Fréquence d'absorption de la liaison PN dans des composés du phosphore tricoordonne

Abstract

The stretching frequency ν PN in trivalent phosphorous compounds varies considerably (790–1010 cm −1), and appears essentially linked to the possibility of conjugative resonance of the nitrogen doublet with the vacant “ d” orbitals of the phosphorous atoms. A low frequency results from the engagement of nitrogen in an unsatured ring (PN linkage practically simple). This effect is more important than those from the substituents of phosphorus, the latter being linked, on the one hand, to the inductive effect (a highly electro-negative substitute lowers the energy phosphorus “ d” orbital and helps the conjugative resonance with the nitrogen) and on the other hand, to the possibilities of the conjugation of the substituent with phosphorus (which lowers the possibility of return of the nitrogen doublet). The PN frequency has been correlated with the barrier to rotation of that linkage determined by Nuclear Magnetic Resonance (Δ G +) and the chemical exchange rate between the methyl-trifluoroacetate and the aminophosphines. The results have been explained in terms of nitrogen—phosphorus orbital overlapping.