Abstract: Geometries and binding energies are predicted at B3LYP/6-311+G* level for the adenine–BX3 (X=F,Cl) systems and four conformers with no imaginary frequencies have been obtained for both adenine–BF3 and adenine–BCl3, respectively, and single energy calculations using much larger basis sets (6-311+G(2df,p)) and aug-cc-pVDZ were carried out as well. The most stable conformer is BF3 or BCl3 connected to N3 of adenine and with the stabilization energy of 22.55 or 20.59kcal/mol at B3LYP/6-311+G* level (BSSE corrected). The analyses for the combining interaction between BX3 and adenine with natural bond orbital method (NBO) and the atom-in-molecules theory (AIM) have been performed. The results indicate that all the conformers were formed with σ–p type interactions between adenine and BX3, in which pyridine-type nitrogen or nitrogen atom of amino group offers its lone pair electron to the empty p orbital of boron atom and the concomitances of charge transference from adenine to BX3 were occurred. Frequency analysis suggested that the stretching vibration of BX3 underwent a red shift in complexes. Adenine–BF3 complex was more stable than adenine–BCl3 although the distance of B–N is shorter in the later. [Copyright &y& Elsevier]