O-H⋯N, C-H⋯O, and O-H⋯π hydrogen bonds between water and 2-methylpyridine

Fourier Transform Infrared (FTIR) spectra of aqueous solutions of 2-methylpyridine (2-mepy) have been analyzed to identify the 2-mepy-water H-bond complexes. As an aid theoretical frequencies of the complexes have been used. Natural Bond Orbital (NBO), Quantum Theory of Atoms In Molecules (QTAIM), and Reduced Gradient (RDG) isosurface analyses have also been performed. The classical O-H⋯N/non-classical C-H⋯O hydrogen bonds follow a single/two-step electron transfer mechanism. The non-classical interaction may have the single-step process in some complexes. The H-index cannot discriminate the classical and non-classical hydrogen bonds. The electron density ρ(r), Laplacian of electron density ∇²ρ(r) follow the second order perturbation energy E(2) values in ordering only the O-H⋯N interactions. The ρ(r)/ ∇²ρ(r) value shows distinct ranges for O-H⋯N and C-H⋯O hydrogen bonds. The range of O-H⋯π interaction, which can also have negative mutual penetration value, approximates to that of C-H⋯O H-bonds.