Role of Electrostatic Interactions in Determining the Crystal Structures of Polar Organic Molecules. A Distributed Multipole Study

Abstract

The effect of using a realistic model for the electrostatic forces on the calculated structures of molecular crystals is explored by using atomic multipoles derived from an SCF 6-31G** wave function. This was tested on a wide ranging database of 40 rigid organic molecules containing C, H, N, and O atoms, including nucleic acid bases, nonlinear optic materials, azabenzenes, nitrobenzenes, and simpler molecules. The distributed multipole electrostatic model, plus an empirical repulsion-dispersion potential, was able to successfully reproduce the lattice vectors and available heats of sublimation of the experimental room temperature structure in almost all cases. Scaling the electrostatic energy to allow for the effect of electron correlation on the molecular charge density generally improved the lattice energies and the calculated structures to a lesser extent. However, omitting the anisotropic multipole moments usually gave very poor, sometimes qualitatively wrong structures, emphasizing the sensitivity of these crystal structures to the electrostatic forces. We also investigated the sensitivity of the structures to the empirical repulsion-dispersion potential parameters by attempting to optimize these. Since the experimental structures are mainly reproduced to within the errors that could be attributed to the use of static minimization and rigid molecules, it appears that going beyond the atomic charge model to a realistic electrostatic model is a key development in the modeling of the crystal structures of polar and hydrogen-bonded molecules.

Tunable recognition of the steroid α-face by adjacent π-electron density

“…For 13 we only used the low-energy conformation that was 9 kJ mol −1 less stable than the alternative configuration of the hydroxyl proton. The intermolecular forces were modeled with CHELPG (41) atomic charges fitted to the B3LYP/6-31G(d,p) electrostatic potential and an empirical exp-6 repulsion-dispersion model with C, N, O, H(-C) and H(-O) parameters obtained from Coombes et al (42). The search for solid-state complexes of pro and 22 included both 1∶1 (results in the SI Appendix) and 2∶1 stoichiometries.…”

Highly Unusual Triangular Crystals of Theophylline: The Influence of Solvent on the Growth Rates of Polar Crystal Faces

“…Coombes et al) 52 and an atomic multipole electrostatic model, with multipoles derived from a distributed multipole analysis 53 of the calculated molecular charge density. Full details of the CSP methodology have been given elsewhere.…”

Determination of the Crystal Structure of a New Polymorph of Theophylline

“…Cc, C2, Pc, P2/c, C2221, Fdd2, Pccn, P41, I41/a, P41212, P31, R-3, P3121 and P61), all with Z`=1. The resulting crystal structures were then re-optimised using the program DMACRYS [39] with intermolecular interactions described by an empirically parameterised exp-6 repulsion-dispersion potential (the FIT potential described by Coombes et al) [33] and an atomic multipole electrostatic model, with multipoles derived from a distributed multipole analysis [40] of the calculated molecular charge density. Calculated lattice energies were found to be particularly sensitive to the orientation of the two methyl groups, so initial crystal structures were generated with two different orientations of the methyl hydrogen atoms; these two sets of structures were then merged and all structures within 10 kJ mol -1 of the lowest energy structure were further optimised, using the CrystalOptimizer method, [41] which combines a quantum mechanical treatment of the intramolecular energy with the atom-atom model of intermolecular interactions.…”

“…Wide tolerances were required because of the typical structural discrepancies between observed crystal structures and the nearest local minimum on the calculated lattice energy surface. [33,34] These errors result in part from limitations in computational models used to represent inter-and intra-molecular interactions in the crystal; all predicted crystal structures considered in the present study were energy minimised using interatomic potentials. Also contributing to the slight differences between predicted and observed structures is the comparison between a temperature-less lattice energy minimum and a measured structure at real temperature.…”