Substituent effects on the P NMR chemical shifts of arylphosphorothionates

Six tris(aryloxy)phosphorothionates substituted in the para position of the aromatic rings were synthesized and studied by ³¹P NMR, X-ray diffraction techniques and ab initio calculations at a RHF/6-31G** level of theory, in order to find the main structural factors associated with the δ³¹P in these compounds. As the electron-withdrawing (EW) ability of the substituents was increased, an ‘abnormal’ shielding effect on δ³¹P of the arylphosphorothionates was observed. The analyses of the geometrical properties obtained through both experimental and theoretical methods showed that a propeller-type conformation is preferred for the arylphosphorothionates, except in the case of the tris(O-4-methylphenyl)phosphorothionate, since one of the aromatic rings is not rotated in the same direction as the other two in the solid state. The main features associated with the δ³¹P NMR of compounds 1-6 were a decrease of the averaged O-P-O angle and mainly the shortening of the PS bond length, which is consistent with an increase of the thiophosphoryl bond order as δ³¹P values go upfield. On the other hand, comparison of the experimental and calculated bond lengths and bond angles involving α bonded atoms to phosphorus of the six compounds suggested that stereoelectronic interactions of the type n_πO-σ*_PS, n_πO-σ*_P-OAr and n_πS-σ*_P-OAr could be present in the arylphosphorothionates 1-6.