Chemical shift, in XPS

The example of B5H9 serves to show how the chemical shift may be used as an aid to determining the stmcture of a molecule and, in particular, in deciding between alternative stmctures. There are many examples in the literature of this kind of application which is reminiscent of the way in which the chemical shift in NMR spectroscopy may be employed. However there is one important difference in using the two kinds of chemical shift. In XPS there are no interactions affecting closely spaced lines in the spectmm, however close they may be. Figure 8.15 illustrates this for the C lx lines of thiophene. In NMR spectroscopy the spectmm becomes more complex, due to spin-spin interactions, when chemical shifts are similar. 

The XPS technique provides identification of oxidation states of metals in supported catalysts, but the determinations are often inexact and require confirmation by other methods. XPS is especially useful for detecting changes in oxidation states of cluster precursors on various oxide supports. For example, the transformation of Rh4(CO)i2 on silica, alumina, MgO, ZnO, and TiOj to yield different surface species such as a raft of Rh(CO)2(OM) or Rh metal aggregates has been inferred from chemical shifts in XPS data (Fig. 5) 39-41). The XPS technique requires ultrahigh vacuum, and instability of the... 

An electronic effect was also used to explain the difference in 1,3-butadiene hydrogenation selectivity observed over various types of nickel catalysts such as Ni(B), Raney nickel, nickel powder from the decomposition of nickel formate, Ni(P), and Ni(S). As discussed in Chapter 12, chemical shifts in XPS binding energies (Aq) for the various nickel species were compared with that of the decomposed nickel catalyst to determine the extent of 1-butene formation as related to the electron density on the metal. The higher the electron density, the more 1-butene formation was favored. 

Figure 7.16 Chart of carbon chemical shift in XPS spectra. (Reproduced with permission from D. Briggs and J.T. Grant, Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications and Surface Spectra Ltd, Chichester. 2003 IM Publications.)...

Section 2.1.3 shows that in an XPS spectrum. X-ray excited Auger peaks are often as prominent as the photoelectron peaks themselves. For many elements, the chemical shifts in Auger peaks are actually greater than the shifts in photoelectron peaks. The two shifts can be combined in a very useful quantity called the Auger parameter a, first used by Wagner [2.30] and defined in its modified form [2.31] as... 

By analyzing the XPS intensities for normal and grazing exit of electrons from the sample surface it was concluded that the pim molecules are lying approximately flat on the copper surface. Note that this orientation of the pim molecule in principle allows for a considerable interaction between the benzene ring and the copper surface. However, it is known from earlier studies that the benzene UPS valence band remains essentially unaltered when benzene is deposited on a copper surface (12) No evidence for a strong 7t-electron-copper interaction is thus observed despite the fact that the benzene molecule is believed to lie flat on the copper surface. Hence, the observation of only minor chemical shifts in the Is levels of the benzene ring carbons is consistent with the proposed orientation (12) of pim on the copper surface. 

The chemical shift of "x- 1.5 eV for the core level peak in PEO, relative to PS, can be attributed to each carbon being attached to an oxygen atom and is consistent with theoretical predictions and experimental results on related low-molecular-weight model compounds. (4, 16) As expected, there is no shake-up peak in The core level spectrum for PEO since the poljroer is fully saturated. (17) These significant differences in XPS spectra of PS and PEO, i.e., the 1.5 eV chemical shift in the core levels, the uniqueness of the ir -it shake-up peak... 

On the other hand, the acceptor number, AN, characterizes the acidity or electron acceptor jpability of a material. It is based on the NMR chemical shift of XP contained in (C H 3P0 when reacting with the acceptor. Each probe selected had a known AN and DN in order to quantitatively "sample" the respective surfaces involved in the composite. Three probes were used to study the fiber surfaces. Chloroform (CHC1,) was used as the acidic probe and had an AN equal to 23.1 and DN equal to 0. Tetrahydrofuran (THF) was used as the basic probe with AN equal to 8.0 and DN equal to 20.0. Ethyl acetate (EA) is considered to be amphoteric with an AN equal to 9.3 and DN equal to 17.1. For the fiber investigations CHCl, THF and EA proved to be satisfactory from a chromatographic standpoint. 

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