degeneracy is lifted lated polarizability, the greater the Raman signal strength in the E and T modes for the C3v and C2v species and we will be. might expect to see splitting of those bands. However, To get an appreciation of this empirical fact, let us look the isotopic mass induced symmetry differences are suf- at the polarized Raman spectra of toluene and acetonitrile ficiently small such that the collisional broadening in the shown in Figures 4 and 5, respectively. Toluene belongs to liquid prevents the splitting from being resolved in the lift- point group Cs and can be thought of as a mono-substi- ing of degeneracy in the E and T modes. The fact that the tuted (D6h). Recall that only the totally symmetric polarization ratio of the isotope components remains very vibrational modes are polarized, and we see from the char- small is further evidence for the lack of a reduction of the acter table of Cs that the Aʹ species is Raman active. Inspec- symmetry from Td to C3v, or C2v. tion of the Raman spectrum of toluene in Figure 4 reveals that the most intense Raman bands in the fingerprint re- The Raman Scattering Strength gion at 521, 786, 1003, 1030, and 1210 cm-1 are all polarized of Totally Symmetric Modes and therefore of Aʹ symmetry. Conversely, those bands that In part I (1), we discussed whether vibrational modes will are not polarized are significantly weaker than the totally be Raman active, active, or silent and found that symmetric vibrational modes. the application of group theory based on the symmetry of Now let’s consider an entirely different molecule with www.spectroscopyonline.com March 2014 Spectroscopy 29(3) 19

chemical bonds in a are all oriented in particular 40,000 254 directions and not all Raman bands can necessarily be 632 detected from one face of a single crystal. Therefore, one 30,000 275 Parallel polarized should not expect to see all Raman bands when examining 238 Perpendicular polarized a single crystal face, particularly if the crystal face exam- 20,000 264 153 ined is a crystallographic X-, Y-, or Z-axis face. The Raman -1 Intensity (counts) 10,000 581 bands at 254, 275, and 632 cm are three of the four al- 323 lowed A1 Raman bands of LiNbO3. Keeping the laser beam 0 fixed with Z-axis polarization but rotating the Raman 100 200 300 400 500 600 700 800 900 1000 Raman shift (cm-1) analyzer 90° to the perpendicular configuration selects the E modes. Note the dramatic difference in parallel and perpendicular polarized Raman spectra obtained from Figure 6: Polarized Raman spectra of the X-face of single crystal LiNbO3 with the incident laser polarized along the Z-axis. single crystals compared to the parallel and perpendicular spectra acquired from a liquid sample. In a liquid you are sampling all orientations of the molecule in space, whereas respect to chemical bonding and molecular structure, one in a crystal the chemical bonds are all oriented in space in that is not aromatic. Acetonitrile belongs to point group accordance with the crystal class to which the compound

C3v, and its spectrum is shown in Figure 5. The most in- or element belongs. tense Raman bands appear at 919, 2253, and 2943 cm-1 and The forms of the Raman polarizability tensors for crys- all three are polarized. Let’s focus on the two bands at 2253 tals are dictated by the symmetry of the crystal. By this we and 2943 cm-1 because of what they tell us about the im- mean that whether a specific Raman polarizability tensor portance of totally symmetric modes and their relationship element is zero or nonzero is dictated by the crystal sym- -1 to Raman signal strength. The 2253 cm band is assigned metry (3). For LiNbO3 the same spectrum will be obtained to the CN symmetric stretch and the 2943 cm-1 band to whether the incident laser beam is polarized along the CH stretching. Both are very intense and one might have Z-axis or the Y-axis if the Raman analyzer is configured supposed that based on functional group character alone perpendicular to the incident polarization for the backscat- that the CN stretch would be far more intense because of tering arrangement. We invite you to watch a short lecture the CN triple bond. Although bond order and functional explaining the polarized Raman spectroscopy of LiNbO3 group identity are indeed important and suggestive regard- and a real time demonstration of polarized Raman spectral ing Raman scattering strength, you can see that the nature acquisition with a single crystal at https://www.youtube. of the symmetry species to which the vibrational mode be- com/watch?v=96ob6wY-6t8. longs can be just as important to Raman signal strength. Crystals from a Reagent Bottle Polarized Raman Spectroscopy Now that you’ve seen the dramatic effects of crystal sym- of Single-Crystal LiNbO3 metry and orientation with respect to the single crystal We now move from liquids to considering single crystals. Raman experiment you should be better prepared to face

LiNbO3 is a technologically important material with a high the challenges of Raman sampling when dealing with poly- nonlinear susceptibility and electro-optic coefficient. At crystalline materials. You can encounter laser polarization room it is a ferroelectric with a crystal struc- and crystal orientation effects similar to that of single ture belonging to the C3v crystal class. The complete set of crystals when analyzing crystalline samples sprinkled onto vibrational modes is given by a glass slide from a reagent bottle. Figure 7 shows reflected light images of rubrene grains on a microscope slide. The

vib = 4A1 (IR,R) + 9E (IR,R) + 5A2 [2] crystal system of rubrene is orthorhombic and the pre- dominant form of these crystals belongs to the D2h crystal where the A1 and E modes are both Raman and infrared class with 4 per unit (4). In fact, you can see active and the A2 modes are silent. LiNbO3 is often used that orthorhombic structure manifest in the crystal habit for optical frequency doubling because of its large d33 non- of grain R_3 in Figure 7. Grain R_1 shows no particular linear coefficient. The conventional approach for second orientation whereas the crystallographic axes and faces of with LiNbO3 is to guide an incoming grain R_3 can be assigned according to its crystal habit laser beam incident upon the X-face of the crystal and po- and are depicted on the grain. larized along the Z-axis. Using that same configuration, po- The Raman active modes of D2h rubrene are species Ag, larized Raman spectra of the X-face with the incident laser B1g, B2g, and B3g wherein the B1g, B2g, and B3g modes have beam polarized along the Z-axis are shown in Figure 6. off-axis nonzero Raman tensor elements for combinations Configuring the Raman analyzer parallel to the incident of all three axes. Therefore, one can expect a rich variety polarization selects the A1 Raman bands. Unlike a liquid of spectra from a collection of single grains randomly in which the molecules are randomly oriented in space, the oriented on the microscope slide. The Raman spectra of 22 Spectroscopy 29(3) March 2014 www.spectroscopyonline.com

Conclusions In this second of a two-part series we presented polarized Raman spectra (a)-40 (b)-40 and discussed the association of the -30 -30 b symmetry species of the normal vi- -20 -20 (0,0,1) brational mode and the depolarization -10 -10 c ratio of Raman scattering. We demon- 0 0 strated that although bond order and a (1,0,0) y (µm) 10 y (µm) 10 functional group identity are indeed 20 20 (1,1,0) important and suggestive regarding 30 5µm 30 5µm Raman scattering strength, the nature 40 40 of the symmetry species to which -40 -20 0 20 40 -40 -20 0 20 40 x (µm) x (µm) the vibrational mode belongs can be just as important to Raman signal strength. The experimental applica- Figure 7: Reflected light images of rubrene grains (a) R_1 and (b) R_3. tion of polarized Raman spectroscopy for single crystals was demonstrated with the uniaxial ferroelectric crystal

LiNbO3. Finally, the Raman spectra of individual grains of rubrene from a 4000 1302 1522 R_3: Polarization in ab plane reagent bottle demonstrated the chal- lenges of Raman sampling when deal- 3000 1004 ing with polycrystalline materials. 1542 1216 1434 220 References R_1 2000 896 (1) D. Tuschel, Spectroscopy 29(2), 14–23 (2014). Intensity (counts) 1000 (2) M. Diem, Introduction to Modern R_3 Vibrational Spectroscopy (Wiley-Inter- science, New York, 1993), p. 181. 200 400 600 800 1000 1200 1400 1600 (3) D. Tuschel, Spectroscopy 27(3), 22–27 Raman shift (cm-1) (2012). (4) J.R Weinberg-Wolf, L.E. McNeil, S. Liu, Figure 8: Raman spectra of different grains of rubrene. and C. Kloc, J. Phys.: Condens. Matter 19, 276204 (15pp) (2007). grains R_1 and R_3 acquired at the tensities of the bands. That last point center of each image and without the is important. If you look carefully, you David Tuschel is use of a Raman analyzer (no polar- will see that the band positions of the a Raman applications ization selection here) are shown in and traces do indeed match; manager at Horiba Sci- Figure 8. The spectrum of R_3, the it is simply the relative intensities of entific, in Edison, New blue trace, was acquired with the inci- the bands that differ, which can lead Jersey, where he works dent laser beam polarized off axis but one to falsely conclude that the two with Fran Adar. David within the ab crystallographic plane. samples are chemically not the same is sharing authorship of Grain R_1 is not oriented in any when in fact they are. this column with Fran. He can be reached obvious way relative to the crystallo- at: [email protected]. graphic axes and its Raman spectrum is the red trace. Here, you see two grains of the same compound taken from the same reagent bottle and yet For more information on this topic, please visit: www.spectroscopyonline.com they produce significantly different spectra with respect to the relative in-

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