A Density Functional Theory Study of Structure of Phosphonic Acid

The molecular structure of the stable conformation of phosphonic acid in gas phase has beencomputed by employing complete geometry optimization in Density Functional Theory(DFT) methods. The methods used for calculations are B3LYP, BP86 and B3PW91 that havebeen studied in two series of basis sets: D95** and 6-31+G(d,p) for hydrogen and oxygenatoms; LANL2DZ for phosphorus. Bond lengths and angles generally agree with the limitedprior theoretical and experimental values. Phosphonic acid exhibits a minimum-energyconformation in which the acidic hydrogens are oriented toward the phosphoryl oxygen andthe distance from it, nearly 2.7-2.8 A. The conformation of phosphonic acid may bedescribed on the basis dipole-dipole interactions plus relatively small internal hydrogenbonding and steric effect. In contrast to earlier studies, little conformational evidence of"anomeric" effects is found in this compound.