Theoretical ab initio study of substituted benzene trimer: Interplay between hydrogen bonding and π–π interactions

A Study of π-π Stacking Interactions and Aromaticity in Polycyclic Aromatic Hydrocarbon/Nucleobase Complexes

We analysed the interactions and aromaticity electron-density delocalisation observed in π-π complexes between the phenalenyl radical and acenaphthylene, and the DNA and RNA nucleobases (adenine, guanine, cytosine, thymine and uracil). Interaction energies are obtained at the M06-2X/6-311++G(2df,p) computational level for gas phase and PCM-water conditions. For both the phenalenyl radical and acenaphthylene, the complexes formed with guanine are the most stable ones. Atoms in molecules and natural bond orbital results reveal weak π-π interactions between both interacting moieties, characterized by bond critical points between C⋅⋅⋅C and C⋅⋅⋅N atoms. Nucleus independent chemical shifts (NICS) indicate the retention of the aromatic character of the monomers in the outer region of the complex. The fluctuation indexes reveal a loss of electron delocalisation upon complexation for all cases except guanine complexes. Nevertheless, the interface region shows large negative NICS values, which is not associated with an increase of the aromaticity or electron-density delocalisation, but with magnetic couplings of both molecules, leading to an unrealistic description of the aromatic behaviour in that region.

Aromatic Amino Acids-Guanidinium Complexes through Cation-π Interactions

Continuing with our interest in the guanidinium group and the different interactions than can establish, we have carried out a theoretical study of the complexes formed by this cation and the aromatic amino acids (phenylalanine, histidine, tryptophan and tyrosine) using DFT methods and PCM-water solvation. Both hydrogen bonds and cation-π interactions have been found upon complexation. These interactions have been characterized by means of the analysis of the molecular electron density using the Atoms-in-Molecules approach as well as the orbital interactions using the Natural Bond Orbital methodology. Finally, the effect that the cation-π and hydrogen bond interactions exert on the aromaticity of the corresponding amino acids has been evaluated by calculating the theoretical NICS values, finding that the aromatic character was not heavily modified upon complexation.