Superatoms. Группа авторов
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Название: Superatoms
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119619567
isbn:
Figure 2.12 Electron affinity of coinage metal atoms decorated with F.
Source: Koirala et al. [51]. © American Chemical Society.
Figure 2.13 Electron affinity (EA) of Au(BO2)n as a function of n (black line).
Source: Adapted with permission from Ref. [52]. © John Wiley & Sons.
Superhalogens and hyperhalogens can be used in the design of novel salts for applications in solar cells, batteries, and hydrogen storage materials. While we discuss these applications in detail in Chapter 10, in the following chapter we show how hyperhalogen concept led to the synthesis of a hypersalt, KAl(BH4)4. Note that Al(BH4)3 is a volatile pyrophoric liquid. Although it contains 16.8 wt % hydrogen, it cannot be used as a hydrogen storage material because of safety concerns. However, by adding one more BH4 unit to Al(BH4)3, an Al(BH4)4 hyperhalogen can be formed. By combining it with a K cation, Knight et al. [53] synthesized KAl(BH4)4 hypersalt, which is solid and nonvolatile under ambient conditions (Figure 2.14).
Figure 2.14 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
Atoms in the Group 16 of the periodic table require two extra electrons to satisfy the octet rule. When isolated, these atoms cannot retain both the electrons due to electron–electron repulsion. However, an atomic cluster could be stable as a dianion if it is large enough to reduce electron–electron repulsion. The question is: how small a cluster has to be so that it can retain two extra electrons without fragmenting or ejecting the second electron spontaneously? Such a cluster could be viewed as a superchalcogen that is stable, yet mimics the chemistry of Group 16 elements. Chen et al. [54] studied this possibility by focusing on M(CN)4 clusters where M is a divalent alkaline earth metal atom (Be, Mg, Ca, Zn, Cd), which contributes two electrons while each CN molecule would need one electron to satisfy the octet rule. The authors calculated the equilibrium geometries and total energies of neutral, monoanionic, and dianionic M(CN)4 clusters using density functional theory. The results are presented in Figure 2.15. The energy gains in adding the first (second) electron to M(CN)4 clusters are 3.13, 2.94, 2.89, 2.78, and 2.59 eV (0.32, 0.97, 1.21, 0.83, and 0.56 eV), respectively, for M = Be, Mg, Ca, Zn, and Cd. The stability of the M(CN)4 2− indicates that the octet rule can be effectively used to rationally design doubly charged species that are stable in the gas phase.
It is interesting to compare the relative robustness of clusters obeying the jellium and octet shell closure rules in the rational design of cluster‐assembled materials. It was discussed earlier that Al13 − cluster, in spite of its being stable and chemically “inert,” coalesces when crystals of KAl13 and [(CH3)4N+][Al13 −] are formed. Note that (CH3)4N+, obeying the octet rule, retains its geometry [36]. Huang et al. [36] studied the stability of crystals composed of (CH3)4N+ cation and B(CN)4 − anion. Note that both molecules satisfy the octet rule. The authors found [(CH3)4N+][B(CN)4 −] to be a stable charge‐transfer transparent salt with a band gap of 6.5 eV and having a diverse range of structural phases. In Figure 2.16 we show the geometry of the isolated [(CH3)4N+][B(CN)4 −] cluster and the optimized geometry of the [(CH3)4N+][Al13 −] crystal having body‐centered cubic structure and Td symmetry. The band gap of the crystal phase is close to that of the HOMO–LUMO gap of the isolated cluster, implying that the electronic structure of the crystal is guided by the properties of the individual cluster building blocks.
Figure 2.15 (a)–(e) are the globally optimized geometries for M(CN)40,1−,2− (M = Be, Mg, Ca, Zn, and Cd) clusters, respectively. Gray, pink, light green, dark green, purple, light blue, and dark blue spheres stand for C, N, Be, Mg, Ca, Zn, and Cd atoms, respectively. The charges on selected atoms are also given.
Source: Chen et al. [54]. © American Chemical Society.
In spite of the success of the octet rule accounting for the stability of clusters composed of light elements, like the jellium model, it has limitations; stable clusters exist even though they do not satisfy the octet rule. These include, for example, NO (which has an odd number of valence electrons), BH3 and BF3 (which are electron deficient), and PCl5, SF4, and SF6 (which are electron rich).
Figure 2.16 (a) Isolated (CH3)4N+Al13− cluster. (b) Optimized body‐centered‐cubic phase of (CH3)4N+Al13− crystal with Td symmetry (same as the molecular point‐group of the respective ions). (c) Total energy and lattice parameter (lattice constants a, b, and c in Å, interaxial angles α, β, and γ in degrees, and volume in Å3) fluctuations during the ab initio molecular dynamics simulation. (d) Snapshot of the structure obtained from the ab initio molecular dynamics (AIMD) simulation for 10 ps after 4 ps to allow the system to reach thermal equilibrium.
Source: Huang et al. [36]. © American Chemical Society.
2.2.3 18‐Electron Rule
Stability СКАЧАТЬ