Vibrational fine structure effects

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]. 

Fluorescence techniques have been used with great success in the study of PEO-fe-PSt micelles [64]. In this study, the effect of polymer concentration on the fluorescence of pyrene present in water at saturation was studied. Three features of the absorption and emission spectra change when micellization occurs. First, the low-energy band of the (S2-So) transition is shifted from 332.5 to 338 nm. Second, the lifetime of the pyrene fluorescence decay increases from 200 to ca. 350 ns, accompanied by a corresponding increase in the fluorescence quantum yield. Third, the vibrational fine structure changes, as the transfer of pyrene from a polar environment to a nonpolar one suppresses the permissibility of the symmetry-forbidden (0,0) band. 

Incomplete coverage of the surface of such a fifth-generation POPAM dendrimer exposes hydrophobic areas of adamantyl units remaining uncomplexed by cyclodextrins on the dendritic outer shell [38]. Pyrenes were used as neutral fluorescence probes to examine whether this might lead to aggregation in water driven by the hydrophobic effect [39a]. Their inclusion in the dendrimer/cyclo-dextrin aggregate leads to changes in fluorescence intensity and in the vibrational fine structure. Formation of excimers was also observed. 

The ultraviolet spectrum of pyrazine in cyclohexane shows maxima at 260 nm (corresponding to a tt-tt transition) and 328 nm (corresponding to a n-7r transition) in each case with vibrational fine structure the coefficients of molecular extinction are 5600 and 1040, respectively.78,79 Substitution of halogen has a bathochromic effect on the ultraviolet spectrum of pyrazine.80 A useful index of the ultraviolet and visible spectra of pyrazine derivatives is available for the period from 1955 to 1963.81 The far-ultraviolet spectrum of pyrazine... 

Most of the last-named difficulty has been removed in a simple mathematical treatment which effectively removes a major part of the high-energy side of the electron beam (Winters et al., 1966). An experimental factor b, accounting for the lack of knowledge of electron energy distribution, has been found to be quite close to that calculated from the Maxwell-Boltzmann distribution. Recently it has been claimed that the 0 0 ionization and vibrational fine structure of acetylene had been detected (J. H. Collins et al., 1968), and this energy distribution difference (E. D. D.) method appears to be both simple and accurate. 

The solvent often exerts a profound influence on the quality and shape of the spectrum. For example, many aromatic chromophores display vibrational fine structure in non-polar solvents, whereas in more polar solvents this fine structure is absent due to solute-solvent interaction effects. A classic case is phenol and related compounds which have different spectra in cyclohexane and in neutral aqueous solution. In aqueous solutions, the pH exerts a profound effect on ionisable chromophores due to the differing extent of conjugation in the ionised and the non-ionised chromophore. In phenolic compounds, for example, addition of alkali to two pH units above the pKa leads to the classical red or bathochromic shift to longer wavelength, a loss of any fine structure, and an increase in molar absorptivity (hyper chromic... 

The absorption spectrum of phenylalanine is shown in Fig. 1. The low intensity absorption peak centered slightly below 2600 A corresponds to a forbidden x it transition. Vibrational fine structure is quite evident in this region. Aside from increased blurring of the fine structure which is to be expected on passing into successively more polar media, change of solvent effects little change in the general size, shape, and location of the absorption envelope. 

At the low-energy side there are three bands centered at 12.0,12.7, and 13.4 eV (96,800, 102,425, and 108,070 cm , respectively). The first band has vibrational fine structure with an parent progression of about 1170 cm. The adiabatic Ip is at 11.56 eV (93,230 cm" ). These three bands can only belong to ionization from either the leg or the 3aig levels the ionization from the leg orbitals yields a positive ion in a degenerate state, so it must be split by the Jahn-Teller effect... 

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 effect of low temperature in sharpening the vibrational fine-structure bands of the absorption spectra of both organic and inorganic compounds has been investigated by various workers. Kronenberger... 

In Section 4 we have examined phenomena related to the nephdauxetic effect — the reduction of the B and C parameters below the free-ion value — and in particular its utility as a measure of covalency and its relationship to the chemical stability of particular oxidation states, whilst in Section 5 we have dealt with the charge-tramsfer spectra on the basis of the usual qualitative molecular orbital theory. Values of the optical electronegativities, Zopt., derived from the Laporte-allowed bands are listed, and correlations with nephelauxetic effects estimated from the d—d transitions are examined. Sections 6 and 7 survey respectively the question of vibrational fine structure, which appears to be limited to the M(IV) complexes, and some general considerations about the operation of the JahrirTeUer effect in hexafluoro compoimds. 

---