Structures and properties of cytosine–BX 3 (X=F, Cl) complexes: an investigation with DFT and MP2 methods

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For the cytosine–BX 3 (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–BF 3 or cytosine–BCl 3, boron atom of BX 3 (X=F, Cl) connected to carbonyl oxygen O 8 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 BX 3 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 BX 3, 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 BX 3 were occurred. Still, there is one hydrogen bond existed in all the conformers of cytosine–BX 3 system and these hydrogen bonds contribute to the stability of the complex systems. Frequency analysis suggested that the stretching vibration of BX 3 underwent a red shift in complexes. Cytosine–BF 3 complex was more stable than cytosine–BCl 3 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–BX 3 have been obtained.