Cluster composition comparisons

Table VIII. Five Midwestern Localities Cluster Composition Comparisons...

The computer can also calculate the possible compositions of ions of a given mass, taking into account only the elements in the molecular formula. For example, if a compound is known to contain only CHON as elements, a fragment detected at m/z 39 can have only C2HN or C3H3 as its elemental composition. More examples can be found in Appendix 5. The computer can also calculate for a given molecular formula the theoretical relative abundances within the isotopic cluster for comparison with experimental values. It can also compare the spectra that it observes with a library of spectra. 

An important parameter for comparison with theory as well as for understanding many properties would be relative binding energies or stabilities. Unfortunately those are hard to assess in the gas phase. One of the few experiments to report thermodynamic binding energies between base pairs is the work by Yanson et al. in 1979, based on field ionization [25], Relative abundances of nucleobase clusters in supersonic beams are an unreliable measure of relative stability for a two reasons First, supersonic cooling is a non-equilibrium process and thus comparison with thermal populations is tenuous at best. Secondly, ionization probabilities may be a function of cluster composition. The latter is certainly the case for multi photon ionization, as will be discussed in detail below. 

It is interesting to note that the base displaced need not be a simple one, that is, dihydrogen is exclusively displaced from [HFe3(CO)9(/u-H)2BH] (equation 10). Indeed, a comparison of equations (10) and (11) demonstrates that cluster composition is important in determining the outcome of a Lewis base displacement reaction on a transition metal-main group element cluster. This implies that control of electronic structure via cluster element composition is a viable means of controlling reactivity. 

In Secondary Ion Mass Spectrometry (SIMS), a solid specimen, placed in a vacuum, is bombarded with a narrow beam of ions, called primary ions, that are suffi-ciendy energedc to cause ejection (sputtering) of atoms and small clusters of atoms from the bombarded region. Some of the atoms and atomic clusters are ejected as ions, called secondary ions. The secondary ions are subsequently accelerated into a mass spectrometer, where they are separated according to their mass-to-charge ratio and counted. The relative quantities of the measured secondary ions are converted to concentrations, by comparison with standards, to reveal the composition and trace impurity content of the specimen as a function of sputtering dme (depth). 

The surface-catalyst composition data for the silica-supported Ru-Rh cuid Ru-Ir catalyst are shown in Figure 1. A similcir plot for the series of silica-supported Pt-Ru bimetallic catalysts taken from ref. P) is included for comparison purposes. Enthalpies of sublimation for Pt, Ru, Rh and Ir are 552, 627, 543, and 648 KJ/mole. Differences in enthalpies of sublimation (a<75 KJ/mole) between Pt and Ru cind between Rh and Ru are virtually identical, with Pt euid Rh having the lower enthalpies of sublimation. For this reason surface enrichment in Pt for the case of the Pt-Ru/Si02 bimetallic clusters cannot be attributed solely to the lower heat of sublimation of Pt. Other possibilities must also be considered. 

The first transition metal derivatives of a Zintl ion was prepared by Teixidor et al. in 1983 in reactions between Pt(PPli4)4 and en solutions of the Eg (E = Sn, Pb) [25, 26]. Despite being the first examples in this important class of clusters, the complexes have yet to be isolated and their structures and compositions remain unknown. The authors propose that complexes have a (PPh3)2PtSng stoichiometry and a nido-ty structure. Based on comparisons with NMR parameters from the past 30 years and the stoichiometry of the reactions described by Teixidor et al., we believe that the Rudolph compounds are most likely 22-electron cZos )-Pf E9Pt (PPh3) complexes. Our rationale is given below. 

One type of the constituent metallocenters in the MoFe protein has the properties of a somewhat independent structural entity. This component, referred to as the FeMo cofactor (FeMo-co), was first identified by Shah and Brill (1977) as the stable metallocluster extracted from acid-denatured MoFe protein. The FeMo-co was able to fully activate a defective protein in the extracts of mutant strain UW45, a protein which subsequently was shown to contain the P clusters but not the EPR-active center. The isolated cofactor accounted for the total S = t system observed by EPR and Mdssbauer spectroscopies of the holo-MoFe protein (Rawlings et al., 1978). Elemental analysis indicated a composition of Mo Fee-8 Se-g for the cofactor, which, if there are two FeMo-co s per a2 2> accounts for all the molybdenum and approximately half the iron in active enzyme (Nelson etai, 1983). Although FeMo-co has been extensively studied [reviewed in Burgess (1990)] the structure remains enigmatic. To date, all attempts to crystallize the cofactor have failed. This is possibly due to the instability and resultant heterogeneity of the cofactor when removed from the protein. Also, there is a paucity of appropriate models for spectral comparison (see Coucouvanis, 1991, for a recent discussion). Final resolution of this elusive structure may require its determination as a component of the holoprotein. 

A quantitative surface compositional analysis requires the comparison of the experimental yield of the individual clusters with corresponding yields obtained theoretically this may be done by numerical simulation of the complex collision process but the accuracy of the result cannot yet be ascertained. The accuracy of the compositional analysis depends to some extent on such poorly known factors as the interatomic potential, ionization cross-sections and quantum-mechanical corrections to a treatment based on classical trajectories. 

Changing the molar Cul to ligand ratio to 2 1 (using the same experimental conditions) has a dramatic effect on the composition of the colorless crystalline material 15. Now elemental analyses were in accordance with the ligation of two Cul units per dithioether. Unfortunately, we failed to obtain X-ray-suitable crystals to elucidate the solid-state structure of this compound. However, comparison of the emission spectra of this extremely luminescent material with those of polymers 1, 9, 10, 16, and 21 (see below), for which the occurrence of Cu4(p3-I)4 clusters has been crystallographically established, suggests the existence of cubane-like Cu4(p3-I)4 units in compound 15. 

Infrared spectroscopy is most useful for identifying a known compound via comparison with published infrared data. In general, it is not possible to determine the structure or composition of a cluster by its infrared spectrum alone, although the spectrum can provide several useful indi-... 

The above literature review gives a comparison of different ways to control selectivity for both homogeneous and heterogeneous catalytic reactions. There are several common features for the four areas of stereoselectivity metal clusters, alloys and poisoning shape selectivity and reaction pathway control. In fact, many times more than one of these areas may be involved in a catalytic system. Some common features for all of these areas include precise control of the structural and compositional properties of the catalysts. This paper serves as an overview for the other manuscripts in this book. Specific review chapters on each of the four areas can be found in reviews that follow by D. Forster et al., K. J. Klabunde et al., M. E. Davis et al., and H. C. Foley and M. Klein et al. 

---