Assignment of bands to vibrations

We are now able to determine the total number of modes of each symmetry species, and to say which of these are infrared- and which are Raman-active. Often we want to go further than this, and assign each band in the spectra first to specific symmetry species, and then to particular modes. We illustrate the first of these stages with the example of PF5 (Table 8.3), which has a trigonal bipyramidal structure of D h symmetry (see character table in the on-line supplement to chapter 2). 

In some cases, overall selection rules leave no doubt. For PF5 only modes of e symmetry are active in both IR and Raman spectra, so any Raman bands coinciding in frequency with IR bands can confidently be 

Symmetry species Number of IR-active modes Number of Raman-active modes Polarization 

---

Assign these bands to vibrational transitions use the notation of Problem 6.29. (The lowest and highest frequency fundamentals are v2 and vy respectively.)... 

Table II lists the Raman peak positions for samples 4 and 11, which are typical of unbleached lignin-rich wood pulps. These peaks have been assigned in terms of contributions by cellulose and lignin. Raman contributions from hemicelluloses were expected to be broad and to occur at wavenumbers where cellulose contributions were detected. In Table II, certain bands are assigned to specific chemical-group vibrations. The assignment of bands to various groups in cellulose was based on literature [18]. The lignin-related assignments are based on the work described in this paper and the authors unpublished work.

In this context, it is nevertheless important to know if )3 turns in proteins make specific contributions to the spectra. The only protein for which such a vibrational analysis has been done is insulin (Bandekar and Krimm, 1980). This protein is a particularly suitable one for such a study, since its structure has been solved (Blundell et al., 1972), it is relatively small, with only four )3 turns, and Raman spectra of single crystals have been reported (Yu et al., 1974). The normal-mode calculations (Bandekar and Krimm, 1980) permit a correlation of previously unassignable bands in the Raman spectrum with turns in the structure, as well as showing that some of the computed )3-turn frequencies lie in spectral regions previously associated exclusively with a-helix modes. These results thus emphasize our previous remarks that caution must be exercised in proposing unique assignments of bands to a-helix and /3-sheet structures in proteins. 

The main disadvantage of the technique is that a minimum protein concentration of 3-5 mg/ml is needed to obtain quality spectra of proteins in H2O solutions. The absolute mass of protein needed is not great because usually less than 50 pi of solution is required to load the sample cell. If solubility is limited, then the protein can be studied at much lower concentrations (around 1 mg/ml) in D2O. However, the researcher must then be aware of the potential difficulties of data interpretation due to the direet effeets of H-D exchange on the vibrational frequencies of amide I eomponent bands (see [11,56]). In some cases, deuteration of the protein makes assignment of bands to different secondary structural types uneertain. This can be a problem if quantitation of seeondary struetural content is needed. However, if all that is required is a global eomparison between a spectrum for an aqueous control sample and that for... 

Polarization effects are another feature of Raman spectroscopy that improves the assignment of bands and enables the determination of molecular orientation. Analysis of the polarized and non-polarized bands of isotropic phases enables determination of the symmetry of the respective vibrations. For aligned molecules in crystals or at surfaces it is possible to measure the dependence of up to six independent Raman spectra on the polarization and direction of propagation of incident and scattered light relative to the molecular or crystal axes. 

For the nitrone group, one would expect the emergence of bands in the vibration spectrum typical of stretching vibrations Vc=jv and %+ 0-. However, while identification of band c=n, appearing in a specific region 1610 to 1530 cm-1, does not seem problematic, the assignment of band is frequently unjustified and... 

The requirements for Raman resonance that must be fulfilled are the following (120,121) (a) total symmetry of the vibrations with respect to the absorbing center, and (b) same molecular deformation induced by the electronic and vibrational excitations. Quantum chemical calculations (41) of the vibrational frequencies and the electronic structure of shell-3 cluster models allowed the assignment of the main vibrational features, as shown in Fig. 7. The 1125 cm-1 band is unequivocally assigned to the symmetric stretching of the Ti04 tetrahedron. 

In an effort to assign the bands to ee and ae isomers the (thixantphos)Rh(CO)2D complex was measured for comparison. Upon H/D exchange, only Vi and v3 shift to lower wavenumbers (respectively 18 and 14 cm-1), and therefore, these two bands are assigned to the carbonyl frequencies of the ee complex. The two bands that do not shift, v2 and v4, belong to the ae complex. From the disappearance of a low-frequency shoulder upon H/D exchange, it can be concluded that one of the rhodium hydride vibrations is partly hidden under v4. 

IR spectroscopy is not the most useful tool for probing the structures of metal amide complexes. The assignment of M—N vibrational modes is not a simple task as such bands can be readily coupled to other vibrational modes involving the NR2 moiety.1... 

Polyethylene has been studied spectroscopically in greater detail than any other polymer. This is primarily a result of its (supposedly) simple structure and the hope that its simple spectrum could be understood in detail. Yet as simple as this structure and spectrum are, a satisfactory analysis had not been made until relatively recently, and even then significant problems of interpretation still remained. The main reason for this is that this polymer in fact generally contains structures other than the simple planar zig-zag implied by (CH2CH2) there are not only impurities of various kinds that differ chemically from the above, but the polymer always contains some amorphous material. In the latter portion of the material the chain no longer assumes an extended planar zig-zag conformation, and as we have noted earlier, such ro-tationally isomeric forms of a molecule usually have different spectra. Furthermore, the molecule has a center of symmetry, which as we have seen implies that some modes will be infrared inactive but Raman active, so that until Raman spectra became available recently it was difficult to be certain of the interpretation of some aspects of the spectrum. As a result of this work, and of detailed studies on the spectra of n-paraffins, it now seems possible to present a quite detailed assignment of bands in the vibrational spectrum of polyethylene. 

---