| Abstract
| - Studies based on ab initio optimized geometries (at B3LYP/6-311+G** and MP2/6-311+G** levels)and on experimental structures retrieved from the Cambridge Structural Database (CSD) revealthat the nucleobases constituting DNA and RNA differ significantly in their aromatic character,as shown by the geometry-based index of aromaticity HOMA that ranges from 0.466 for thymineto 0.917 for adenine, based on B3LYP/6-311+G** calculations, and 0.495−0.926, respectively, ifbased on the MP2/6-311+G** level. Aromaticity of the bases decreases markedly with an increaseof the number of double-bond CX (X = N, O) substituents at the rings. H-bonds involving COgroups in Watson−Crick pairs cause an increase of the aromatic character of the rings.
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