Topological properties of hydrogen bonds and covalent bonds from charge densities obtained by the maximum entropy method (MEM)

Abstract

Charge densities have been determined by the Maximum Entropy Method (MEM) from the high-resolution, low-temperature (T ≃ 20 K) X-ray diffraction data of six different crystals of amino acids and peptides. A comparison of dynamic deformation densities of the MEM with static and dynamic deformation densities of multipole models shows that the MEM may lead to a better description of the electron density in hydrogen bonds in cases where the multipole model has been restricted to isotropic displacement parameters and low-order multipoles (l max = 1) for the H atoms. Topological properties at bond critical points (BCPs) are found to depend systematically on the bond length, but with different functions for covalent C—C, C—N and C—O bonds, and for hydrogen bonds together with covalent C—H and N—H bonds. Similar dependencies are known for AIM properties derived from static multipole densities. The ratio of potential and kinetic energy densities |V(BCP)|/G(BCP) is successfully used for a classification of hydrogen bonds according to their distance d(H...O) between the H atom and the acceptor atom. The classification based on MEM densities coincides with the usual classification of hydrogen bonds as strong, intermediate and weak [Jeffrey (1997). An Introduction to Hydrogen Bonding. Oxford University Press]. MEM and procrystal densities lead to similar values of the densities at the BCPs of hydrogen bonds, but differences are shown to prevail, such that it is found that only the true charge density, represented by MEM densities, the multipole model or some other method can lead to the correct characterization of chemical bonding. Our results do not confirm suggestions in the literature that the promolecule density might be sufficient for a characterization of hydrogen bonds.

Conformational Preferences of 2-Acylpyrroles in Light of FT-IR and DFT Studies

“…H-bonds can be classified according to atomic electronegativity criteria derived from a report by Desiraju and Steiner [80] and according to spectroscopic, structural and thermodynamic properties of H-bonds proposed by Jeffrey [81]. Jeffrey has noticed for the first time that some intercorrelated parameters such as H-bond energies, geometries, stretching frequencies of the proton donating group are sufficient to classify the interactions as strong, moderate and weak hydrogen bonds.…”

Theoretical Study on Interactions between N-Butylpyridinium Nitrate and Thiophenic Compounds

“…For hydrogen bonds, there is a correlation between the interaction distances and topological parameters at the BCPs. 44,45 Here, the existence of such a correlation has been checked for configurations [ …”

Topological Properties of Chemical Bonds from Static and Dynamic Electron Densities

“…The topological characterization of polar bonds such as C–O bonds appeared not to be straightforward, which has also been reported earlier for static electron densities. 39 – 42 In low-density regions, as present for hydrogen bonds, static and dynamic densities show similar values for properties ρ (BCP) and 2 ρ (BCP) within the complete temperature range studied.…”

“…A simple characterization of hydrogen bonds does not categorically require the employment of multipole parameters, while it is strongly recommended for the purpose of correlating bond lengths of hydrogen bonds with their topological properties at BCPs. 42 , 43 Topological properties of hydrogen bonds can be accurately characterized by ρ INV .…”