Symmetric stretch bending interactions

Table 3 shows the nonadiabatic levels of B2 symmetry and their absorption intensities between 16000 and 16300 cm l. The equilibrium symmetric-stretch and bending coordinates of the electronic species are different by about 6 and 24%, respectively [17], whereas the antisymmetric stretch is equal. Therefore, the conical intersection preferably couples a2B2(vi,v2,0) combination states with their X Ai partners, but this interaction is perturbed by the A B2 antisymmetric-stretch species. Above 15000 cm l, the nonadiabatic intensity distribution is thus modulated by the maxima due to n> states with large A B2 symmetric stretch-bending character, whereas A B2 pure overtones are much weaker (e.g. bands 409 and 415 of Table 3). As the energy increases, these vibronic interactions give rise to a more and more irregular spectrum.

Spectral assignments have been made as follows. The spectra for p polarized IR for all the surfactants studied (Fig. l(a-f)) exhibit strong intensities for the methylene asymmetric stretch (d") at 2930 cm in agreement with the value observed in the IR spectrum, (2925 cm [45]. Peaks of moderate intensity are observed for the methylene symmetric (d ) and methyl symmetric (r" ") stretches at 2848 and 2872 cm" respectively. A weak methylene Fermi resonance (dpa) at 2900 cm resulting from interaction of an overtone of the methylene bending mode with the methylene symmetric stretch, is observed as a shoulder of the methylene asymmetric stretch. This can be compared to the methylene Fermi resonanee in polymethylene appearing in the IR (d" (it)FR) at 2898-2904 cm" and in the Raman (d" (0)FR) at 2890 cm" 1 [46,47]. 

For polymethylene chains, the origin of these complexities may be described in terms of the appropriate binary combinations involving methylene bending modes interacting with the infrared or Raman-active symmetric stretching fimda-mental (154,155). Two levels of Fermi resonance interactions, intramolecular and intermolecular, need be distinguished, however. Unexpectedly, the Raman-active bands observed for different intermolecular packings are quite different (Fig. 16). 

Several internal modes of the triflate anion have been employed to study ionic interactions in nonaqueous solutions. In Raman studies, the S—O symmetric stretching mode was the most frequently used [228,236,251,252,254,255], although the CFg symmetric bending at 766 cm (free ion) has been likewise analyzed [253,254]. Infrared studies, on the other hand, are usually centered on the S—O antisymmetric stretching mode, Vio(E) [236,244,246,248-250,254,258]. Because it is a doubly degenerate normal mode, the analysis of the band shape is usually rather complex. In alkaline earth triflates, Bakker [249] has used successfully the CF3 symmetric bending IR band at 752 cm (free ion) to evaluate the ionic pairing. 

Twice the bending frequency happens to come very close to the symmetric stretching frequency. It therefore interacts with the symmetric stretch. 

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