QTAIM study of strong H-bonds with the O-H•••A fragment (A = O, N) in three-dimensional periodical crystals

The relationship between the d(H center dot center dot center dot A) distance (A = O, N) and the topological properties at the H center dot center dot center dot A bond critical point of 37 strong (short) hydrogen bonds occurring in 26 molecular crystals are analyzed using the quantum theory of atoms in molecules (QTAIM). Ground-state wave functions of the three-dimensional periodical structures representing the accurate experimental geometries calculated at the B3LYP/6-31G** level of approximation were used to obtain the QTAIM electron density characteristics. The use of an electron-correlated method allowed us to reach the quantitatively correct values of electron density rho(b) at the H center dot center dot center dot A bond critical point. However, quite significant differences can appear for small absolute values of the Laplacian (< 0.5 au). The difference between the H center dot center dot center dot O and H center dot center dot center dot N interactions is described using the rho(b) versus d(H center dot center dot center dot A) dependence. It is demonstrated that the values of parameters in this dependence are defined by the nature of the heavy atom forming the H center dot center dot center dot A bond. An intermediate (or transit) region separating the shared and closed-shell interactions is observed for the H-bonded crystals in which the bridging proton can move from one heavy atom to another. The crystalline environment changes the location of the bridging proton in strong H-bonded systems; however, the d(O-H)/d(H center dot center dot center dot O) ratio is approximately the same for both the gas-phase complexes and molecular crystals with a linear or near-linear O-H center dot center dot center dot O bond.