On the transferability of QTAIMC descriptors derived from X-ray diffraction data and DFT calculations: substituted hydropyrimidine derivatives.

The combined study of electron-density features in three substituted hydropyrimidines of the Biginelli compound family has been fulfilled. Results of the low-temperature X-ray diffraction measurements and density functional theory (DFT) B3LYP/6-311++G** calculations of these compounds are described. The experimentally derived atomic and bonding characteristics determined within the quantum-topological theory of atoms in molecules and crystals (QTAIMC) were demonstrated to be fully transferable within chemically similar structures such as the Biginelli compounds. However, for certain covalent bonds they differ significantly from the theoretical results because of insufficient flexibility of the atom-centered multipole electron density model. It was concluded that currently analysis of the theoretical electron density provides a more reliable basis for the determination of the transferability of QTAIMC descriptors for molecular structures. Empirical corrections making the experimentally derived QTAIMC bond descriptors more transferable are proposed.

Molecular and crystal properties of ethyl 4,6-dimethyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate from experimental and theoretical electron densities.

The electron density and electronic energy densities in ethyl 4,6-dimethyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate have been studied from accurate X-ray diffraction measurements at 110 K and theoretical single-molecule and periodic crystal calculations. The Quantum Theory of Atoms in Molecules and Crystals (QTAMC) was applied to analyze the electron-density and electronic energy-density features to estimate their reproducibility in molecules and crystals. It was found that the local electron-density values at the bond critical points derived by different methods are in reasonable agreement, while the Laplacian of the electron density computed from wavefunctions, and electron densities derived from experimental or theoretical structure factors in terms of the Hansen-Coppens multipole model differ significantly. This disagreement results from insufficient flexibility of the multipole model to describe the longitudinal electron-density curvature in the case of shared atomic interactions. This deficiency runs through all the existing QTAMC bonding descriptors which contain the Laplacian term. The integrated atomic characteristics, however, suffer noticeably less from the aforementioned shortcoming. We conclude that the electron-density and electronic energy QTAMC characteristics derived from wavefunctions, especially the integrated quantities, are nowadays the most suitable candidates for analysis of the transferability of atoms and atomic groups in similar compounds.