Davydov shift

Deviations from OGM were recognized early on spectroscopic properties of molecular crystals Davydov shifts and splittings of absorption bands in molecular crystals are clear deviations from OGM and were rationalized based on the excitonic model (EM) [10, 14, 15, 16, 17]. This same model proved extremely successful to describe the complex and technologically relevant spectroscopy of molecular aggregates, i.e. of clusters of molecules that spontaneously self-assemble in solution or in condensed phases [IS]. Much as it occurs in molecular crystals, due to intermolecular electrostatic interactions the local bound electron-hole pair created upon photoexcitation travels in the lattice and the corresponding wave function describes an extended delocalized object called an exciton. We explicitly remark that the Frenkel picture of the exciton, as a bound electron-hole pair, both residing on the same molecule, survives, or better is the basis for the excitonic picture. The delocalization of the exciton refers to the fact that the relevant wave function describes a Frenkel exciton (a bound e-h pair) that travels in the lattice, and this is of course possible even when electrons and/or holes are, separately, totally localized. In other terms, the EM describes localized charges, but delocalized excitations. 

The energy difference I/ , -F2I=2 V12 is known as Davydov or exciton splitting, Figure 8.3. The shift of energy levels gives rise to new bands in the absorption spectrum denoted as the upper and lower Davydov (exciton) components. These components are the H- and J-bands observed in absorption spectra of molecular aggregates. 

Fig. 4.8. Temperature dependences of shifts (a) and widths (b) for Davydov-split spectral lines of local vibrations in the 2x1 phase of CO molecules adsorbed on the NaCI(lOO) surface.

A. A. Davydov, G. K. Boreskov, T. M. Yurieva and N. A. Rubene, Associative mechanisms of water gas shift reaction , Doklady Akademii Nauk USSR, 1977, 236, 1402. 

Three double complex salts [Pt(CNR)4][Pt(CN)4](R = Me, Et or Bu ) exhibit low-energy electronic absorption bands in the solids which are absent in solution spectra of the anion or cation complexes with simple counter-ions.171 These bands, which are responsible for the intense colour of the solids, are ascribed to M - L charge transfer which has been red-shifted by Davydov interaction between anion and cation. Cation-anion association was also observed in MeCN solution and association constants were measured. 

The conclusion that palladium particles in zeolites may carry a partial positive charge follows from the IR study of CO adsorption. This adsorbate can be considered to be a probe of the electronic state of palladium. Namely, the shift toward higher frequencies of the CO linear band (for Pd°-CO it appears at <2100 cm ) reflects a decrease in the back donation of electrons from Pd to CO. Along with such an interpretation, Figueras et al. (138) detected the presence of electron-deficient Pd species in Pd/ HY but not in Pd/Si02. More recently, Lokhov and Davydov (139) confirmed the presence of positively charged Pd species apart from Pd° in reduced (at 300°C) Pd/Y samples and ascribed a 2120- to 2140-cm"1 band to Pd+-CO complexes (Fig. 7). Similarly, Romannikov et al. (140) report that adsorption of CO on Pd/Y samples reduced at 300°C produces IR bands at >2100 cm 1 ascribed to Pd+-CO and Pdzeolite protons, because the IR band of the zeolite O-H group decreases when CO is released and increases when CO is added to the cluster (141, 142). 

Kinetic evidence for synergic adsorption of carbon monoxide and water on the low-temperature shift catalyst Cu/ZnO/Fe203 was obtained by van Herwijnen and deJong (113), and IR spectra of surface formate were detected on several oxide catalysts, including CuO/MgO, at temperatures as low as 20 JC and pressures of 20 Torr, as reported by Davydov et al. (104). Decomposition of the surface formate to C02 and H2 occurred at 100-150°C over the Cu/MgO catalyst and at 250 300°C over the MgO catalyst, and the promotion effect of copper was attributed to the formation and decomposition of a labile surface formate (HCOO)2Cu. Ueno et al. (117) have shown earlier that surface formates are formed on zinc oxide, from CO and H20 as well as from C02 and H2, and hence an associative mechanism of the shift and reverse-shift reaction, involving formate intermediate, is believed to operate on many oxide catalysts. 

The main features that must be considered are the magnitude and symmetry of the static potential in the H2O lattice site and intermolecular coupling (even between molecules of adjacent primitive unit cells) of the water bands. The former is shown by large frequency shifts of the water bands and decreased intramolecular coupling of the stretching vibrations (see Sect. 4.2.7) compared to those of free water molecules (see Table 3) and is discussed in Sects. 4.2-4.4 in more detail. The latter also produces frequency shifts and the so-called correlation field (Davydov) splitting of the water bands. 

In a subsequent paper, Munn [98] showed that the frequency-dependent local-field tensors accounted for the shift of the poles of the linear and nonlinear susceptibilities from the isolated molecular excitation frequencies to the exciton frequencies. The treatment also described the Davydov splitting of the exciton frequencies for situations where there is more than one molecule per unit cell as weU as the band character or wave-vector dependence of these collective excitations. In particular, the direct and cascading contributions to x contained terms with poles at the molecular excitation energies, but they canceled exactly. Combining both terms is therefore a prerequisite to obtaining the correct pole structure of the macroscopic third-order susceptibility. Munn also demonstrated that this local field approach can be combined with the properties of the effective or dressed molecule and can be extended to electric quadrupole and magnetic dipole nonlinear responses [96]. 

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