Experimental and Theoretical Charge Density Distribution in a Host Guest System: Synthetic Terephthaloyl Receptor Complexed to Adipic Acid
ABSTRACT The experimental charge density distributions in a host-guest complex have been determined. The host, 1,4-bis[[(6-methylpyrid-2-yl)amino]carbonyl]benzene (1) and guest, adipic acid (2). The molecular geometries of 1 and 2 are controlled by the presence in the complex of intermolecular hydrogen bonding interactions and the presence in the host 1 of intramolecular hydrogen bonding motifs. This system therefore serves as an excellent model for studying noncovalent interactions and their effects on structure and electron density, and the transferability of electron distribution properties between closely related molecules. For the complex, high resolution X-ray diffraction data created the basis for a charge density refinement using a pseudoatomic multipolar expansion (Hansen-Coppens formalism) against extensive low-temperature (T = 100 K) single-crystal X-ray diffraction data and compared with a selection of theoretical DFT calculations on the same complex. The molecules crystallize in the noncentrosymmetric space group P2(1)2(1)2(1) with two independent molecules in the asymmetric unit. A topological analysis of the resulting density distribution using the atoms in molecules methodology is presented along with multipole populations, showing that the host and guest structures are relatively unaltered by the geometry changes on complexation. Three separate refinement protocols were adopted to determine the effects of the inclusion of calculated hydrogen atom anisotropic displacement parameters on hydrogen bond strengths. For the isotropic model, the total hydrogen bond energy differs from the DFT calculated value by ca. 70 kJ mol(-1), whereas the inclusion of higher multipole expansion levels on anisotropic hydrogen atoms this difference is reduced to ca. 20 kJ mol(-l), highlighting the usefulness of this protocol when describing H-bond energetics.
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ABSTRACT: The first systematic electronic resolution study of a series of urea-based anion receptor complexes is presented. The hydrogen bonding in these multi-component systems was fully characterised using Bader's Quantum Theory of Atoms In Molecules (QTAIM) with the strength of the various N-Hanion hydrogen bonds quantified and the individual contributions of different intermolecular forces to the overall receptor: anion interaction derived by comparison of the charge densities in the related complexes. The strength of the N-Hanion hydrogen bonds was correlated to the basicity of the anion and related to the structure of the receptors. The geometric criteria used to identify hydrogen bonding interactions in standard resolution X-ray diffraction studies were shown to be valid for stronger interactions. However, these geometric criteria are less reliable and lead to assumptions that are not necessarily upheld when applied to weaker intermolecular interactions. The presence of these could only be confirmed by charge density studies. The effect that changes to the receptor substitution pattern have on the entire supramolecular system is illustrated by the differences in the electrostatic potential distributions and atomic charges across the series. The application of systematic high resolution studies to rationalise a variety of host-guest systems has been demonstrated.Physical Chemistry Chemical Physics 04/2014; 16(22). DOI:10.1039/c3cp54858a · 4.20 Impact Factor