Computational study of pyrylium cation-water complexes: hydrogen bonds, resonance effects, and aromaticity

Hydrogen bonds formed between the pyrylium cation and water were characterized by means of geometric, energetic and electronic parameters through calculations done with the B3LYP/6-31+G(d,p) method. The wavefunctions were analyzed by the Natural Bond Orbitals (NBO), Natural Steric Analysis (NSA), Natural Resonance Theory (NRT), and Atoms in Molecules (AIM) methods. The energy decomposition method proposed by Xantheas was employed. The vibrational frequencies and the intensity of the C-H stretching bands were studied. The Nucleus Independent Chemical Shifts (NICS) and Harmonic Oscillator Model of Aromacity (HOMA) methods were used to verify the aromaticity of the pyrylium cation and the studied complexes. Complexation results in small alterations in the equilibrium geometry of the monomers. Energetic analysis allowed us to verify the stability order of the studied complexes and the intensity of the hydrogen bonds taking place between the monomers. Small alterations in the electronic structure of the monomers occur, indicating that the interaction between pyrylium and water is weak and little contributes to increasing the cation resonance.