Jahn-Teller effect interpretation

Jahn-Teller Effect and Vibronic Coupling in Biomolecules Experimental Findings and Theoretical Interpretation... 

The measurement and interpretation of the d—d spectra of copper(II) complexes have provided a highly fruitful field for coordination chemists in recent years. There are two reasons for this first, copper(II) has a one hole electronic configuration which should render the d—d spectra amenable to theoretical anal3mis. Secondly, the structural chemistry of copper(II) encompasses a remarkable variety of stereochemistries and coordination numbers. This can to some extent be rationalised in terms of the Jahn-Teller effect, and a vast amount of crystallographic data has been accumulated. Indeed, it is probably true to say that more structural parameters have been measured for the coordination compounds of copper(II) than for those of any other transition metal ion. Such data are, of course, a prerequisite for extensive theoretical studies. Moreover, the d—d spectra are sensitive to the detailed geometry about the metal atom, and interest in the ligand field splitting naturally accompanies stmctural studies of copper(II) compounds. 

If these vibrational frequencies belong to the same excited electronic states, then clearly the C-C stretching frequencies did not increase as would be expected upon excitation from the ICg orbital and the C-C bond did not become shorter. However, if the symmetry of the excited state is only C2h (or less), it is conceivable that the C-C bond which would become shorter when the electron leaves the Icg orbital would still become longer, even much more longer when the lowering of symmetry is achieved (cf. Caldwell and Gordon [84-86]). Then how about the Jahn-Teller effect Pearson and Innes did not find any sure manifestation of it and so suppose that it must be very slight and so the observed vibrational fine structure can be interpreted in terms of totally symmetric vibrations only. 

The new results by Buenker et al. [87] constitute a challenge to the interpretation of the ethane spectrum what became traditional in the last 30 years. It clearly indicates the need for more theoretical work with an even more extended basis set and a computation of the potential surfaces for all the relevant vibrational motions, implying a closer look at the consequences of the Jahn-Teller effect in the degenerate states as well as possible vibronic couplings between close-lying excited states. Equally needed are experimental works carried out at even higher resolution than has hitherto been possible more information on the vibrational and rotational structures are badly needed. This concerns not only the absorption spectra but also the electron-impact and photoelectron spectra. 

The degeneracy of the ground state is lifted, and at low temperatures there is a slow electronic relaxation between levels. A Jahn-Teller effect need not be invoked. The theory evolved to interpret the MgO data has led to a method for determining the quadrupole moment of Fe (see Chapter 5) [52]. 

Although most of the work on this series of doped alkali halides has dealt with T1(I), several studies have been conducted on Pb(II) doped systems (63-68). From this work, it is clear that for the most part, the same trends that have been observed and explained in the T1(I) case are relevant to Pb(ll) as well that is, the same basic series of bands is observed in the absorption spectrum, and these bands are due to a combination of intraatomic and CT transitions. The C band is a multiplet that has been interpreted in terms of the Jahn-Teller effect... 

We interpret this behavior as caused by excited vibronic levels vdiidi are partly occupied at higher temperatures. After all the angular dependence of the g tensor in the /3-phases [Eq. (30)] still indicates the presence of a static antiferrodistortive pattern as in the T modifications udiidi is, however, superimposed by a planar dynamic Jahn-Teller effect The g(ic) parameters, characterizing the planar delocalization in the excited state ( planar dynamics ) [Eq. (31)] and the static antifenodistortive order [Eq. (25)], respectively, are identical. Only the small lower munetry component of the unit cells vthich induces Ihe cos 2 9 term in Eq. (30) allows to suggest the superimposed planar dynamic behavior as discussed. Further evidence for a delocalization of this type is obtained from the EPR line width vdiidi exhibits a pure cos ... 

P=90°, 02/04=10, C/B=4) [56] are shown in Fig. 24. These diagrams can be used for the assignment of the main transitions in UV/VIS spectra, as will be exemplified in Sect. 4.1.2.3. However, for a detailed interpretation of the fine structure, a refinement of the ion position and of the term splitting due to the Jahn-Teller effect and spin-orbit coupling is necessary. 

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