Electronic spectra adsorbed compounds

The EELS spectrum of DMBM [Fig. 39(a)] and the IR spectrum of solid DMBM in KBR [Fig. 39(b)] are very similar except for the absence of the sulfhydryl SH stretch in the EELS spectrum (2500 cm-1). [The difference in appearance of the EELS and IR spectra results from several causes the resolution of EELS (80 cm-1) was poorer than that of IR (4 cm-1) EELS emphasizes electron impact scattering while IR emphasizes dipole scattering, an experimentally valuable distinction and the intrinsic line widths may be broader for the adsorbed layer than for the solid compound.] Assignments of the EELS bands are proposed in ref. 81. Absence of the SH stretch from EELS spectra of DMBM is evidence for removal of the sulfhydryl hydrogen during adsorption... 

If the difference in atomic number between the absorber element and the backscattering element is >10 and if only one kind of element backscatters, EXAFS spectra can be analyzed readily to provide local structural data on adsorbed species. However, because the electron mean free path, thermal and static disorder parameters (Debye-Waller factors), and coordination number for an absorber environment cannot be determined a priori with sufficient accuracy, EXAFS data for suitable reference compounds of known molecular structure must be used to help interpret the EXAFS spectrum for an interfaeial region. 

A selective interaction with adsorbed tetrazine molecules is revealed when the adsorbent is brought into contact with the vapors of nitrobenzene, or benzoquinone. The spectrum is radically changed (52). This is presumably due to the formation of intermolecular addition compounds between the electron donating and accepting molecules in the adsorbed state. 

Another widely accepted viewpoint on the catalytically active sites in the silica-alumina is that they are aprotonic acids, viz., electrophilic A1 atoms with unfilled p-shell, the electron density being shifted from them towards the three surrounding 0 atoms (72). Such an electron deficiency confers on the A1 atom an afiinity towards an unshared electron pair of the basic adsorbed molecule, i.e., the properties of a Lewis acid. In fact, the addition compound, obtained on sorption of aniline vapor by a sublimed AICI3 film in a high vacuum exhibits the same shift towards the spectrum of benzene, as is the case with a protonic acid (73). 

The complex excitation mechanism essentially states that the action spectrum of surface photochemistry should be referenced to the transitions of the adsorbate-substrate complex rather than the gas phase spectrum. As a first approximation the complex is taken as being analagous to the corresponding organometallic compound. Alternatively the perhaps known electronic structure of the adsorbate may be referenced to an electronically similar gas phase... 

Vibrational spectra obtained by electron energy-loss spectroscopy (EELS) of LD adsorbed at Pt(lll) were as shown in Figure 4A. Also shown in the Figure are the locations and relative intensities of bands in the mid-infrared spectrum of solid LD in KBr. The EELS spectra contain peaks corresponding to most of the IR bands of solid LD, although not all of the bands are resolved, and absence of the phenolic OH stretches is to be expected (see catechol, below). Table 3. Shown for comparison are the EELS spectra of the amino acids L-tyrosine (TYR), L-cysteine (CYS), phenylalanine (PHE) and alanine, and of the related compounds dopamine (DA) and... 

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