Structural and vibrational study on the acid, hexa-hydrated and anhydrous trisodic salts of antiviral drug Foscarnet

Abstract

The structural and vibrational properties of acid species, hexa-hydrated and anhydrous trisodic salts of antiviral drug foscarnetin gas phase and in aqueous solution have been studied in this work by using the hybrid B3LYP method with the 6-31G* and6-311++G** basis sets. The properties in solution were carried out with the self consistent reaction force (SCRF) method by usingthe integral equation formalism variant polarised continuum (IEFPCM) and SD models while the complete vibrational assignmentsfor those three species were performed in both media by using the experimental available infrared spectrum of hexa-hydratedtrisodic salt and the scaled quantum mechanical force field (SQMFF) methodology. The natural bond orbital (NBO) studies suggestthat the hexa-hydrated salt in solution is most stable than the anhydrous one in the same medium but in gas phases the anhydroussalt shows a higher stability in solution. The atoms in molecules (AIM) analyses have revealed the ionic characteristics of the O---Na bonds in both salts supporting the higher stability of the hexa-hydrated salt in solution. The evaluation of the frontier orbitalsshow that the anhydrous salt is the most reactive species in solution, as supported by its higher solvation energy and volumevariation. Apparently, the presences of phosphate group in foscarnet probably increase its activity when it is used as drug. Theexperimental infrared bands observed in the hexa-hydrated species at 1059 and 983 cm-1 are clearly attributed to the stretchingmodes of phosphate group while the strong bands at 1445 and 1333 cm-1 are associated to the stretching modes of carboxylategroup. In addition, the force constants for the carboxylate and phosphate groups are reported.