A DATABASE AND MOLECULAR ORBITAL STUDY OF INTRAMOLECULAR STRONG HYDROGEN BONDS IN METAL-DIGLYOXIME SYSTEMS. Kevin W. McConnell and Ronald F. See, Department of Chemistry, Saint Louis University, St. Louis, MO 63103

The nature of strong hydrogen bonds has long been of interest, both as a test to theories of chemical bonding and, more recently, as a key step in the hydrogen transfer mechanism of certain enzymatic catalysis. For O^..H^..0 systems, strong hydrogen bonds may be defined as those with a distance of less than 2.8 Å between the oxygen atoms. Metal-diglyoxime complexes provide an excellent opportunity to compare strong hydrogen bonds, as changes in both steric and electronic effects are quantifiable. A search of the Cambridge Structure Database has resulted in approximately 150 known metal-diglyoxime structures, each containing two strong intramolecular hydrogen bonds, ranging from 2.40 to 2.80 Å between oxygen atoms. Results of this search suggest that electron-withdrawing substituents on the glyoxime backbone favor stronger intramolecular hydrogen bonds; a possible explanation for this trend will also be presented. In order to test theoretical predictions against experimental results, molecular orbital calculations (ZINDO) were performed on nickel diglyoxime model compounds containing the substituents -CH₃, -NH₂, -OH and -F. The results of these calculations are not totally consistent with the empirical trends identified through the database search. These results will be analyzed in terms of Brown's bond-valence theory, as well as Gilli's resonance-assisted hydrogen bonding concepts.