Quantum crystallographic charge density of urea

TL;DR

This paper presents a quantum crystallographic charge density model of urea that improves upon traditional methods by capturing shared charge and providing more accurate atomic displacement parameters, validated against high-resolution experimental data.

Contribution

It introduces a quantum theory-based charge density model refined with ultra high-resolution X-ray data, offering a new approach that surpasses traditional spherical atom models.

Findings

Quantum model matches data as well as multipole models with fewer parameters.

The quantum model reveals new information about charge distribution not captured by multipole models.

Hydrogen displacement parameters in the quantum model align with neutron data.

Abstract

Standard X-ray crystallography methods use free-atom models to calculate mean unit cell charge densities. Real molecules, however, have shared charge that is not captured accurately using free-atom models. To address this limitation, a charge density model of crystalline urea was calculated using high-level quantum theory and was refined against publicly available ultra high-resolution experimental Bragg data, including the effects of atomic displacement parameters. The resulting quantum crystallographic model was compared to models obtained using spherical atom or multipole methods. Despite using only the same number of free parameters as the spherical atom model, the agreement of the quantum model with the data is comparable to the multipole model. The static, theoretical crystalline charge density of the quantum model is distinct from the multipole model, indicating the quantum model provides substantially new information. Hydrogen thermal ellipsoids in the quantum model were very similar to those obtained using neutron crystallography, indicating that quantum crystallography can increase the accuracy of the X-ray crystallographic atomic displacement parameters. The results demonstrate the feasibility and benefits of integrating fully periodic quantum charge density calculations into ultra high-resolution X-ray crystallographic model building and refinement.