QTAIMC study of the X–H/H⋯O bond order indices (X = O, N, C) in molecular systems

The O–H/H⋯O bond order indices are evaluated at the B3LYP level using the Quantum Theory of Atoms in Molecules and Crystals (QTAIMC). The parameters of bond critical points, such as electron density, principal electron-density curvatures, kinetic and potential energy densities, are calculated for 32 molecular systems comprising of 40 covalent O–H bonds and 69 hydrogen H⋯O bonds of different strength. Different models for the bond order index are tested using the Cioslowski–Mixon approach as a standard. A uniform description of the bond order indices for the covalent and H-bonds in the O–H⋯O fragment, ranging from 0.97 to 2.91 Å, is achieved using the electron density and electronic kinetic energy density features. The model describing the H-bond indices of the X–H⋯O fragment (X = O, N and C) in terms of electron density and potential energy density is also suggested.