Superatoms. Группа авторов

Superatoms - Группа авторов


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      2.2.2.1 Superalkalis and Superhalogens

      Alkali atoms, with outer electronic configuration of ns 1, have one excess electron while halogen atoms, with outer electronic configuration of ns 2 np 5, need one extra electron to achieve the octet shell closure of their respective ionic cores. As a result, both the atoms are reactive. Gutsev and Boldyrev [44, 45] were the first ones to use the octet rule to design superalkali and superhalogen clusters that not only mimic the chemistry of alkali and halogen atoms, respectively, but also surpass their properties. The ionization potentials of superalkalis are lower than those of the alkali atoms while the electron affinities of superhalogens are higher than those of the halogen atoms. The composition of superalkali is M k + 1X, where M is an alkali atom and X is an atom with valence k. An example of a superalkali is Li3O whose ionization potential of 3.54 eV [46] is smaller than that of the Li atom, namely, 5.39 eV. The composition of a superhalogen, on the other hand, is MY k + 1, where Y is a halogen atom and M is a metal atom with valence k. A typical example of a superhalogen is LiF2, which has an electron affinity of 5.45 eV [47]. This is much larger than the electron affinity of F, namely 3.4 eV. The reason for these superior properties is inherent in the nature of the distribution of electrons. Note that the phase space occupied by the outer electrons increases with cluster size. In superalkalis, this makes it easier to remove an electron, hence leading to a lower ionization potential. In a superhalogen, on the other hand, the increased phase space for the electron distribution causes a reduction in electron–electron repulsion; hence, leading to a higher electron affinity. That superhalogens can promote unusual reactions was already realized by Bartlett in 1962, long before Gutsev and Boldyrev coined the word. Bartlett and coworkers showed that O2 and noble gas atoms such as Xe can be ionized by using PtF6 and estimated its electron affinity as 6.8 eV [48, 49]. The fact that clusters can mimic the chemistry of alkali and halogen atoms with superior properties provides new opportunities to design supersalts with superalkalis and superhalogens as building blocks [50].

Schematic illustration of electron affinity of coinage metal atoms decorated with F.

      Source: Koirala et al. [51]. © American Chemical Society.

Schematic illustration of electron affinity (EA) of Au(BO2)n as a function of n (black line).

      Source: Adapted with permission from Ref. [52]. © John Wiley & Sons.

Schematic illustration of al(BH4)3 (left panel) and KAl(BH4)4 (right panel).

      Source: Adapted with permission from Knight et al. [53]. © American Chemical Society (courtesy of D. Knight and R. Zidan, private communication).

      2.2.2.2 Superchalcogens


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