Frequencies intramolecular vibrations

In the examples smdied so far, the photoinduced short-time dynamics of a molecular system has been governed by a few high-frequency intramolecular vibrational modes that strongly couple to the electronic transition, a situation that... 

Although the general descriptions of RLT and electron transfer are similar, very different types of vibration are involved in each case. In the former case, the accepting modes are high-frequency intramolecular vibrations, whereas in the second case the major role is played by a continuous spectrum of polarization phonons in condensed media [Dogonadze and Kuznetsov, 1975], The localization effects associated with the low-frequency part of the phonon spectrum (mentioned in the previous section) still do not show up in electron transfer reactions due to asymmetry of the potential. 

In Eq. (5b), /zw is the frequency and S is the unitless displacement of the high-frequency intramolecular vibrational mode which is coupled to the ET process,... 

Large-amplitude, low-frequency intramolecular vibrations may lower the molecular symmetry of the average structure from the higher symmetry of the equilibrium structure. Some examples from metal halide molecules are shown in Figure 3-47. 

An important achievement of the early theories was the derivation of the exact quantum mechanical expression for the ET rate in the Fermi Golden Rule limit in the linear response regime by Kubo and Toyozawa [4b], Levich and co-workers [20a] and by Ovchinnikov and Ovchinnikova [21], in terms of the dielectric spectral density of the solvent and intramolecular vibrational modes of donor and acceptor complexes. The solvent model was improved to take into account time and space correlation of the polarization fluctuations [20,21]. The importance of high-frequency intramolecular vibrations was fully recognized by Dogonadze and Kuznetsov [22], Efrima and Bixon [23], and by Jortner and co-workers [24,25] and Ulstrup [26]. It was shown that the main role of quantum modes is to effectively reduce the activation energy and thus to increase the reaction rate in the inverted... 

For the case of atom-diatom systems with very high frequency intramolecular vibrations, the available results indicate that vibrational predissociation is slow, such that its rate cannot be measured using the available methods. Nevertheless, many of these systems are also of interest in the present context since rotational predissociation can also be important. Systems such as He-HF and Ne-HF, which already have been studied spectroscopically S are also well suited to these types of measurements. 

Fig. 5.5. Normal modes of the 4 atom hydrogen bonded (DF)2. The 6 vibrational degrees of freedom naturally separate into high frequency (intramolecular) and low frequency (intermolecular) modes. The intermolecular modes correspond to stretches and bends of the hydrogen bond, with typical frequencies of a few hundred cm s. The intramolecular modes, however, correspond to stretches of the covalent DF bonds and therefore absorb near the 3000cm free DF frequency. Shown at right is a schematic diagram illustrating how the far-IR intermolecular modes may be observed in the near-IR via combination bands built on the high frequency intramolecular vibrations.

The phonon frequencies Vpho on in proteins have the following characteristic values at acoustic modes they lie between 10" and 10" s" ( 0.0004-0.004 eV) in a polypeptide chain the acoustic modes are generated by vibrations, which cause changes in the relative distances and orientations of the side chains. The upper limit is given by the high-frequency intramolecular vibrational modes of hydrogen atoms with a frequency of 10 s" ( 0.4 eV). At body temperatures (k T ks 310 K 0.03 eV), the acoustic modes will be most active in the scattering of electrons. Therefore for the subsequent considerations Vphonon = 10 s has been taken as the characteristic value. 

Very fast processes like high frequency intramolecular vibrational modes contribute mainly to the Stokes shift occuring in an electron transfer process, but they are not necessarely very specific for the dynamics of proteins. 

Ceo lattice expands upon the introduction of alkali atoms with large ionic radii. This also implies that the appearance of superconductivity in 50 fullerides has been most reasonably interpreted in terms of electron-phonon coupling to the high frequency intramolecular vibrations which is nearly invariable as a function of temperature. 

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