Low-coordinated cations

Shallow electron traps (Sect. 2.3.10). Some specific sites at the MgO surface exhibit electron affinities of about 1 eV and can bind electrons. Among these sites, there are low-coordinated cations, like an Mg 3g corner site, divacancies, and other morphological sites like the reverse corner. 

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Schoeller WW (2003) Donor-Acceptor Complexes of Low-Coordinated Cationic p-Bonded Phosphorus Systems. 229 75-94... 

It is informative to begin the further discussion with a consideration of the various cations which will be the aim of the following sections. The most prominent examples of the low-coordinated cations in phosphorus chemistry are the phosphenium (phosphanyhum) cations. They were scrutinized in recent reviews [32-35], and hence will not be discussed in details in the present review. 

The third chapter (W.W. Schoeller) reviews the latest developments in the area of donor-acceptor complexes of low coordinated cationic n-bonded phosphorus systems, a topic which attracts the interest of many researchers, both from theoretical and experimental points of view. 

Figure 6 Examples of low-coordinate cationic aluminum compounds.

Low-coordinate cationic organoaluminum(in) compounds (two to four coordinate) 286... 

The singly trapped electron center is paramagnetic and produces a characteristic EPR powder pattern. The electron trapping site, labeled Mg in Equation 1.51, can either be a single low-coordinated cation n = 1) or a small array of surface cations n > 1), while the proton is stabilized by a single 0 anion as a surface hydroxyl group (0 -r H —> OH ). A typical X-band cw-EPR spectrum for this excess electron center on polycrystalline MgO is shown in Figure 1.16. 

Low-coordinated cations ( 3.1). These are Mg ions with a number of neighbors lower than on the flat (100) terraces. To this category belong therefore four-coordinated ions located at step and edge sites, Mg%c, and the three-coordinated ions located at comers, kinks, etc. Mg 3c. 

The study of the vibrational modes of small adsorbed molecules provides therefore a very useful tool to detect and titrate low-coordinated cation sites in UHV cleaved single crystals as well as on high surface area polycrystalline materials. This does not apply only to CO but also to other molecules for instance, excellent correlations between theory and experiment have been found for the v(CN) bands of CD3CN adsorbed on Mg4c and Mgs/ cations of MgO... 

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