Phosphorus-sulfur bond order in phosphorothioate anions

The question of negative charge localization between phosphorus-bonded oxygen and sulfur in aqueous solutions of phosphorothioate anions has long attracted attention in organophosphorus chemistry. The /sup 18/O- and /sup 36/S-induced isotopic perturbations of /sup 31/P NMR signals support a structure with double-bonded O and single-bonded S. The /sup 18/O-induced perturbation of the /sup 31/P NMR chemical shift of 0.041 ppm corresponds to a P-O bond order of nearly 2, on the basis of the perturbation of 0.021 ppm for single-bonded O. On this basis it would be expected that the P-S bond order should be close to unity. This is supported by the observed perturbation of /sup 31/P NMR signals by isotopic sulfur. Measurements of the previously unresolved /sup 36 -34/S isotope shift of 0.003 ppm enable the authors to estimate the expected isotopic shift for the /sup 36 -32/S with sulfur singly bonded to phosphorus. The expected shift is about 0.006 ppm, or twice the value for /sup 36 -34/S. The value for doubly bonded sulfur is 0.018 ppm in agreement with an earlier result. Therefore, the upfield shift of 0.0065 ppm observed for the anion indicates that P-S bond order is near 1, and the negative charge ismore » largely localized on the sulfur atom.« less