Bond Order and Charge Localization in Nucleoside Phosphorothioates

Abstract

In the recent literature on nucleoside phosphorothioate anions the structural formulas show a double bond between phosphorus and sulfur and a single bond between phosphorus and oxygen with a negative charge localized on oxygen. However, a review of physical data on these compounds shows the reverse to be the case; that is, in phosphorothioate anions the P-S bond is a single bond with a negative charge localized on sulfur, while the P-O bond order for exocyclic and nonbridging oxygens is greater than 1, approaching 2 in O-alkyl phosphorothioate monoanions and O,O-dialkyl phosphorothioates. The P-O bond orders in phosphorothioate dianions and trianions approach 1 ½ and 1 ⅓, respectively, owing to delocalization of negative charge among two or three oxygens. These conclusions are based on bond lengths obtained from x-ray crystallographic data and electron diffraction, the magnitudes of the effects of 18 O on the 31 P-nuclear magnetic resonance chemical shifts of phosphorus in nucleoside [ 18 O]phosphorothioates, the p H -dependence of 17 O-NMR chemical shifts in [ 17 O]phosphate and [ 17 O]thiophosphate, the vibrational spectra of thiophosphate di- and trianions, and the pK a (dissociation constant) values for phosphoric and thiophosphoric acids.