Influence of intermolecular interactions on multipole-refined electron densities

Abstract

This work examines the effect of intermolecular interactions on molecular properties derived from simulated X-ray diffraction data. Model X-ray data are computed from a superposition of ab initio molecular electron densities in the crystal, as well as from periodic crystal Hartree-Fock electron densities, for the hydrogen-bonded systems ice VIII, formamide and urea, as well as the weakly bound acetylene. The effects of intermolecular interactions on the electron density are illustrated at both infinite and finite data resolution, and it is concluded that multipole models are capable of quantitative retrieval of the interaction density, despite the known shortcomings of the radial functions in the model. Multipole refinement reveals considerable enhancement of the molecular dipole moment for hydrogen-bonded crystals, and negligible change in molecular second moments. Electric field gradients at H nuclei are significantly reduced in magnitude upon hydrogen bonding, and this change is also faithfully represented by the rigid pseudoatom model.

Electron Densities and Related Properties from the ab-initio Simulation of Crystalline Solids

“…Figure 4 reports total and interaction ED maps obtained from HF CRYSTAL calculations with a very good (TZPP) GTO set. The picture is very similar to that provided by Spackman et al [69]: with respect to the superposition of molecular densities the most notable feature is a build-up of charge in front of the terminal oxygen, due to the population of the hydrogen bonds with the neighboring molecule and to the corresponding de-population of the N-H intramolecular bonds. [10], but allows both inter-and intra-molecular changes of the ED to be recorded.…”

“…Gatti has recently reviewed the power of this theory using precisely urea as a test case [10] and extending the analysis performed in a pioneering ab-initio HF study [68]: in particular, he demonstrated the ability of topological analysis to describe quantitatively environmental effects on the ED, both as concerns the intermolecular and, indirectly, the intramolecular region (for instance, the appreciable change of the dipole moment of the molecule in the crystal with respect to its gas-phase value). The important discussion about the detectability of environmental effects from diffraction data by Spackman et al [69] is also worth mentioning.…”

Effective molecular polarizabilities and crystal refractive indices estimated from x-ray diffraction data

“…3 closely resembles that of the interaction density for urea ͑the difference between crystal Hartree-Fock calculation and superposition of isolated molecules͒ in Fig. 2 of Spackman et al 63 ͑see also Fig. 7 of Docherty et al 64 ͒, and hence many of the observed effects of wave function fitting for urea can be attributed to hydrogen bonding.…”

An analysis of the experimental and theoretical charge density distributions of the piroxicam–saccharin co-crystal and its constituents

“…The temperature factors of hydrogen atoms were anisotropically modelled based upon discussions by Hoser et al, 29 Spackman et al [38][39][40] and Koritsansky et al 41 These studies have observed dissimilarities in the topological analysis of weak interactions such as H-bonds , van der Waals forces and π-π stacking interactions. 30 To observe the effect of applying calculated anisotropic temperature factors for hydrogen atoms during multipole refinement, anisotropic temperature factors were calculated 31 9 Our findings show that the use of anisotropic temperature factors…”