Towards quantitative analysis of intermolecular interactions with Hirshfeld surfaces

Enhancements to the properties based on Hirshfeld surfaces enable quantitative comparisons between contributions to crystal packing from various types of intermolecular contacts.

Reassessment of large dipole moment enhancements in crystals: a detailed experimental and theoretical charge density analysis of 2-methyl-4-nitroaniline

The molecular dipole moment of MNA in the crystal has been critically reexamined, to test the conclusion from an earlier experimental charge density analysis that it was substantially enhanced due to a combination of strong intermolecular interactions and crystal field effects. X-ray and neutron diffraction data have been carefully measured at 100 K and supplemented with ab initio crystal Hartree-Fock calculations. Considerable care taken in the measurement and reduction of the experimental data excluded most systematic errors, and sources of error and their effects on the experimental electron density have been carefully investigated. The electron density derived from a fit to theoretical structure factors assisted in the determination of the scale and thermal motion model. The dipole moment enhancement for MNA in the crystal is much less than that reported previously and only on the order of 30-40% (approximately 2.5 D). In addition to the dipole moment, experimental deformation electron density maps, bond critical point data, electric field gradients at hydrogen nuclei, and atomic and group charges all agree well with theoretical results and trends. Anisotropic modeling of the motion of hydrogen atoms, integral use of periodic ab initio calculations, and improved data quality are all aspects of this study that represent a considerable advance over previous work.

Controlling the Confinement and Alignment of Fullerene C70 inpara-Substituted Calix[5]arenes

In toluene fullerene C(70) forms 2:1 complexes with p-benzylcalix[5]arene (1) and p-phenylcalix[5]arene (2), [C(70) subset1(2)].6(C(7)H(8)) and [C(70) subset2(2)].7(C(7)H(8)). The fullerene molecules are completely shrouded by two calix[5]arenes in addition to terminal benzyl groups from other supermolecules, [C(70) subset1(2)], and solvent. Within the capsule-like supermolecules the calixarenes have distinctly different arrangements relative to the principal axis of the fullerene; for [C(70) subset1(2)].6(C(7)H(8)) the oxygen planes of the two calixarenes are skewed by 37.0 and 47.5 degrees , whereas in [C(70) subset2(2)].7(C(7)H(8)) the principal axes of the fullerene and the two encapsulating calixarenes are more closely aligned with the corresponding angles at 9.7 and 8.6 degrees , and features a pentaphenyl inter-calixarene embrace. The Hirshfeld surfaces of these two complexes have been investigated for a detailed understanding of the orientation and nature of interactions of C(70) with the cavitand-type molecules and toluene.

Dipole and quadrupole moments of molecules in crystals: A novel approach based on integration over Hirshfeld surfaces

Elegant expressions are derived for the computation of dipole and quadrupole moments of molecules using the electrostatic potential and electric field evaluated on an oriented molecular surface. These expressions are implemented for Hirshfeld surfaces, applied to various molecular crystals, and compared with the results from the quantum theory of atoms in molecules. The effect of intermolecular interactions is also explored by examining the differences between electrostatic moments derived from a periodic Hartree-Fock electron density and an electron density resulting from a superposition of noninteracting molecules. The enhancement of the dipole moment for hydrogen bonded molecular crystals is typically 30%-40% and shown to be largely independent of the partitioning scheme. Dipole moments calculated from Hirshfeld surfaces systematically underestimate those from zero-flux surfaces, a result attributed to the translation of the Hirshfeld surface relative to the zero-flux surfaces for these molecules. For acetylene and benzene, the differences between a crystal calculation and the sum of noninteracting molecules are small, and both partitioning schemes yield quadrupole and second moment results in close agreement.

Physicochemical aspects of host-guest compounds

The macro properties of crystalline inclusion compounds depend on their structures. Their thermal stabilities are a function of the strength and the directionality of the various nonbonded interactions occurring in the host-guest assembly. Their lattice energies, as measured by the method of atom-atom potentials, correlate with the thermodynamics of the guest-release reactions and the selectivity that a given host displays for a particular guest. The kinetics of solid-host:vapor-guest reactions and of guest exchange are important in our understanding of catalytic processes. Crystal engineering, in which materials of predetermined properties may be synthesized, is still at the empirical stage.