Correlations of NHN hydrogen bond energy with geometry and 1H NMR chemical shift difference of NH protons for aniline complexes.

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.