Clusters positively charged

When Rutherford allowed the radiation to pass between two electrically charged electrodes, he found that one type was attracted to the negatively charged electrode. He proposed that the radiation attracted to the negative electrode consists of positively charged particles, which he called a particles. From the charge and mass of the particles, he was able to identify them as helium atoms that had lost their two electrons. Once Rutherford had identified the atomic nucleus (in 1908, Section B), he realized that an a particle must be a helium nucleus, He2+. An a particle is denoted or simply a. We can think of it as a tightly bound cluster of two protons and two neutrons (Fig. 17.5). 

The magnitude of the positive charge on all Au cluster compounds can be rationalized by simple MO calculations . The reactivity and structure of the... 

The charge distribution determined within clusters by CNDO has been reported for only a few cases. Let us consider only one cluster, the 13-atom fee cluster with only two geometrically different types of atom. There is a center atom with 12 nearest neighbors, and there are 12 surface atoms each with 4 nearest neighbors. At the equilibrium bond length (0.34 nm) the center atom has a net positive charge, but this situation is reversed at the bulk experimental distance (0.288 nm). 

Fig. 14 Potential energy profile for stepwise and concerted mechanisms with (solid lines) and without (dotted lines) an attractive interaction between the caged fragments in the product cluster. The case of the reduction of a neutral substrate is represented. It can be transposed for reductions of a positively charged substrate or for oxidations of neutral or negatively charged substrates.

The GIAO-MP2/TZP calculated 13C NMR chemical shifts of the cyclopropylidene substituted dienyl cation 27 show for almost all carbon positions larger deviations from the experimental shifts than the other cations 22-26. The GIAO-MP2/TZP method overestimates the influence of cr-delocalization of the positive charge into the cyclopropane subunit on the chemical shifts. Electron correlation corrections for cyclopropylidenemethyl cations such as 27 and 28 are too large to be adequately described by the GIAO-MP2 perturbation theory method and higher hierarchies of approximations such as coupled cluster models are required to rectify the problem. 

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