We have examined the reactivity and saturation of small gold clusters (cations, neutrals and anions) towards several molecules and find that specific small gold clusters exhibit a pronounced variation in their reactivity towards hydrogen, methane and oxygen. The reactivity not only depends strongly on cluster size but also on the cluster charge state. For example, small (n < 15) gold cations react readily with Dz, but no evidence of reaction is observed for the anions under our experimental conditions. Similar behavior is seen for methane. With oxygen only even atom (odd electron) anions are reactive, and Auto is the only cation which exhibits evidence of reaction. The global features (small cluster cations reactive towards H2, CH4, but large ones not reactive, odd electron anions reactive towards O2) are qualitatively explained by appealing to a simple frontier orbital picture. The uptake of deuterium and methane on gold clusters also exhibits a pronounced size dependence with D/Au varying from a high of 3 for the dimer to zero for clusters containing more than 15 Au atoms. Comparison of the methane and deuterium saturation behavior leads us to suggest that methane is dissociated and bound as CH 3 and H.

aqueous phase on the organic liquid, however, has not been commented upon nor rigorously assessed. To a first approximation, one assumes that the "interfacial" organic liquid behavior parallels that of the bulk phase. How do these observations compare to the phospholipid bilayer-water interface?Consider first the acyl chain region. In the center of a bilayer, e.g., composed of DMPC, the viscosity has been determined to be about 1.5 P, a value about 200 times greater than that of water and equivalent (approximately) to that of a light oil.26 However, from the center of the bilayer to the surface, 13C NMR has been used to demonstrate a 100-200-fold gradient of disorder (expressed as microviscosity).22 The hydrocarbon chains near the headgroups are much more crystalline, therefore, than in the center of the bilayer. The liposomal interfacial transfer process initially involves, as a result, solute "extraction" from an organic environment that is much more structured than a simple liquid. This situation may be reasonably expected to result in a significant slowing of the phase transfer process.At the headgroup region of a liposome, a very complex structure exists. The phosphatidylcholine headgroup binds water tightly: calorimetric experiments, for example, indicate that 10 mol of water per mole of synthetic lecithin is unfreezable at 0 °C;23 using (25) Guy, R.

Our studies of small (n=2−40) gas phase transition metal clusters (cations, neutrals and anions) have revealed a number of size-dependent chemical and physical properties. This paper will discuss results involving activated dissociative molecular chemisorption of hydrogen (deuterium) and small alkanes on cationic, neutral and anionic platinum and gold clusters. For example, we have yet to find any size gold cluster anion to exhibit measurable reactivity towards di-deuterium, but reactivity is observed on small (n<15) gold cations and very small (n<9) neutral gold clusters. Methane or ethane chemisorption occurs most readily only on small Pt and Au cluster cations and least rapidly, if at all, on cluster anions. The larger Pt and Au clusters are found to be non-reactive towards the alkanes under our experimental conditions.At the other extreme, at steady state, small clusters of Pt, Rh, Ni, Pd, V, Nb, and Ta are hydrogen “rich” exhibiting H/M stoichiometrics >> 1. These results are consistent with a limited data base on supported clsutes obtained via EXAFS measurements and have important implications for catalytic reactions involving hydrogen and light hydrocarbons.

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The effect of charge state on niobium cluster chemisorption kinetics is explored via measurement of the relative rates of D2 activation by Nb−x, Nbx, and Nb+x containing up to 28 atoms. The presence of the + or − charge is found to have only a minor effect on rate for the majority of the clusters, with the reactivity of the ions being generally within a factor of 2.5 of the corresponding neutrals. The excess charge does, however, have a profund influence on reaction rate for a number of clusters in the 7≤x≤16 size range, which may be indicative of the importance of cluster electronic structure in the chemisorption process. Kinetic data for Nb9, Nb12, and Nb+12 are found to deviate significantly from the expected pseudo-first-order behavior, suggesting the existence of structural isomers for these species. The anomalous behavior for Nb9 and Nb12 was not observed in previous neutral Nbx chemisorption studies. The maximum uptake of D2 by niobium clusters is found to be essentially independent of charge state, but varies strongly with x, consistent with the presence of a high barrier to D–D bond activation for certain of the clusters.