Charge-density analysis and electrostatic properties of 2-carboxy-4-methylanilinium chloride monohydrate obtained using a multipolar and a spherical-charges model

Abstract

The new crystal structure of 2-carboxy-4-methylanilinium chloride monohydrate was determined by X-ray diffraction and refined using three different electrondensity models. In the first model, the ELMAM2 multipolar electron-density database was transferred to the molecule. Theoretical structure factors were also computed from periodic density functional theory calculations and yielded, after multipolar-atoms refinement, the second charge-density model. An alternative electron-density modelling, based on spherical atoms and additional charges on the covalent bonds and electron lone-pair sites, was used in the third model in the refinement versus the theoretical data. The crystallographic refinements, structural properties, electron-density distributions and molecular electrostatic potentials obtained from the different charge-density models were compared. As the number of variables refined in the different models is the same, the R factor is a good indicator of refinement quality. The R factor is best for multipolar modelling, presumably because of the greater flexibility and larger number of parameters to model the electron density compared to the sphericalcharges model. The electrostatic potentials around the molecule show a high correlation coefficient between the three models. research papers 454 Noureddine Dadda et al. Charge-density analysis using different models

A critical analysis of dipole-moment calculations as obtained from experimental and theoretical structure factors

“…compared to MM theo , as shown also by the residual maps (Fig. 6), a result in agreement with Dadda et al (2012). Restriction of hyd was found to be crucial in limiting the influence of peripheral H atoms in the dipole-moment magnitude (models 2j-2l in Table S3).…”

“…The residual density was fitted in a [0., 1] Å interval around the C-C bonding electron-density peak of the amino acid in the plane perpendicular to the C-C bond to avoid effects of the neighbouring C atoms. The Slater function coefficients and vir (r) curve are shown in Dadda et al (2012).…”

“…Virtual-atom model (VIR theo ). The virtual-atom model (Dadda et al, 2012) introduces a simplification in the multipolar model and was designed by us to be routinely used in macromolecular modelling. This approach describes the electron density as a superposition of real and virtual spherical atoms:…”

“…In the VIR exp model (Dadda et al, 2012), the calculations were performed based on the geometry obtained after high/low-order refinement of experimental data. The H atoms were moved to the neutron positions and refined with these H-X distances constrained in the final cycles, i.e.…”

The generalized invariom database (GID)

“…ELMAM has seen applications on a number of small-molecule (e.g. Domagała et al, 2011;Dadda et al, 2012) and protein structures (e.g. Housset et al, 2000;Jelsch et al, 2000).…”