Coupling effects, vibronic

The majority of photochemistry of course deals with nondegenerate states, and here vibronic coupling effects aie also found. A classic example of non-Jahn-Teller vibronic coupling is found in the photoelection spectrum of butatiiene, formed by ejection of electrons from the electronic eigenfunctions [approximately the molecular orbitals). Bands due to the ground and first... 

Fe-4S] clusters, the asymmetric electron distribution apparent in Fig. 8B results essentially from the interplay between valence delocalization and vibronic coupling effects 113). 

It must be borne in mind, however, that vibronic coupling effects (33) among CT and (tt,tt ) states may substantially alter the picture portrayed... 

Since transition from is strongly forbidden (which cannot be circumvented by vibronic coupling effects since an octahedral molecule has no t- g-type vibrations necessary, for this coupling), the spectrum is due to a... 

Figure 1 illustrates the three cases of JT vibronic coupling effects in the simplest situation of a double degenerate or pseudodegenerate state interacting with only one... 

Thus the SB produced by chemical bonding, intermolecular interactions, and chemical reactions can be regarded as triggered by the same mechanism of JT vibronic coupling effects as in the SB of structural phase transitions in crystals. 

The corrective terms are of the order DSl ki20/(ESt — ESi) and effectively couple the vibrons and S, v. For anthracene, these terms are of the order of >/10, and we leave them aside in a first analysis. Thus, we are led to a global shift (DSl) of all the vibronic levels, without coupling. On the contrary, the monoexcitation condition leads to the condition 2 = 0 in (2.87), with the following result ... 

This expression means both a differential shift and coupling of the vibronic states. In dipolar excitons the D term is very important, and (2.89) explains why the calculations reported in refs.59,63 clearly overestimate the collective coupling effect Their D term is comparable to J (500cm" ) and adds to it. 

To evaluate the collective coupling effects, we make the assumption that the only efficient mechanism of coupling is that introduced by Jk of Section II.B.l.c. Thus, we neglect the terms discarded in order to pass from (2.87) to (2.88), which result from the interactions with the other electronic states (in particular S2), and which may introduce differential stabilization of the vibronic levels. 

Detailed investigations have been made of the octahedral [UCle] ion. Its spectrum is largely vibronic in nature, with electronic transitions accompanied by vibrations of the complexion (odd-parity modes-the Tiu asymmetric stretch and and T a deformations). Here, as in other cases, overlap of bands from different states occurs because of the similarity in crystal-field and spin-orbit coupling effects. Its spectrum can be altered by destroying the centre of symmetry (e.g., by hydrogen bonding), which enables pure electronic transitions to be observed, and alters band patterns in multiplets. 

For a long time stable distortions were thought to be the only evidence for a JT effect. However the papers by Moffitt et al. Longuett-Higgins et al. Bersuker and O Brien showed that, even for weak coupling, the vibronic level scheme was much different from the one obtained in an isolated electronic system. 

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