Intramolecular vibrational enhancement

Alternatively, such prepared excited states may prove useful photochemically under particular circumstances. This is especially true for local-mode-type molecules [461, 462], that is, molecules for which vibrational eigenstates resemble localized excitation in individual bonds. As an example, in the case of HOD, the large frequency difference between the OH and OD oscillators is such that intramolecular vibrational relaxation does not destroy the localized excitation. (Similar effects arise if one excites a resonance state that displays local behavior see, for example, Ref. [463] for an ABA-type molecule.) As shown theoretically [464, 465], and confirmed experimentally [53-60], preparation of the OH stretch followed by an excitation laser leading to dissociation gives a marked enhancement of the H atom photodissociation in many molecules. 

Table 3.59 Infrared intensities (km mol ) of the intramolecular vibrational modes of H CO-HCL calculated at the SCF level, and the enhancement ratio as compared to the monomer . ...

Finally, one interesting way in which electrical effects are interlaced with vibrational effects is in hydrogen bonding. Intermolecular electrical interaction appears to be a dominant factor in understanding red shifts and transition moment enhancements in intramolecular vibrations of hydrogen-bonded molecules [90, 119, 120]. Undoubtedly, there will be other types of molecular problems that are revealed to be dependent on electrical features. The capability for ab initio study of electrical properties paves the way for this to happen. 

The fact that using excitation in the CT absorption a number of Raman bands appears in the intermolecular mode region,while the intramolecular bands practically disappear, can be understood on the basis of the dimeric model. The modulation of the transfer integral t by the intermolecular vibrations, particularly the antiphase translational modes, provides an efficient mechanism for intensity enhancement at resonance with the CT transition. No such mechanism is operative for the intramolecular vibrational modes. 

Now, in aromatic hydrocarbons intramolecular skeletal vibrations, rather than C—H vibrations, dominate the vibronic coupling contribution to the term J m = — . Furthermore, intermolecular vibrations will have negligible effect on the coupling of the electronic states of interest. Thus, in the case of internal conversion, where the (relatively large) matrix elements are solely determined by intramolecular vibronic coupling, no appreciable medium effect on the nonradiative lifetime is to be expected. On the other hand, intersystem crossing processes are enhanced by the external heavy atom effect, which leads to a contribution to the electronic coupling term. 

One of the most prominent hydrogen-bonded systems is DNA. Despite numerous experimental and theoretical investigations on vibrational spectra of nucleic acid bases [7-13], information on inter- and intramolecular interactions in base pairs and DNA oligomers is still limited [14-25]. A recent example is the work on single adenine-uracil (AU) base pairs in the Watson-Crick geometry in solution, which showed an enhancement of vibrational energy... 

Most of the reported compounds which show this effect are organic molecules (179-184), where restriction of intramolecular rotation is generally accounted for being the main cause for AIE (175,185). The enhancement is therefore mostly related to the reduction of the nonradiative rate constant rising from the vibrational and rotational mode of the appended groups to the chromophores, which in the aggregates are strongly prevented. 

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