Following a recent experimental report on the synthesis and thermal stability of imidazopyridopyrimidine-containing structures with the ability to form four hydrogen bonds, the geometries and relative stabilities of the pairs of model bases have been studied using the HF, MP2 and B3LYP approaches. The H-1 NMR chemical shifts of the base pairs have been estimated by the SOS-DFPTIGLO method and employed in the analysis of the experimental proton spectra. (C) 2004 Elsevier B.V. All rights reserved.