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Название: Superatoms
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119619567
isbn:
Source: Zaho et al. [104]. © John Wiley & Sons.
2.3.4 Tetra‐Anions and Beyond
Recently Hong and Jena [105] developed a universal model to examine the stability of multiply charged anions capable of carrying more than three extra electrons by tailoring their proximity to the closed shell as well as the size and the electron affinity of the terminal groups. They discovered several thermodynamically stable tetra‐ and penta‐anions containing as few as 50 and 80 atoms, respectively. The starting point is to realize that the extra electrons in a stable multiply charged cluster must reside in a set of bound states. Assuming a spherical potential well with a given depth V and radius r, the number of bound states depends on a positive coefficient a where V ≥ a/r 2. The larger the coefficient a, the greater are the number of bound states. A deeper or wider potential well can hold more bound states.
To form a stable tetra‐anion, the authors chose two stable dianions. Recall that M(CN)4 2− (M = Mg, Ca, Sr, Ba) and BeB11(CN)12 3− are known to be very stable dianion and trianion, respectively. By removing a CN− from BeB11(CN)12 3−, Hong and Jena combined BeB11(CN)11 2− with a series of M(CN)4 2− clusters and optimized the resulting geometries (Figure 2.33). BeB11(CN)11M(CN)4 4− (M = Ca, Sr, Ba) clusters were found to be stable with the fourth electron bound by 0.79, 0.20, and 0.25 eV, respectively. Similarly, the fourth electron affinity of Be2B22(CN)23 4− was found to be 1.48 eV. A similar procedure led to the discovery of Be2B22(CN)23Ca(CN)4 5− penta‐ion with a fifth electron affinity of 30 meV. The level of theory used to predict the composition, geometry, and electron affinity of the above tetra‐ and penta‐anions is same as that was used to predict the electron affinity of B12(CN)12 2−. As mentioned above, this prediction has now been experimentally verified. Thus, we believe that the predicted stable tetra‐ and penta‐anions can be found and the synergy between theory and experiment can lead to the focused discovery of other multiply charged anions, potentially opening a new chapter in materials chemistry.
Figure 2.33 (a) Evaluation of the size (r = 9.09 Å) needed for a stable tetra‐anion using the estimated a4 and the maximal V (15.85 eV) of the known trianions (in green). The red dashed line is the estimated threshold line for tetra‐anions. The green solid line is the threshold line for trianions. (b) Demonstration of using the known multiply charged clusters with proper sizes to form new clusters with higher negative charge states. To form the penta‐anion, certain CN− terminal group, as indicated under shades, needs to be knocked off. (c) Evaluation of the size (r = 11.04 Å) needed for a stable penta‐anion using the estimated a5 and the maximal V (19.86 eV) of the newly found tetra‐anions (in red). The purple dashed line is the estimated threshold line for penta‐anions. Boron is in pink, beryllium in light yellow, carbon in gray, nitrogen in blue, calcium in dark yellow, strontium in green yellow, and barium in brown.
Source: Fang and Jena [105]. © John Wiley & Sons.
2.4 Conclusions
In this chapter we have focused on the design of superatoms, which are atomic clusters of specific size and composition and whose chemistry mimics that of the atoms in the periodic table. The concept of superatoms is that their free electrons occupy a new set of orbitals that are defined by the entire group of atoms in the cluster, instead of by each atom separately. If these superatomic orbitals have the same symmetry as that of the atoms, one can think of a new class of materials with superatoms as the building blocks. Because of their specific size, cluster‐assembled materials may have properties different from those of the corresponding atom‐assembled materials. A classic example is C60 fullerene‐based material vs diamond and graphite. All are made of only carbon atoms but their properties are very different because of the way carbon atoms are arranged.
The central question is how to design these superatoms so that they are stable and maintain their structure when assembled. We discussed various electron‐counting rules that have been used for nearly a century to explain the stability of atoms and the compounds they form. Just as superatomic orbitals mimic atomic orbitals, it is expected the same electron‐counting rules that apply to atoms may also apply to superatoms. Indeed, we demonstrated how the octet rule for low atomic number species, the 18‐electron rule for transition metals, the 32‐electron rule for rare earth metals, the Wade‐Mingos rule for boron‐based systems, and the aromaticity rules for organic molecules can be used to design not only stable neutral but anionic species. More importantly, we demonstrated how multiple electron‐counting rules can be used simultaneously to design negative ions carrying up to five extra electrons that are stable against fragmentation or auto‐ejection of the electron.
In Chapter 10, we will discuss how the superatoms can be used to promote unusual chemistry such as making noble gas atoms form chemical bonds at room temperature and accessing high oxidation states of metal atoms. The potential of cluster‐assembled materials in storing hydrogen, catalyzing reactions, serving as building blocks of super‐ and hyper‐salts, electrolytes in Li‐ and Na‐ion batteries, and moisture‐insensitive hybrid perovskite‐based solar cells is also discussed.
References
1 1 Jena, P. and Sun, Q. (2018). Super atomic clusters: design rules and potential for building blocks of materials. Chem. Rev. 118: 5755–5870. See also references 1‐146 in Chapter 1.
2 2 Khanna, S.N. and Jena, P. (1992). Assembling crystals from clusters. Phys. Rev. Lett. 69: 1664–1667.
3 3 Khanna, S.N. and Jena, P. (1995). Atomic clusters: building blocks for a class of solids. Phys. Rev. B 51: 13705–13716.
4 4 Kroto, H.W., Heath, J.R., O’Brien, S.C. et al. (1985). C60: buckminsterfullerene. Nature 318: 162–163.
5 5 Kratschmer, W., Lamb, L.D., Fostiropoulos, K., and Huffman, D.R. (1990). Solid C60: a new form of carbon. Nature 347: 354–358.
6 6 Knight, W.D., Clemenger, K., de Heer, W.A. et al. (1984). Electronic shell structure and abundances of sodium clusters. Phys. Rev. Lett. 52: 2141–2144.
7 7 Mayer, M.G. (1948). On closed shells in nuclei. Phys. Rev. 74: 235–239.
8 8 Abegg, R. (1904). Die Valenz und das Periodische System. Versuch einer Theorie der Molekularverbindungen (the valency and the periodical system ‐ attempt on a theory of molecular compound). Z. Anorg. Chem. 39: 330–380.
9 9 Lewis, G.N. (1916). The atom and the molecule. J. Am. Chem. Soc. 38: 762–785.
10 10 Langmuir, I. (1919). The arrangement of electrons in atoms and molecules. J. Am. Chem. Soc. 41: 868–934.
11 11 Langmuir, I. (1921). Types of valence. Science 54: 59–67.
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