Noble guests in organic cages – encapsulation of noble gases by cryptophane

Selective Noble Gas Inclusion in Pentagon-Dodecahedral X20-Cages

Using DFT-based computational chemistry calculations (ωB97XD/def2-tzvp//ωB97XD/def2-svp/svpfit + ZPE(ωB97XD/def2-svp/svpfit)), binding energies of noble gases encapsulated in a series of dodecahedrane molecules (general formula: X20H20 where X = C, Si, Ge, Sn and Pb, and X20 where X = N, P, As, Sb and Bi) were calculated to learn about the noble gas selectivity. Based on calculated binding energies, the Sn20H20 cage can best accommodate noble gases with a medium size radius (Ar and Kr), while the Pb20H20 dodecahedrane cage is best suited for noble gases with the larger radii (Xe and Rn). On the other hand, from the elements of the V main group of the periodic table, the Bi20 cage has shown the best results to selectively encapsulate Ar and Kr, with the amounts of energy being released being -5.24 kcal/mol and -6.13 kcal/mol, respectively. By monitoring the geometric changes of all here-reported host cages upon encapsulating the noble gas guest, the host has shown minor to no flexibility, testifying to the high rigidity of the dodecahedrane structure which was further reflected in very high encapsulating energies.

Noble-gas compounds: A general procedure of bonding analysis

This paper accounts for a general procedure of bonding analysis that is, expectedly, adequate to describe any type of interaction involving the noble-gas (Ng) atoms. Building on our recently proposed classification of the Ng-X bonds (X = binding partner) [New J. Chem. 44, 15536 (2020)], these contacts are first distinguished into three types, namely, A, B, or C, based on the topology of the electron energy density H(r) and on the shape of its plotted form. Bonds of type B or C are, then, further assigned as B-loose (B^l) or B-tight (B^t) and C-loose (C^l) or C-tight (C^t) depending on the sign that H(r) takes along the Ng-X bond path located from the topological analysis of ρ(r), particularly at around the bond critical point (BCP). Any bond of type A, B^l/B^t, or C^l/C^t is, finally, assayed in terms of contribution of covalency. This is accomplished by studying the maximum, minimum, and average value of H(r) over the volume enclosed by the low-density reduced density gradient (RDG) isosurface associated with the bond (typically, the RDG isosurface including the BCP) and the average ρ(r) over the same volume. The bond assignment is also corroborated by calculating the values of quantitative indices specifically defined for the various types of interactions (A, B, or C). The generality of our taken approach should encourage its wide application to the study of Ng compounds.