Bands and Electronic Spectra

It is a somewhat formidable undertaking to make an energy-band calculation for a crystal such as a-quartz with nine atoms per primitive cell a full calculation has only recently been completed by Chelikowsky and Schliiter (1977), who used a self-consistent pseudopotential method. The results of this calculation are shown 

If 0 = 0, as in /1-cristobalite, the orbitals in the bonding unit become particularly simple and are illustrated in Fig. 11-10. The B orbital can be thought of as 

Orbitals of the bonding unit when it i.s taken to be straight () = 0), as in /1-cristobalite. This approximation simplifies the understanding of the Si02 spectra. The dark dot at. the center of each combination of orbitals is the oxygen nucleus. 

Of particular interest are the optical spectra. Chclikow.sky and Schluter calculated the Joint density of states for direct transitions (which would be proportional to C2 were the dipole matrix elements all equal). sec Section 4-A -with the result shown at the bottom of Fig. 11-12. It bears little resemblance to the experimental Cj curve (uppermost in the figure), for a number of reasons. Tlie prominent peak at 10.4 eV appears to be an cxciton peak (See Section 6-H), as had been stiggested earlier by Platzoder (1968) on the basis of observed temperature dependence. Pantelidcs and Harrison took this peak to result from interband transitions, since it lay at an enci gy above the photoconductivity threshold of 9 eV (DiStephano and Eastman, 1971b) that would rule out the possibility that the peak represents a simple exciton, but not that it represents an excitonlikc 

The ailculalcd joint density of states in fact ri.ses smoothly through the energy of the next two peaks in the experimental C2 plot. Only when Chelikowsky and Schliiter incorporated calculated dipole matrix elements (the middle curve, labelled 1 2(10) theory in Pig. 11-12), did those two peaks show up. Variations of the matrix elements with energy presumably arise from interference between the dipole matrix elements for different bonding units in the primitive cell, in which case the structure in C2 should not be interpreted in terms ofa single bonding unit. 

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The interpretation of vibrational and electronic spectra of these complexes is much more complicated. Whereas IR spectra generally show only bands of low intensity, tbe Raman spectra often display intense and characteristic lines. Rigorous assignments have not been published so far, but the most simple M(/i2-S), ring systems can easily be... 

Table 69 Main Bands in the IR and Electronic Spectra of [Cr(NCX)s]3 Complexes...

The Mo(RDta)4 complexes do not react with I2 and decompose upon treatment with Br2 (482). An extensive study of the EPR and electronic spectra of the Mo(R2Dtc)4 complexes has been reported (483). An extended Htickel MO calculation has been carried out for the [Mo(H2Dtc)4]+ cation (485). The electronic absorptions of low intensity between 13 and 20 kK were assigned to CT bands. The magnetic moments of these compounds 1.7 BM are indicative of one unpaired electron. The Curie-Weiss law is followed for these compounds (485). 

Electron Density. Electron density calculations for 1,6-naphthyridine have been made for comparison with those of other nitrogenous heterocycles and for the rationalization of ionization phenomena and electronic spectra.676 840 1126 1173 Electron Spin Resonance. Radicals derived from 1,6-naphthyridine have been studied,1329 especially with respect to their ESR spectra.1083 Infrared/Raman Spectra. Assignments for the major bands in IR/Raman spectra of 1,6-naphthyridine and related compounds have been reported.44 1124 1251 Ionization. Theoretical calculations for the protonation of 1,6-naphthyridine have been carried out for comparison with those of the other naphthyridines and related heterocycles.806 813... 

Characteristic vibration bands are presented in Chapter 4.01.3.7, Tables 26 and 27. Further information is also available in Section 4.27.2.3.6 (68AHC(9)165, p. 202, 70SA(A)2057). The IR and electronic spectra of the metal complexes of bismuthiol I and II (thiadiazole derivatives) have been published (80BSF(l)45l). 

H Norrstrom and A Teder. Absorption Bands in Electronic Spectra of Lignins. Part 1. Lignins from Alkaline Cooks on Spruce. Svensk Papperstidn. 74 85-93, 1971. 

Absorption bands in electronic spectra are usually broad the absorption of a photon of light occurs in sslO s whereas molecular vibrations and rotations occur more slowly. Therefore, an electronic transition is a snapshot of... 

Moreover, it is remarkable that at least four bands in the photoelectron spectrum exhibit vibrational fine structure. Thus, the ion possesses as many stable excited states. Only one of them, the first one is due to ionization from a n-orbital. (This makes the fine structure observed in both the photoelectron and electronic spectra of ethane less surprising.) Vy, U2. and V3 are Raman active, but v is both Raman and infrared inactive and its frequency had to be determined by indirect methods (ref. 96). 

The proposed calculation procedure will be first tested by analysing in detail the effects of the solvent polarity on the structure and electronic spectra of the simple merocyanine Ml. Afterwards, the selected calculation procedure will be applied to the more complex dyes M2 and M3, characterized by equal length of the conjugated path connecting the donor and acceptor group, but exhibiting opposite solvatochromic effects. To be precise, the acyclic merocyanine M2 shows, like the simpler chromophore Ml, positive solvatochromism [25] (i.e. bathochromic shift of the first absorption band on increasing solvent polarity),... 

Physical Measurements. For the electrolyses, a Wenking potentiostat model 70TS1 and a Koslow Scientific coulometer model 541 were used. Voltammetry with wax-impregnated graphite and rotating platinum electrodes was performed as described elsewhere (7, 8). IR and electronic spectra were measured on Perkin-Elmer 225 and Cary 14 instruments. X-band ESR spectra were recorded at room temperature on a JEOL MES-3X spectrometer. Phosphorus-31 NMR spectra were recorded in the pulse mode on a Varian XL-100 instrument at 40.5 MHz using a deuterium lock, or on a Bruker HFX-90 instrument at 36.43 MHz using a fluorine lock. 

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