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 experimental electron density rho(r), its Laplacian del(2)rho(r), the molecular dipole moment, the electrostatic potential T(r), and the intermolecular 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 experimental results have been compared with theoretical densities from DFT calculations on the isolated molecule and with fully periodic calculations. The crystal structure of (1) consists of zigzag ribbons extended along one cell axis and formed by molecules connected by both (OHO)-O-... and (CHO)-O-... interactions, while in a perpendicular direction, adjacent molecules are linked by short (CHO)-O-... intermolecular contacts. An extensive, quantitative study of all the intra- and intermolecular (HO)-O-... interactions, based not only on geometrical criteria, but also on the topological analysis of rho(r), as well as on the evaluation of the pertinent energetics, allowed us (i) to assess the mutual role of (OHO)-O-... and (CHO)-O-... interactions in determining molecular conformation and crystal packing; (ii) to identify those (CHO)-O-... contacts which are true hydrogen bonds (HBs); (iii) to determine the relative hydrogen bond strengths. An experimental, quantitative evidence is given that (CHO)-O-... HBs are very similar to the conventional (OHO)-O-... HBs, albeit generally weaker. The comparison between experimental and theoretical electric dipole moments indicates that a noticeable charge rearrangement occurs upon crystallization and shows the effects of the mutual cooperation of HBs in the crystal. The total intermolecular interaction energies and the electrostatic energy contribution obtained through different theoretical methods are reported and compared with the experimental results. It is found that the new approach proposed by Spackman, based on the use of the promolecular charge density to approximate the penetration contribution to intermolecular electrostatic energies, predicts the correct relative electrostatic interaction energies in most of the cases.