Polarized Spectra of Single Crystals

In the preceding section, the 30 normal vibrations of the Bravais cell of calcite crystal have been classified into symmetry species of the factor group The results (Table 1.26) show that three intramolecular (A2m + 2 J, three translatory lattice (A2u + 2 ) and two rotatory lattice (A2 + vibrations are IR-active, whereas three intramolecular (Aig + 2Eg), one translatory lattice (Eg), and one rotatory lattice (Eg) vibrations are Raman-active. In order to classify the observed bands into these symmetry species, it is desirable to measure infrared dichroism and polarized Raman spectra using single crystals of calcite. 

Suppose that we irradiate a single crystal of calcite with polarized infrared radiation whose electric vector vibrates along the c axis (z direction) in Fig. 1.42. Then the infrared spectrum shown by the solid curve of Fig. 1.48 is obtained [142]. According to Table 1.21, only the A2 vibrations are activated under such conditions. Thus, the three bands observed at 885(v), 357(0, and 106(r) cm are assigned to the species. The spectrum shown by the dotted curve is obtained if the direction of polarization is perpendicular to the c axis (x,y plane). In this case, only the vibrations should be infrared-active. Therefore the five bands observed at 1484(v), 706(v), 330(0, 182(0, and 106(r) cm are assigned to the species. Here, v, t, and r denote intramolecular, translatory lattice, and rotatory lattice modes, respectively. 

Polarized Raman spectra provide more information about the symmetry properties of normal vibrations than do polarized infrared spectra [143]. Again consider a single 

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Fig. IS. Polarized spectra of single-crystal Ba[Pt(CN)4] 4 H20 (295 K, 1 bar)30,85, ll3). The intensities of the differently polarized emission spectra can be compared. The spectra are not corrected, neither due to the response of the apparatus nor due to reabsorption effects. Excitation X, = 364 mm, E c spectral resolution 10 cm-1 crystal thickness for the absorption measurements 10 pm...

Experimentally, the direction of a transition moment in a molecule can he evaluated by four methods (i) polarized spectra of single crystals, (ii) fluorescence or phosphorescence polarization, (iii) spectra of molecules embedded in stretched Aims, and (iv) spectra of molecules oriented by external fields. Only relative directions of the transition moment can be determined by means of the last three methods, whereas the polarized spectra of single crystals give the absolute direction of the moments if the crystal structure is known. The first method has been applied to the study of the electronic structure and spectra of several pyrimidine bases of nucleic... 

These workers have proposed that when heme proteins are crystallized, low-spin forms are favored in the equilibrium high-spin form low-spin form, and changes in conformation can also be involved. [A discussion of spin forms has been given by Kotani (1964).] The use of polarized spectra of single crystals permitted an assignment of all the absorption bands in the spectra. 

Developments in instrumentation have allowed measurements of the single-crystal CD spectra of [(+ )D-Cr(en)3]3+ doped in [Ir(en)3]Cl3 between 7 and 293 K. Transitions to the excited states 4T2, 47i, ZE, 27i and 2T2 were observed.418 The discussion refers to much earlier work. The polarized electronic spectra of single crystals of several trans-Cr [XY(en)2] complexes have been assigned and ligand field parameters evaluated.419 CD, absorption and circularly polarized emission spectra have been reported for [Cr(en)3]3+ in the region of the A2g 2EgTig transitions.420... 

Figure 10.1 Comparisons of visible to near-infrared spectra of calcic pyroxene in transmitted and reflected light. Polarized absorption spectra of single crystals are correlated with the reflectance spectrum of a powdered sample of the same mineral (cf. fig. 5.14).

Abs-Wurmbach, I., Langer, K. Oberhansli, R. (1985) Polarized absorption spectra of single crystals of the chromium-bearing clinopyroxenes kosmochlor and Cr-aegerine-augite. Neues Jahrb. Mineral. Abh., 152,293-319. 

Bums, R. G., Huggins, F. E. Abu-Eid, R. M. (1972b) Polarized absorption spectra of single crystals of lunar pyroxenes and olivines. Moon, 4,93-102. 

Fig. 14. Polarized reflectance and absorption spectra of single-crystal Ba[Pt(CN)4) 4 H20 (295 K, 1 bar). The fi c polarized reflectance spectra and the corresponding calculated absorption spectra are taken from Ref. 136, the Elc polarized ones from Ref. 135, 138. The high energy structures in the reflectance spectra are enlarged by a factor of 10 (use left hand scale)...

Fig. 24. Polarized emission of single-crystal Ba[Pt(CN)4] 4 H20 for different temperatures68, M). The intensities of the spectra, recorded at the same temperature, are comparable, when multiplied by the given factors. Excitation 364 nm (E c)...

Fig. 33. Polarized emission spectra of single-crystal Ba[Pt(CN)4] 4 H20 at different hydrostatic pressures (T = 295 K)133). The emission intensities at different pressures cannot be compared. The excitation wavelength was varied with pressure to fit approximately the maximum of the E c polarized reflectance. For the high pressure investigations a modified sapphire cell of Bridgman s opposed anvil type was used. The pressure was determined by the amount of red-shift of the Rt- and R2-lines167) of ruby crystals placed around the sample...

Figure 45. s NEXAFS spectra of Cl in dichloroanthracene. (a) powder spectrum (b), (c), and (d) represent polarized S spectra of single crystal collected along the X, Y and Z axes of the molecule, respectively (shown in the inset). [Used with permission from Smith et al. 1994. American Chemical Society.]. 

Figure 6. Percent transmission (r)-vs.-frequency traces (a) of the polarized infrared spectra of single-crystal ot-PbNg in the symmetric stretching mode region, at 90 K. The dotted curve indicates polarization parallel to the a axis and the solid-line curve indicates polarization parallel to the c axis (b) Raman Spectrum of a-Pb(N3)2 in the v region.

Figure 16. Polarized lattice-mode Raman spectra of single-crystal a-PbNe for the following orientations (a) z xx)y, (b) z xy)y, (c) z xz)y, (d) z yz)y. The region 30-80 cm" was run at a spectral slit width of 3 cm while the region 60-300 cm was run at a spectral slit width of 6 cm ...

Figure 22. Unpolarized Raman spectra of single-crystal KNCS and (xz) polarized Raman spectra of single-crystal KN3, at different temperatures. A spectral slit width of 4-5 cm was used for the measurements. Peaks marked m in the low-temperature spectrum of KNCS are probably due to two-phonon combinations.

As is shown in Table III-26, normal coordinate analysis predicts that the highest-frequency CO stretching band belongs to the species in the unidentate and the A, species in the bidentate complex. Elliott and Hathaway studied the polarized infrared spectra of single crystals of [Co(NHj)4C03]Br and confirmed these symmetry properties. As will be shown later for nitrafo complexes, Raman polarization studies are also useful for this purpose. 

The vibrational spectra of alkali fluorometallates have been examined in considerable detail. In the hexafluorometallates Li2ZrFe and M2MF6 (M = Rb or Cs M = Zr or Hf) the octahedral [MFe] " anion occupies a site of Dsd symmetry, and consequently the triply degenerate normal modes [V3 V4 V5 (f2g) and (t2u) in Oh] are split into a and e components. Vibrational frequencies (Table 19) have been assigned on the basis of polarized Raman and polarized IR reflectance spectra of single crystals, and assignments have been confirmed by normal coordinate analyses. 

Raman spectra of single crystals of ZSM-39 were obtained by Vergilov and Valtchev [863]. These spectra exhibited a highly polarized line at 3610 cm which was ascribed to a structure-bound hydroxy group. Also, vibrations of the framework and template cations (tetramethylammonium and trimethylammo-nium) were discussed. 

Perutz, M. F., Absorption spectra of single crystals of haemoglobin in polarized light. Nature 143 731, 1939. 

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