In this computational work, we propose to use the NMR chemical shift difference of NH₂ protons for 1:1 complexes formed by aniline and nitrogen-containing proton acceptors for the estimation of the hydrogen bond energy and geometry (N⋯H and N⋯N distances). The proposed correlations could be applied to other aromatic amines as well, in a gas phase, a solution, or a solid state, for both inter- and intramolecular hydrogen bonds. We considered a set of 21 complexes with the NHN hydrogen bond without proton transfer, including hydrogen bonds from weak to medium strong ones (2–21 kcal/mol), with neutral or anionic bases and with sp³ and sp² hybridized nitrogen proton acceptors. For each complex apart from direct hydrogen bond energy calculation, we have tested several other ways to estimate the energy: (a) using a correlation between NH stretching band intensity and hydrogen bond energy and (b) using correlations between electron density properties at (3, −1) bond critical point (quantum theory of atoms in molecules analysis) and hydrogen bond energy. Besides for the studied type of complexes, we obtained refined linear correlations linking the local electron kinetic (G) and potential (V) energy densities with the hydrogen bond energy.

In this study, we present results of a detailed topological analysis of electron density (ED) of 145 halogen-bonded complexes formed by various fluorine-, chlorine-, bromine-, and iodine-containing compounds with trimethylphosphine oxide, Me₃PO. To characterize the halogen bond (XB) strength, we used the complexation enthalpy, the interatomic distance between oxygen and halogen, as well as the typical set of electron density properties at the bond critical points calculated at B3LYP/jorge-ATZP level of theory. We show for the first time that it is possible to predict the XB strength based on the distance between the minima of ED and molecular electrostatic potential (ESP) along the XB path. The gap between ED and ESP minima exponentially depends on local electronic kinetic energy density at the bond critical point and tends to be a common limiting value for the strongest halogen bond.