AN AB-INITIO STUDY OF THE P-C BOND ROTATION IN PHOSPHORUS(V)-STABILIZED CARBANIONS - THE PHOSPHORYL VERSUS THIOPHOSPHORYL GROUP

The potential energy surface for the P-C bond rotation in the P-methylthioxophosphonic diamide anion (8(-)) has been computed at MP4(SDQ)/6-31+G*//6-31+G*. An extensive basis set evaluation up to the MP4(SDTQ)/6-311+G**//MP2/6-31+G** level for four rotamers of 8(-) led to the selection of the above basis set for optimal performance. Results were compared to the previously studied and recomputed phosphoryl analog(7(-)). The experimentally determined higher P-C bond rotational barrier for the thio species is found by our calculations as well, and its roots can be traced by structural comparisons, an isodesmic equation, and NBO analysis. The isodesmic equation for the stabilization of a carbanion by the phosphoryl groups yields energies (P=O, -40, P=S, -47 kcal/mol) in the vicinity of those for the strongest carbanion-stabilizing heteroatomic groups (pi accepters like BH2 or AlH2). The NBO method indicates a stronger back-bonding from the oxygen lone pairs into the sigma* (P-N) orbitals than from those of sulfur. This effect destabilizes the ground state (GS) geometry in which the lone pair on carbon interacts with the same sigma* orbitals. Inclusion of molecules having substituents on phosphorus with a higher (fluorine) and a lower (hydrogen) electronegativity than nitrogen reveals a more general picture. With the electronegative substituents on phosphorus, the favorable carbon lone pair stabilization in the GS of the thio derivatives combines with similar stabilizing interactions for both chalcogen analogs in the TS's resulting in higher rotational barriers for the sulfur species. With electropositive substituents, the opposite effects are observed.