The hydrogen bond interaction in [AH 3-H 3O] +. radical cations for A = C, Si, Ge, Sn and Pb is studied at the CCSD(T)/6-311G++(3df,2pd)//MP2/6-31G++(d,p) level of theory. Two unusual hydrogen bond structures are found to be stable: the single-electron (SEHB) and the proton-hydride (PHHB) ones. The latter structures have been found to easily evolve to a [AH 2H 2O] +.-H 2 complex, from which H 2 can be eliminated. © 2004 Elsevier B.V. All rights reserved.