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 similar to7 eV and it decreases at similar to4 eV upon replacement of its twelve oxygen atoms with sulphur atoms. The vertical ionization potential (IP) of CB and TCB are rather large, being similar to7 and 6 eV, respectively. The vertical electron affinity (EA) of CB calculated in the gas-phase is negative (about similar to2 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. (C) 2004 Elsevier B.V. All rights reserved.