Superatoms. Группа авторов
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Название: Superatoms
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119619567
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Pykko and Runenberg [55] showed that an all‐metal cluster, Au12W, with a HOMO–LUMO gap of 3.0 eV is very stable due to the 18‐electron rule. Here, 12 Au atoms contribute 12 electrons while W atom (3d 5 4s 1) contributes 6 electrons. This prediction was later verified in photoelectron spectroscopy experiment by Wang and collaborators [56]. A further proof of the 18‐electron rule can also be seen by measuring the electron affinity of Ta@Au12. Note that with 17‐valence electrons, Ta@Au12 needs one extra electron to satisfy the 18‐electron shell closure rule. Indeed, the measured electron affinity of 3.76 eV makes Ta@Au12 an all‐metal superhalogen [57]. Chen et al. calculated the structure and stability of M@Au12 2− (M = Ti, Zr, Hf) to see if these clusters can be stable and thus can be regarded as superchalcogens. The results are given in Figure 2.17. Note that all these structures are dynamically stable. However, Ti@Au12 2− is unstable against an electron loss by 0.23 eV while M@Au12 2− (M = Zr, Hf) dianions are stable against the second electron loss by 0.05 eV, due to their increased size.
Figure 2.17 (a)–(c) are the optimized geometries for MAu120,1−,2− (M = Ti, Zr, and Hf) clusters, respectively. Yellow, dark red, purple, and blue spheres stand for Au, Ti, Zr, and Hf atoms, respectively.
Source: Chen et al. [54]. © American Chemical Society.
2.2.4 32‐Electron Rule
The 32‐electron rule applies to clusters containing early 5f elements where complete shell closure of s 2 p 6 d 10 f 14 orbitals give them stability and chemical inertness. The discovery of empty icosahedral Zintl ions such as Pb12 2− and Sn12 2− [58, 59] motivated Dognon et al. [60] to study the stability of endohedral clusters Pu@Pb12. With an electronic configuration of [Rn] 5f 6 7s 2 for Pu and [Xe] 4f 14 5d 10 6s 2 6p 2 for Pb, Pu@Pb12 constitutes a 32‐electron system. The calculated HOMO–LUMO gap of 1.93 eV and the binding energy of 22.17 eV measured against the ionic dissociation limit confirm that the stability of Pu@Pb12 arises due to the 32‐electron shell closure. In Figure 2.18 we compare the orbital energies of Pu@Pb12 with that of Pb12 2−. One can see the strong participation of the central atom orbitals in bonding.
A few years later, Ghanty and coworkers [61] showed that the stability of Pu embedded in a C24 cage also follows the 32‐electron rule, with 8 electrons contributed by Pu and 24 π electrons contributed by the C24 fullerene. The authors found that the C2 symmetry of the empty C24 fullerene transforms to D6d symmetry, once encapsulated with the Pu atom. The HOMO–LUMO gap of 1.83 eV of the bare C24 cage changes to 3.26 eV following Pu encapsulation. The binding energy of the Pu@C24 clusters measured with respect to atomic fragments is 6.77 eV. Other 32‐electron systems studied recently include An@C28 [62], Pu@Sn12 [63], (U@Si20)6− [64], and actinide‐encapsulated fullerene systems [65], U@C28 [66], Ln(CO)8 − (Ln = Tm, Yb, Lu) [67], and superatomic CBe8H12 cluster [68].
Figure 2.18 Orbital energies of Pu@Pb12 and Pb122−. The latter have been shifted to make the HOMOs equal.
Source: Dognon et al. [60]. © John Wiley & Sons.
2.2.5 Aromaticity Rule
Aromaticity rule was developed by Huckel [12, 13, 69] to account for the stability of planar conjugated molecules such as C6H6. Based on a molecular orbital theory, it was shown that planar conjugated monocyclic polyene that has (4n + 2) (n = 0, 1, 2, . . . ) π or nonbonding electrons will be aromatic and stable. For benzene, n = 1. Because of the high stability, the aromatic molecules have low electron affinities, which seldom exceed 1.17 eV. Indeed, the electron affinity of C6H6 is −1.29 eV [70]. Jena and coworkers [71, 72] showed that the aromaticity rule can be used to design superatoms with electron affinities that can even exceed the electron affinity of Cl. This is accomplished by replacing H atoms in C6H6 by more electronegative atoms as well as by changing composition of the hexagonal core. In Figures 2.19 and 2.20, we show the globally optimized geometries of C6H6 − x F x and BC5H6 − x F x (x = 1–6) computed by Driver and Jena [73] using density functional theory. The corresponding electron affinities are given in Table 2.2. Note that the electron affinities of C6H6 − x F x and BC5H6 − x F x steadily increase with x. The computed electron affinity, 0.75 eV of C6F6 agree well with the experimental value of 0.86 ± 0.03 eV [74, 75]. Realizing that the electron affinity of C5H5, which lacks one electron to be aromatic, is +1.80 eV, Jena and coworkers replaced one of the C atoms in C6H6 by a B atom. The resulting BC5H6 lacks one electron to be aromatic, and its electronic affinity of 2.31 eV is indeed high. Further replacement of H atoms by F atoms allows the electron affinity to rise systematically, reaching a value of 3.24 eV for BC5F6. Thus, an aromatic molecule can mimic the chemistry of a halogen atom by suitable tailoring of its core and/or the ligand atoms.
Figure 2.19 Ground state geometries of neutral and anionic C6H6 − xFx. The gray, white, and blue spheres correspond to carbon, hydrogen, and fluorine, respectively.
Source: Driver and Jena [73]. © John Wiley & Sons.
The aromaticity rule has been extended to inorganic systems, providing further opportunities to design a new class of superatoms. One of these is an all‐metal cluster, MAl4 − (M = Li, Na, Cu). Using experiment and ab initio calculations, Li et al. [76] compared the measured photoelectron СКАЧАТЬ