The Modes of Stretching and Bending

The simplest types, or modes, of vibrational motion in a molecule that are infrared active— those, that give rise to absorptions— are the stretching and bending modes. 

Copyright 2013 Cengage Learning. AU Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. 

When two vibrational frequencies (vi and V2) in a molecule couple to give rise to a vibration of a new frequency within the molecule, and when such a vibration is infrared active, it is called a combination band. This band is the snm of the two interacting bands (Vcomb = Vj + Vj). Not all possible combinations occur. The rules that govern which combinations are allowed are beyond the scope of our discussion here. 

Difference bands are similar to combination bands. The observed frequency in this case results from the difference between the two interacting bands = Vi - V2). 

One can calculate overtone, combination, and difference bands by directly manipulating frequencies in wavenumbers via multiplication, addition, and subtraction, respectively. When a fundamental vibration couples with an overtone or combination band, the coupled vibration is called Fermi resonance. Again, only certain combinations are allowed. Fermi resonance is often observed in carbonyl compounds. 

Al ough rotational frequencies of the whole molecule are not infrared active, they often couple with the stretching and bending vibrations in the molecule to give additional fine structure to these absorptions, thus further complicating the spectrum. One of the reasons a band is broad rather than sharp in the infrared spectrum is rotational coupling, which may lead to a considerable amount of unresolved fine structure. 

---

The Modes of Stretching and Bending 18 Bond Properties and Absorption Trends 20 The Infrared Spectrometer 23... 

For aromatic hydrocarbon molecules, in particular, the main acceptor modes are strongly anharmonic C-H vibrations which pick up the main part of the electronic energy in ST conversion. Inactive modes are stretching and bending vibrations of the carbon skeleton. The value of Pf provided by these intramolecular vibrations is so large that they act practically as a continuous bath even without intermolecular vibrations. This is confirmed by the similarity of RLT rates for isolated molecules and the same molecules imbedded in crystals. 

First attempts to model the vibrational spectrum of polymeric sulfur have been reported by Dultz et al. who assumed a planar zig-zag chain structure [172]. The calculated vibrational DOS was in qualitative agreement with the observed Raman spectrum of fibrous sulfur. However, some details of the spectrum like the relative intensities of the modes as well as the size of the gap between stretching and bending vibrations could not be reproduced exactly by this simplified model [172]. 

The (Mo-Mo) alg fundamental, which was first associated with the intense band at 406 cm 1 in the Raman spectrum of Mo2(02CCH3)4 (30), is well above the vibrational frequency found for the Mo-Mo stretching mode in the Mo2Xg ions. Normal coordinate analyses of the Mo2(02CCH3)4 infrared and Raman vibrational data produced a calculated Mo—Mo force constant of 3.6 to 3.9 mdyne A 1 for several possible assignments of the Mo-0 stretching and bending modes (30). 

The connection of the 36 hydrogen atoms to the C60 cage lowers the molecular symmetry and activates Raman scattering from a variety of initially forbidden phonon modes (Bini et al. 1998). In addition, the appearance of the C-H stretching and bending modes and those related to various isomers of C6)0n%, results in a very rich Raman spectrum. The comparison of the phonon frequencies for live principal isomers of C60I f 6, obtained by molecular dynamics calculations, with experimentally observed phonon frequencies has led to the conclusion that the material prepared by the transfer hydrogenation method contains mainly two isomers, those with symmetries DM and S6 (Bini et al. 1998). 

This means that the Ag symmetry stretching modes and the A symmetry bending mode will be Raman active, while the Bu symmetry stretching and bending modes will be infrared active. Similarly, theAu symmetry out-of-plane deformation mode will be infrared active. 

Vibrational energy exchange between the asymmetric stretch and the coupled symmetric stretch and bending modes of C02 [59],... 

FIGURE 8-7 Dipole changes of stretching and bending modes as influenced by polarization in the base. 

Figure 10.6 Major vibrational modes for a non-linear group the CHj. Characteristic stretching and bending vibrations in and out of the plane. In IR spectroscopy, the position and the intensity of the bands are modified by associations between molecules, solvent polarity, etc.

---