Atomic Clusters: Structure, Reactivity, Bonding, and Dynamics

Abstract

Atomic clusters lie somewhere in between isolated atoms and extended solids with distinctly different reactivity patterns. They are known to be useful as catalysts facilitating several reactions of industrial importance. Various machine learning based techniques have been adopted in generating their global minimum energy structures. Bond-stretch isomerism, aromatic stabilization, Rener-Teller effect, improved superhalogen/superalkali properties, and electride characteristics are some of the hallmarks of these clusters. Different all-metal and nonmetal clusters exhibit a variety of aromatic characteristics. Some of these clusters are dynamically stable as exemplified through their fluxional behavior. Several of these cluster cavitands are found to be agents for effective confinement. The confined media cause drastic changes in bonding, reactivity, and other properties, for example, bonding between two noble gas atoms, and remarkable acceleration in the rate of a chemical reaction under confinement. They have potential to be good hydrogen storage materials and also to activate small molecules for various purposes. Many atomic clusters show exceptional opto-electronic, magnetic, and nonlinear optical properties. In this Review article, we intend to highlight all these aspects.

Keywords: Confinement; Electrides; Firefly algorithm; Fluxionality; Hydrogen storage; Particle swarm optimization; aromaticity.

FIGURE 1

Randomly generated configurations of (A) B₅ and (B) B₆ and their convergence to their respective global minimum energy structures (Bond lengths are provided in Å). (Adapted with permission from Mitikiri et al., 2018. Copyright© 2021, John Wiley & Sons, Inc.).

FIGURE 2

Structures of N₄ ^2- and N₆ ^4- clusters obtained at the end of the PSO run, post-processing step, and geometry-constrained optimization computed at the B3LYP/6-311 + G(d) level. (Reprinted from Mitra et al., 2021 with permission from Theoretical Chemistry Accounts, Springer Nature. Copyright© 2021, Springer-Verlag GmbH, DE.).

FIGURE 3

Global and local minimum energy structures of C₅ cluster obtained at the end of the post-processing step computed at the B3LYP/6-311+ G(d,p) level. (Reprinted from Mitra et al., 2021 with permission from Theoretical Chemistry Accounts, Springer Nature. Copyright© 2021, Springer-Verlag GmbH, DE.).

FIGURE 4

Energy profiles (in a.u.) and Nucleus-Independent Chemical Shift (NICS) values on Al₄ ^2- where 4(A), 4(B), and 4(C) represent plots of Energy vs. Particle serial numbers arranged in an increasing order of energy, NICS vs. Particle serial numbers arranged in an increasing order of energy and NICS vs. Energy, respectively. (Reprinted from Mitra et al., 2020 with permission from Theoretical Chemistry Accounts, Springer Nature. Copyright© 2020, Springer-Verlag GmbH Germany.).

FIGURE 5

Optimized structures of B₄₀, Ng@B₄₀, C _2v, and D _2d isomers of Ng₂@B₄₀ optimized at the ωB97X-D/def2-TZVP level. (Adapted from Pan et al., 2018 with permission from the PCCP Owner Societies.).

FIGURE 6

The surface representation of the optimized geometries of the guest encapsulated OA, where the guests are: (A) He, (B) Ne, (C) Ar, (D) Kr, (E) Xe, (F) He₂, (G) Ne₂, (I) Ar₂, (J) Kr₂, (K) Xe₂, (L) C₂H₂, (M) C₂H₄, (N) C₂H₆, (O) CO, (P) CO₂, (Q) H₂, (R) N₂, (S) NO, and (T) NO₂. (Adapted from Chakraborty et al., 2016 with permission from Theoretical Chemistry Accounts, Springer Nature. Copyright© 2016, Springer-Verlag Berlin Heidelberg.).

FIGURE 7

Optimized geometries of the guest encapsulated CB[7] systems at the ωB97X-D/6-311+G(d,p) level of theory. (Reproduced from Pan et al., 2017 with permission from the PCCP Owner Societies.).

FIGURE 8

Optimized geometries of noble gas encapsulated CB[6] complexes at the ωB97X-D/6-311G(2d,p) level. (Reprinted with permission from Pan et al., 2015. Copyright© 2015, American Chemical Society.).

FIGURE 9

NCI plots of Ng_n@CB[6] complexes. (Reprinted with permission from Pan et al., 2015. Copyright© 2015, American Chemical Society.).

FIGURE 10

Optimized geometries of MB₁₂ ^- and ligand bound MB₁₂ ^- at the PBE/def2-TZVPPD level, and their transition state structures for the internal rotation of the B₃ ring. Bond distances are provided in Angstrom unit. (Reprinted with permission from Saha et al., 2017. Copyright© 2017, American Chemical Society.).

FIGURE 11

Deformation density plots of the pairwise orbital interactions in LCoB₁₂ ^- (OC, CO, and NN) systems at the revPBED3/TZ2P//PBE/def2-TZVPPD level. Energies are provided in kcal/mol. (Reprinted with permission from Saha et al., 2017. Copyright© 2017, American Chemical Society.).

FIGURE 12

Optimized geometries of the clathrate hydrates along with their maximum possible H₂ molecule encapsulated complexes. (Adapted with permission from Chattaraj et al., 2011. Copyright© 2011, American Chemical Society.).

FIGURE 13

Optimized structures of C₅Li₇ ⁺, M₅Li₇ ⁺, M₄Li₄ (M = Si, Ge) and their H₂-trapped analogues at the M06/6-311+G(d,p) level. (Adapted from Pan et al., 2012b with permission from the PCCP Owner Societies.).

FIGURE 14

Optimized structures of studied super-alkali ions and their hydrogen-trapped analogues at the M052X/6-311+G(d) level. (Adapted from Pan et al., 2012b with permission from the PCCP Owner Societies.).

FIGURE 15

Optimized structures of (HF)₂@C_n (n = 60, 70, 80, 90) and free (HF)₂ at ωB97X-D/6-31G level. (Reprinted from Khatua et al., 2014b with permission from Elsevier. Copyright© 2014, Elsevier B.V.).