Structures and stabilities of endo- and exohedral dodecahedrane complexes (XΓ₂₀H₂₀ and XC₂₀H₂₀, X = H⁺, H, N, P, C^-, Si^-, O⁺, S⁺)

B3LYP/6-31G* computations predict the relative energies and stabilities of the endohedral (XΓ₂₀H₂₀) and exohedral (XC₂₀H₂₀) dodecahedrane complexes (X = H⁺, H, N, P, C^-, Si^-, O⁺, S⁺). H⁺ does not bind endohedrally but bridges a C-C bond exohedrally; the proton affinity is 185.3 kcal/mol. Except for O⁺, all other guest species (H, N, P, C^-, Si^-, S⁺) are minima at the cage center. The H-atom inclusion energy is similar to that of helium (36.3 vs 38.0 kcal/mol), whereas the other endohedral complexes have much higher inclusion energies (125-305 kcal/mol). In all cases, the endohedral complexes are energetically less favorable than their exohedral isomers. C₂₀H₂₁ has a cage-ruptured structure, whereas N, P, and their isoelectronic analogues have exohedral structures and bind as doublet states to broken cage C-C bonds. Endohedral H, N, C^-, O⁺, and S⁺ preserve their unencapsulated ground states, whereas P and Si^- interact strongly with the cage and lose their atomic ground-state character.