Density functional study of cucurbituril and its sulfur analogue

The molecular and electronic structures of cucurbituril (CB) and its sulfur analogue, thia-cucurbituril (TCB), have been investigated by means of density functional theory (DFT) calculations. The HOMO–LUMO (H–L) energy gap of CB is ∼7 eV and it decreases at ∼4 eV upon replacement of its twelve oxygen atoms with sulphur atoms. The vertical ionization potential (IP) of CB and TCB are rather large, being ∼7 and 6 eV, respectively. The vertical electron affinity (EA) of CB calculated in the gas-phase is negative (about −2 eV), thus suggesting that its anion is thermodynamically unstable. On the other hand, formation of the TCB anion might be favoured in electron-donating solvents since its calculated vertical EA is close to zero. An analysis of the Kohn–Sham frontier orbitals of CB and TCB indicates that neither the HOMO nor the LUMO is degenerate.