Cluster compounds ionization potential

Figure 7. Dependence of organosilicon compounds chemisorption energy on the silica surface corrected for OH precursor formation on the inverse difference between ionization potential of silica surface cluster and electron affinity of the compounds. Figure taken from [49].

Owing in part to the relatively recent development of cluster chemistry, there is an almost complete lack of thermodynamic data for cluster compounds. The measurement of appearance potentials of positively charged ions by mass spectrometry has offered a technique for ready measurement of this parameter, but the high values obtained for ionization potentials and heats of formation of ions by this method cast some doubt on its accuracy (358). It would appear that ions are generated in excited states (22). It may be expected that there will be a considerable increase in the thermodynamic investigations of clusters in the near future, so as to provide quantitative data on which to base theoretical considerations. 

Figure 5 Third ionization potential I3 versus atomic number for the lanthanide series. Elements marked in red form clusters, elements marked in blue form stable divalent compounds with 4f 5d° electronic configurations...

The pattern as seen in Figure 5 may be transferred to a periodic table of the rare earth elements, see Figure 6. Only elements underlaid in red form clusters. The lower I3 is, the easier it is to produce cluster complexes. Elements underlaid in blue form stable divalent compounds, for example EUCI2 the divalent state with the electronic configuration 4f 5d° (with n =7, 14, 6, 13 for R = Eu, Yb, Sm, Tm) has the highest stability and, thus, is the easiest to achieve when the third ionization potential is the highest. The divalent chemistry of these elements is alkaline-earth and saltlike this is described in The Divalent State in Solid Rare Earth Metal Halides. 

MALDI is the dominant ionization source choice for MSI however, it is not without drawbacks. The application of MALDI matrix on top of the tissue surface complicates the analysis by adding potentially isobaric matrix cluster ions that may obscure the drug compound. Careful selection of MALDI matrix can reduce the matrix effect. An alternative would be to either use the water native to the tissue as a matrix such as with IR-MALDI or altogether eliminate the need for matrix. DESI is an atmospheric pressure technique that permits the direct analysis of surface samples, including tissue sections, with minimal sample preparation (Takats et al., 2004 Cooks et al., 2006). In contrast to MALDI—MSI, no matrix is required however, the spatial resolution for DESI—MSI is worse when compared to MALDI or SIMS imaging experiments. 

Older soft ionization methods that are related to MALDl—such as fast atom bombardment (FAB), hquid secondary ion mass spectrometry (LSIMS), or field desorption (FD)— in principle have potential for the generation of intact gas-phase ions from noncovalent complexes. In particular, the extensive clustering often found in FAB-MS, which is generally thought of as a nuisance, can be viewed as evidence for this. However, the limitation of these methods lies in their inabiUty to ionize very large molecules. It is generally very difficult or impossible to obtain useful mass spectra from compounds with molecular weights above a few thousand daltons. For this reason, FAB-MS, LSIMS, and FD-MS only play a very minor role in this field. 

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