Band assignments symmetry

The skeleton vibrations. C3NSX, CjNSXj. C NSXY, or C NSXj (where X or Y is the monoatomic substituent or the atom of the substituent which is bonded to the ring for polyatomic substituents), have been classified into suites, numbered I to X. A suite is a set of absorption bands or diffusion lines assigned, to a first approximation, to a same mode of vibration for the different molecules. Suites I to VIII concern bands assigned to A symmetry vibrations, while suites IX and X describe bands assigned to A" symmetry vibrations. For each of these suites, the analysis of the various published works gives the limits of the observed frequencies (Table 1-29). 

Whether the molecule is a prolate or an oblate asymmetric rotor, type A, B or C selection mles result in characteristic band shapes. These shapes, or contours, are particularly important in gas-phase infrared spectra of large asymmetric rotors, whose rotational lines are not resolved, for assigning symmetry species to observed fundamentals. 

The picosecond IR absorption spectrum of the tS state in the fingerprint region is different in w-heptane and in acetonitrile. The spectrum recorded for Si tS in the nonpolar solvent w-heptane is consistent with a species that has a center of symmetry. In acetonitrile, the spectrum exhibits additional weak bands near 1570, 1250, and 1180 cm which are approximately at the same frequencies as strong Raman bands assigned to in-plane vinylic vibrational modes in 5i. This result was taken to suggest a molecular structure for 5i that lacks a center of symmetry in acetonitrile. However, because the intensities of these three bands are weak, it was concluded that either the polarization of 5i or the contribution from polarized S structures to all of the S structures in acetonitrile may be small. 

In the case of naphthalene, transitions to the two lowest excited states (again, often indicated with Lb and La) are two-photon forbidden, as in benzene. However, due to vibronic coupling, the Lb band is visible in the 2PA spectrum of naphthalene in the 575-650 nm region (see Fig. 5), while La gains intensity in the IPA spectrum and peaks around 275 nm [44-46], but is basically absent from the 2PA spectrum this is again in line with predictions based on the pseudoparity of the states. Polarization ratio data were used to aid the band assignment. A weak 0-0 peak of the Lb band can actually be seen in the 2PA spectrum (at 630.5 nm for naphthalene in cyclohexane [45] and at 631.8 nm in carbon tetrachloride [47]), probably because of local perturbation of the symmetry due to the solvent environment or other effects [44,45]. The 2PA... 

An intensive study of the vibrational (IR) spectrum of Zn(acac)2 and its lsO, Zn and Zn labelled derivatives has also been made, and the bands assigned on the basis of the isotopic shifts assuming localized symmetry.745... 

In their original paper (2) on the structure of Fe5C(CO)l5, Dahl and co-workers assigned two bands in the infrared spectrum of hydrocarbon solutions of the cluster, at 790 and 770 cm-1, to vFeC modes. This assignment has been confirmed by a recent study of the infrared spectra of the series M5C(CO)15, (M = Fe, Ru, Os) (78). The room temperature spectra of the compounds (Table II) in the solid state are quite similar to each other, comprising three bands assigned as the a, and e modes (split in the solid state) expected for the C4 symmetry of the isostructural clusters. At low temperature the ruthenium and osmium clusters exhibit five absorptions associated with M-C stretches, whereas the iron cluster retains its room temperature spectrum. This is ascribed to the presence of two types of cluster molecule in the crystal lattices of the ruthenium and osmium clusters which are isostructural with, but not isomorphous with, the iron analog in which all the molecules are identical. 

The infrared active v (CH2), v (CH2), 8 (CH2), and yr (CH2) fundamentals can be readily assigned as a result of the extensive spectroscopic studies on hydrocarbons which have been undertaken [Sheppard and Simpson (795)]. In addition, because of the polarized radiation studies on single crystals of normal paraffins [Krimm (95)], it is possible to assign uniquely the components of the doublets found in the spectrum for these bands to symmetry species. Similarly, the Raman active va(CH. ), vs(CH2), (CHg), v+ (0), and v+ (n) fundamentals can be unambiguously assigned, the latter two on the basis of normal vibration calculations... 

As stated in the preceding section, depolarization ratios of Raman bands provide valuable information about the symmetry of a vibration that is indispensable in making band assignments. Figure 1-18 shows a coordinate system which is used for measurements of depolarization ratios. A molecule situated at the origin is irradiated from the y-direction with plane polarized light whose electric vector oscillates on the yz-plane (Ez). If one observes scattered radiation from the x-direction, and measures the intensities in the... 

Another example is provided by a series of octahedral MX Y6 type ions (n = 0-6), where M is Pt(IV), Os(IV), and Ir(IV), and X and Y are halogens. Preetz and co-workers (4) prepared these mixed-halogeno ions and assigned their IR/Raman bands based on point group symmetry. Table 4-3 shows the point group and classification of IR/Raman-active fundamental vibrations. Figure 4-3 shows the IR/Raman spectra and band assignments of the [PtCl Br6 ]2 series. It should be noted that these ions exhibit v(PtCl),... 

The most comprehensive analysis of /3-diketones125, 1265 proves the Cs symmetry is the correct one for the /3-diketones involving CH3, Ph and CF3 groups. The band assignments are given in Table 12. The effect of deuteriation of the olefinic proton is also noticeable on the modes v(C=C) and v(C-C), especially the latter, but it is the effect of deuteriation on the hydrogen bond which has far reaching implications. 

Under these conditions, the bands assigned to SiFs disappeared, and intense bands grew in near 730 and 480 cm, which were assigned to the SiFs anion. The observation of three bands above 800 cm , the Si-F stretching region, demonstrated that the symmetry of the anion in the ion pair is not strictly the D3j trigonal bipyramid structure anticipated from valence bond considerations. Rather, the local symmetry is lowered by the alkali metal cation, either activating an otherwise forbidden... 

The direct comparison of adsorbate modes with the modes for the free ions in solution, without considering symmetry changes in the adsorbed state, is not the correct procedure for band assignment. One must use the elements of group theory with the aid of the surface selection rule in order to make a proper spectral analysis. 

Overall, very few data are available on the solvent dependence of the CO-stretching fundamentals. Few correlations have been sufficiently well-established to be used for band assignment. It has been found, however, that if the CO-stretching frequencies of the compound Mn2(CO)jo were plotted against those of the derivatives Mn2(CO)gL2 in a series of nonpolar solvents, linear correlations were only found when fundamentals belonging to the same symmetry species were compared (259). These observations were consistent with the band assignment proposed for the derivatives, and served as a check for the correct assignment. 

The purpose of this section is to draw attention briefly to two interesting examples of progressions involving both totally symmetric and non-totally symmetric modes. First, the resonance Raman spectra (50) of various salts of the [AuBr4]" ion (D4h symmetry) show progressions in the i i (Aig) Au—Br stretching frequency based on one quantum of V2 (Big) (up to 2 f j) and also based on a quantum of J 4( 2g) (up to 1 4 + 5 J i). Excitation was at 457.9 nm, which lies near the absorption maximum (22,500 cm ) of a broad band assigned (51) to a superposition of two electric-dipole-allowed transitions, both of the type a dx2 yj(Au) - 7r p(Br),... 

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