Vibronic structure/progression

Umax 15,150 cm , 4550) is both red-shifted and of ca 2.5 times the intensity of the similar band in Mo2Mc4(PMe3)4. At low temperatures, vibronic structure of this band has a 400-cm progression, corresponding to v(MoC) substituted derivatives show a 360-cm progression, assigned to the v(MoC) -I- v(MoMo) combination. The parent compound is thus electronically different from the substituted compounds, and is similar to C2(C=CH)4. This is also demonstrated by its sensitivity to irradiation at 15,150 cm (substituted compounds are stable under these conditions). 

The presence of vibronic structure in the emission spectra of ions in solids does not depend on the nature of the particular ion alone, but also on the nature of its surroundings. In the case of ns2 ions the strength of the spin-orbit interaction relative to the Jahn-Teller effect determines whether the progression will be in v1 or v2. However, it is not usually possible to observe any vibronic structure at all due to deviations from high symmetry (pseudo Jahn-Teller effect in the ground state). Our present understanding is of a qualitative nature. Further progress is hampered by the fact that the presence of vibronic structure in the spectra is for these ions more exception that rule. 

Crystal-field spectroscopy of dn ions is being extended nowadays to the near-infrared region with interesting results. Vibronic structure is nowadays used to obtain information on the deformation of the excited state. Also in case of the closed-shell d° complexes the excited state appears to be strongly distorted. Among the latter class especially the linear species show efficient luminescence. Molecular-orbital calculations are in progress and will probably yield interesting results. 

PW2 have been obtained from vibronic structure analysis together with the Too value60. Eight well-resolved emission bands in the 710-395 nm region have been observed under photoexcitation of Pbl261. Some of them can belong to vibrational progressions of the 35i-1Ai transition in PW2. 

I. W(CO)s(pyridine) 3. The electronic spectra of this molecule show vibronic structure, but only one progression is apparently observed. (In fact, the progression does not arise from one normal mode its assignment led to the discovery of the MIME which is discussed in Section V.) Because the spectrum cannot be further resolved under any of the conditions which have been tried and because insufficient detail is available from the electronic spectra, Raman information must be used in order to calculate the distortions. This example provides an excellent illustration of how the combination of pre-resonance Raman and electronic spectroscopy are used in conjunction to obtain the desired information. This molecule is also an excellent example for discussion because the distortions provide a detailed basis for interpreting the orbitals involved in the electronic transition and because the relationship between distortions and the molecule s photochemical reactivity can be compared. 

K2[PtCl4] is shown in Figure 43. In the best-fit calculated emission spectrum, the Aij mode (329cm and the Bi, mode (304cm ) combine to produce the MIME frequency in the main progression. No other vibrational mode was found that could combine with the Aj, mode to produce the spacing of 315cm . Very small contributions from other modes are possible, but they only serve to decrease the resolution of the vibronic structure and do not improve the fit. 

A striking feature in the vibronic structure of the low temperature emission spectrum of ruthenocene is a repetitive pattern of clusters of bands. The pattern consists of the main peak and three side-peaks which are separated from the main peak by about 47 cm , 65 cm , and 112 cm . The first two side peaks are often not well resolved. The separation between the bands within a cluster is less than the energy of any normal mode. A possible explanation for the side-peaks could be phonon wings on the main 333 cm progression. However, the spectra obtained from organic glasses contain the same repetitive pattern. Thus the structure must arise from molecular normal modes and not from crystal lattice modes. 

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