Structures and properties of cytosine-BX3 (X=F, Cl) complexes: an investigation with DFT and MP2 methods

For the cytosine-BX3 (X=F, Cl) systems, geometries and binding energies were calculated using the B3LYP method of DFT and MP2 method of ab initio with 6-311+G* or 6-311++G* bases sets. Three isomers were found for each system, and then single-point energy evaluations were performed using the larger basis sets (6-311+G(2df,p) with DFT method. In the most stable isomers of cytosine-BF3 or cytosine-BCl3, boron atom of BX3 (X=F, Cl) connected to carbonyl oxygen O8 of cytosine with a stabilization energy of −78.71 and −69.62 kJ/mol at B3LYP/6-311+G* level (BSSE corrected) respectively. The analyses for the combining interaction between BX3 and cytosine with the atom-in-molecules theory (AIM) and natural bond orbital method (NBO) have been performed. The results indicate that all the isomers were formed with σ-p type interactions between cytosine and BX3, in which pyridine-type nitrogen or carbonyl oxygen 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 cytosine to BX3 were occurred. Still, there is one hydrogen bond existed in all the conformers of cytosine-BX3 system and these hydrogen bonds contribute to the stability of the complex systems. Frequency analysis suggested that the stretching vibration of BX3 underwent a red shift in complexes. Cytosine-BF3 complex was more stable than cytosine-BCl3 complex although the distance of B-Y (Y=N, O) is shorter in the later. Still, the mechanisms of conversion between different isomers of Cytosine-BX3 have been obtained.