The H-bond interaction of the adenine with HCl is investigated using the combined matrix-isolation FT-IR and theoretical density functional theory. Theoretical calculations indicate that three closed complexes of the type III (proton transfer orion pair H-bond), each containing two hydrogen bond, i.e., N-1(+)-H...Cl-...H-NH; N3-H...Cl-...H-N-9 and N-7(+)-H...Cl-...H-NH, have been obtained. The experimental spectra are consistent with this prediction. (C) 2004 Elsevier B.V. All rights reserved.