ELECTRONIC-STRUCTURE AND CONFORMATIONAL PROPERTIES OF (CARBOXY-ALKENYL)-PHOSPHONIC ACIDS

The general conformational properties and electronic structure of (car boxy-alkenyl)-phosphonic derivatives were determined at RHF/STO-3G() level. In all the series, low rotation barriers were found for the two C=C/P=O conformers. In the compounds in which the interactions betwee n the carboxylic and phosphonic moieties are smaller, the most stable conformers are the C=C/P=O s-cis ones. In most of the conformers, the C=C/C=O system presents the disposition s-cis. The Z-(2-carboxy-vinyl) and Z(2-carboxy-propenyl) phosphonic acids present intramolecular hyd rogen bonds, existing in at least four conformer with internal hydroge n bonds. These last compounds were more rigorously studied at RHF/3-21 G() and RHF/6-31G(*) levels. The most stable conformer shows a trans structure for the C=C/P=O angle, with an intramolecular hydrogen bond located between the hydroxylic hydrogen of phosphonic group and the ca rbonyl oxygen of carboxylic moiety. A secondary conformer is found wit h a double intramolecular hydrogen bond between two hydroxylic hydroge ns of the phosphonic moiety and the oxygen of carboxylic bond. Another secondary conformer appears with an intramolecular hydrogen bond betw een the oxygen of the phosphoryl bond and the hydroxylic hydrogen of t he carboxylic group. A study of the topology of charge densities is ca rried out. This analysis reveals bonds with an ionic participation. A very weak pi conjugation, variable with the conformers, is found in th e C=C/P=O system, as well as a strongly polarized P=O partial triple b ond. The intramolecular hydrogen bonds give rise to cyclic structures. (C) 1997 Elsevier Science B.V.