PHOSPHORUS–CARBON DIAD TAUTOMERISM IN PHOSPHONIUM COMPOUNDS. THEORETICAL STUDY

Phosphorus-carbon diad tautomerism was studied in phosphonium compounds with such strong electron acceptor substituents as cyano-, nitro-, fluorosul phonyl-, and trifluoromethyl- sulphonyl groups using semiempirical, ab initio, and DFT methods. It was shown that in the gas phase all studied monosubstituted species are in the phosphoryl form and no enol forms should be detectable. In contrast, for doubly substituted compounds enol forms should be well detectable and in some cases even predominant. Comparison of different calculation methods indicated that ab initio (HF/6-31 + G*) and DFT (B3LYP/6-311 + G**) calculations give close results. Both applied semiempirical methods (PM3 and MNDO/d) seem to overestimate strongly the stability of the enol form. It was also found that the MNDO/d method gives unrealistic results for compounds containing both hypervalent phosphorus and sulphur.