Cluster compounds electron affinity

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].

A wide range of thermochemical properties can be measured, including not only proton affinity or gas-phase basicity, but also electron affinity, ionization energy, gas-phase acidity and cation affinity Entropy changes upon attachment of an ion to a molecule are also accessible and provide information on both the nature of the bonding and fragmentation mechanisms in cluster ions, especially in biological compounds. Thermochemical determinations by the kinetic method also provide very useful structural information e.g., two-electron three-center bond has been observed in the gas phase by means of the kinetic method. " In the last years, the kinetic method has been also applied to characterize chiral ions in the gas phase. 

One of the major applications of the CURES-EC procedure has been in calculating the electron affinities of the carbon clusters C [46]. Carbon clusters are common species in nature, since they are a product of combustion and contribute to environmental problems. They have been observed in the interstellar medium. The fuller-enes represent one of the most recent discoveries of a new form of carbon. Carbon clusters have many forms—linear chains, cyclic compounds and the fullerenes— because carbon can form covalent bonds. The NIST tables give 54 electron affinities values for C 14 for Si 17 for Ge 45 for Sn and 55 for Pb . Only the structures for the carbon compounds from n = 3 to 30 are identified [12]. 

The electron affinities of the linear and monocyclic C clusters have been calculated using the CURES-EC method and agree with the experimental values. The electron affinities of the Si , Ge , Sn , and Pb vary with N in a manner similar to those of the cyclic C compounds. Up to N = 15 to 20, the values are considerably lower than the bulk work function. This behavior is different from that for metallic clusters, where the electron affinity is a function of A-1/3. However, the data for Pb taken up to N = 204 can be extrapolated to the work function. 

At ion source pressures on the order of 0.5 Torr and ion source temperatures of approximately 373 K, the rate constants for electron attachment and proton abstraction suggest that there are an adequate number of collisions in the ion source to permit equilibria to be sufficiently established. This is a prerequisite in order to assume a Boltzmann distribution of internal energies of the anions (or cations). Thus thermochemical data, such as electron affinities and the proton affinities of anions, can be used to calculate the energetics of these reactions (75). The cluster adduct anion [M- -C]" (Reactions 7.39-7.41) have third-order rate constants. Where comparisons can be made, the magnitude of positive- and negative-mode third-order rate constants are similar (69,103). The clustering reactions are important in NlCl spectra for polar compounds in the presence of polar molecules such as water and alcohol. 

---