Chemical bonding and structure-reactivity correlation in Meldrum's acid: a combined experimental and theoretical electron density study.

Chemical bonding in Meldrum's acid (MA) based on the experimental electron density obtained from high-resolution X-ray diffraction data at 20 K and from solid state theoretical calculations at the experimental molecular geometry have been analyzed by using the Quantum Theory of Atoms in Molecules. The total electron density was modeled with use of the Hansen-Coppens multipole formalism, and features associated with both intra- and intermolecular bond critical points, topological bond orders, atomic charges, and the electrostatic potential have been characterized and used to understand structure-reactivity relationships. The acidic methylene hydrogen atoms carry modest positive charges. A notable feature is the presence of an intramolecular H...H interaction that imparts stability to the boat conformation. A correlation between the topological bond order and the nature of the chemical bonds in MA illustrates the fact that elimination of carbon dioxide/acetone is an important feature of the chemistry of MA. The surface electrostatic potential and the charge distribution rationalize the sites of nucleophilic/electrophilic attack.