Analysis of Non-Covalent Interactions Present within a Series of Nucleobase Dimers from the Perspective of Quantum Topology and Aromaticity

“Atoms in Molecules” formalism is implemented to assess the non-covalent interactions present within a series of homo and heterodimers of the natural DNA nucleobases to critically analyze the characteristics and the energetics of the interaction lines. Furthermore, the Nucleus independent chemical shift (NICS) descriptor is employed to demarcate the role of aromaticity in dictating the specifics of the said non-covalent interactions as well as the overall spatial architectures. Although, the typical observation is that the N─H····N and the N─H····O hydrogen bonds (HB) are resonance assisted (partially covalent) and ionic respectively, deviation from this trend is also apparent. Moreover, it is found that the conventional notion of resonance assisted HBs in a dimeric structure consisting of aromatic skeletons (i.e., an increase in thearomaticity of the ring containing the HB donor moiety coupled with a concomitant decrease in the aromaticity of the ring containing the HB acceptor atom/group) is not strictly applicable to the nucleobases. The influence of steric factor in the stabilizations associated with the studied HBs is justified via Energy Decomposition Analysis. Finally, the analyses address the conundrum of whether the unnatural nucleobase 7-azaindole is really a good nucleobase mimic.