On the interplay between CH...O and OH...O interactions in determining crystal packing and molecular conformation : an experimental and theoretical charge density study of the Fungal Secondary Metabolite Austdiol (C12H12O5)

The total exptl. electron d. r(r), its Laplacian .del.2r(r), the mol. dipole moment, the electrostatic potential j(r), and the intermol. interaction energies have been obtained from an extensive set of single-crystal X-ray diffracted intensities, collected at T= 70(1) K, for the fungal metabolite austdiol (1). The exptl. results have been compared with theor. densities from DFT calcns. on the isolated mol. and with fully periodic calcns. The crystal structure of (1) consists of zigzag ribbons extended along one cell axis and formed by mols. connected by both OH...O and CH...O interactions, while in a perpendicular direction, adjacent mols. are linked by short CH...O intermol. contacts. An extensive, quant. study of all the intra- and intermol. H...O interactions, based not only on geometrical criteria, but also on the topol. anal. of r(r), as well as on the evaluation of the pertinent energetics, allowed us (i) to assess the mutual role of OH...O and CH...O interactions in detg. mol. conformation and crystal packing; (ii) to identify those CH...O contacts which are true hydrogen bonds (HBs); (iii) to det. the relative hydrogen bond strengths. An exptl., quant. evidence is given that CH...O HBs are very similar to the conventional OH...O HBs, albeit generally weaker. The comparison between exptl. and theor. elec. dipole moments indicates that a noticeable charge rearrangement occurs upon crystn. and shows the effects of the mutual cooperation of HBs in the crystal. The total intermol. interaction energies and the electrostatic energy contribution obtained through different theor. methods are reported and compared with the exptl. results. It is found that the new approach proposed by Spackman, based on the use of the promol. charge d. to approx. the penetration contribution to intermol. electrostatic energies, predicts the correct relative electrostatic interaction energies in most of the cases.