Fine vibrational structures

The UV spectra of quinoxalines have been examined in several solvents. In cyclohexane, three principal absorptions are observed (Table 2). In hydroxylic solvents the vibrational fine structure disappears and in methanol or water the weak n- ir transitions are obscured by the intense ir->ir transition (79HC(35)l). 

Vibrational fine structure in the electronic spectra of transition metal compounds an experimental survey. M. Cicslak-Golonka, A. Bartecki and S. P. Sinha, Coord. Chem. Rev., 1980, 31, 251-288... 

Vibrational fine structure was resolved for n=l-3 and 6 [88]. In particular, the observed frequencies allow the identification of both ring and chain isomers of 85 and 87 . It is of interest to note that the only wavenumber measured for the neutral 8e structure (570 32 cm ) is significantly higher than both the calculated and observed Raman wavenumbers of the D3d isomer of 8e and falls in a pronounced gap of the spectrum of this isomer. 

The newly-developed capability to observe metal-metal vibrational fine structure in the valence ionizations of quadruply bonded dimers is illustrated for the delta-bond ionization of Mo2(02CCH3)if. Observation of this structure provides direct information on the bonding influence of an electron in a delta-bonding orbital by showing the significant changes in metal-metal force constant and bond distance that occur when that electron is removed. 

The possibility of obtaining this direct information has not been discussed previously in this context because vibrational fine structure is not generally observed in the ionizations of molecules of this size, and has never before been observed for any transition metal-metal vibrational mode. Our breakthrough in demonstrating that this fine structure can be observed has followed from several developments of our instrumentation. The details of these developments have recently been published along with our report of the first observations of metal-ligand vibrational fine structure in the ionizations of metal carbonyls QJ. 

The important point to remember is that an electron in the delta-bonding orbital of M02(O2CCH3)if has a substantial influence on the strength of the metal-metal interaction. This influence is directly evidenced by the metal-metal vibrational fine structure observed with ionization from the delta orbital, which shows a lowering of the metal-metal stretching frequency and a lengthening of the equilibrium metal-metal bond distance. 

Finally, the implication is made that the sigma ionization is also in the region of the pi ionization for the molybdenum dimers. We have recently carried out an examination of the pi ionization of M02(02CCH3)i+ in which we observe clear vibrational fine structure across the band. This observation is not expected if two different ionizations, the sigma and pi, are overlapped in the same envelope. Thus, I do not see any evidence at this stage that the sigma ionization is in the region of the pi ionization for these complexes. 

Turner and coworkers [7] applied the photoelectric effect to gases. By using the sharp UV line from a helium resonance they could even resolve vibrational fine structure of the electron levels. The subject is outside the scope of the present book we refer to excellent books on the subject [7, 8]. 

Figure 9.15 Morse curve of a diatomic molecule X2 showing vibrational fine structure...

Figure 9.17 shows a spectrum of benzene (IV) in cyclohexane, which clearly shows small peaklets superimposed on a broader band (or envelope). These peaklets are called vibrational fine structure. In benzene, they are caused by excitation from v" = 0 to v = 1, v = 2, etc. Consideration of Figures 9.15 and 9.16 suggests that excitation from v" = 0 to v = 1 requires less energy than from v" = 0 to v = 2, so the excitation v" = 0 -> v = 1 occurs on the right-hand side of the figure, i.e. relates to processes of lower energy. 

In the electronic transitions in visible and ultraviolet region with liquids or solutions, we do not get vibrational bands along with rotational fine structure, but we get a continuous broad electronic band and hence such curves do not give much valuable information. This is because the vibrational fine structure gets suppressed due to overlapping of vibrational spacings. 

Explain the intensity pattern of absorption spectra and the occurrence or absence of vibrational fine structure. 

Figure 2.6 gives the UV-visible spectrum of a very dilute solution of anthracene (Figure 2.7) in benzene, which clearly shows small fingers superimposed on a broader band (or envelope). These fingers are called the vibrational fine structure and we can see that each finger corresponds to a transition from the v = 0 of the ground electronic state to the v = 0, 1, 2, 3, etc. vibrational level of the excited electronic state. 

Radiative transitions may be considered as vertical transitions and may therefore be explained in terms of the Franck-Condon principle. The intensity of any vibrational fine structure associated with such transitions will, therefore, be related to the overlap between the square of the wavefunctions of the vibronic levels of the excited state and ground state. This overlap is maximised for the most probable electronic transition (the most intense band in the fluorescence spectrum). Figure... 

Vibrational fine structure is absent from the excimer emission because the Franck-Condon transition is to the unstable dissociative state where the molecule dissociates before it is able to undergo a vibrational transition. In the case of the monomer emission, all electronic transitions are from the v = 0 vibrational level of M to the quantised vibrational levels of M, resulting in the appearance of vibrational fine structure. 

Absorption spectra can provide information relating to the energy of an excited singlet state. This corresponds to the lowest 0-0 vibrational transition in the electronic absorption spectrum. When the vibrational fine structure is evident, the energy of the excited singlet state is readily determined, but when the 0-0 band cannot be located, the value can be taken from the region of overlap of the absorption and fluorescence spectra. 

In contrast to borazine, the three corresponding excited singlet states of benzene have a much wider spread of absorbing wavelengths and exhibit easily distinguished vibrational fine structure. Many photolysis experiments have been performed using laser lines tuned to selective excite a particular vibrational level of a particular excited state of benzene. Such experiments are more difficult with borazine. The triplet states of benzene have been located experimentally and quantum yields for fluorescence and phosphorescence at various wavelengths and pressure conditions have been determined. 

Finally we note some other properties of these allowed transitions of the lanthanide ions. From Table 1 it becomes clear that in general the 4f—5d bands have a smaller band width than the c.t. transitions, typical values being 1000 and 2000 cm-i, respectively. In this connection it is interesting to find that at low temperatures the 4f- -5d absorption and emission bands often show a distinct and extended vibrational fine structure [Ce3+ (25), Tb + (25), Eu2+ (14, 26), Yb2+ (27)], whereas c.t. transitions do not. From this it seems probable that in the excited c.t. state the interaction between the lanthanide ion and its surroundings is stronger than in the excited 4f 5d state. This is not imexpected. As far... 

Steadily in the order 359, 385, 395, and 402 nm. The emission spectra exhibit a clearer vibrational fine structure than the absorption spectra. For spiro-sexiphe-nyl, 35b, a detailed analysis shows that the vibrational splitting of 0.20 eV corresponds to a phenyl breathing mode in the Raman spectrum [108]. If for spiro-sexiphenyl the outer biphenyl moieties are fixed parallel as in 4-Spiro (43), the absorption maximum is shifted from 346 to 353 nm (amorphous films) and the fluorescence maximum from 420 to 429 nm, maintaining the Stokes shift. The corresponding spectra are shown in Figure 3.17. The absorption signal at 310 nm in the spectrum of 43 can be attributed to the terminal fluorene moieties. The quantum yields for the fluorescence in the amorphous film are 38% for 35b and as high as 70 10% for 43 [89]. 

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