Vibrational modes asymmetry

In order for the electrical component in electromagnetic radiation to interact with a bond, the bond must have a dipole. Thus symmetrical bonds such as those in O2 or Nj do not absorb infrared radiation. However, the majority of organic molecules have plenty of asymmetry. In even small organic molecules the modes of vibration are complex. This is illustrated by the vibrational modes which can occur in a... 

Garbutt (5) made a detailed study of the ESR of CH radicals stabilized on PVG. The temperature dependences of the hyperfine interaction, llnewldths and line asymmetries of CH radicals on PVG were reported (33). Comparisons were made with Schrader s calculated values (34) based on a model of Incomplete orbital following in the vibrational mode (out-of-plane bending mode) for a planar CH radical. The temperature range studied was... 

Fig. 6.3. Absorption spectrum of Bi donors in CZ silicon between 460 and 573cm 1 (2p i is truncated). Note the broadening and asymmetry of 2po lines compared to the other lines, due to interaction with lattice phonons, and the parity-forbidden Sdo line. [Bi] is 2 x 1015 cm-3 and the frequency of the interfering Oi vibrational mode is 64.1 meV or 517cm 1 (after [35])...

Figure 2. Vibrational modes of the CO2 moleeule, the symmetrie stietehing vibration (coi), the asymmetrie stietehing vibration ((03), and the two bending vibrations ((02). Note that coi only involves movement of oxygen atoms, whereas (02 and (03 involve movement of both oxygen and earbon atoms. This results in a larger frequeney shift (Av) for the coi vibration.

So, of the 9 eartesian displaeements, 3 are of ai symmetry, 3 of b2,2 of bi, and 1 of a2- Of these, there are three translations (ai, b2, and b i) and three rotations (b2, b i, and a2). This leaves two vibrations of ai and one of b2 symmetry. For the H2O example treated here, the three non zero eigenvalues of the mass-weighted Hessian are therefore of ai b2, and ai symmetry. They deseribe the symmetrie and asymmetrie streteh vibrations and the bending mode, respeetively as illustrated below. 

While being very similar in the general description, the RLT and electron-transfer processes differ in the vibration types they involve. In the first case, those are the high-frequency intramolecular modes, while in the second case the major role is played by the continuous spectrum of polarization phonons in condensed 3D media [Dogonadze and Kuznetsov 1975]. The localization effects mentioned in the previous section, connected with the low-frequency part of the phonon spectrum, still do not show up in electron-transfer reactions because of the asymmetry of the potential. 

For metal eoverages at whieh the enhaneement of the Raman signal due to dipolar plasmon resonanees is observed, the bands at around 1606 em (in both PTCDA and DiMe-PTCDI) beeome asymmetrie towards the low Ife-queney side in all the investigated systems exeept for Mg/PTCDA. This line-shape asymmetry is likely to be related to a Fano resonant eoupling between the moleeular eleetronie levels and the plasmons in the metallie elusters modulated by the moleeular vibrations [21]. Interestingly, the mode at 1606 em stems from a breathing mode of the earbon rings. 

---