Radicals containing both a phosphoranylidene and a ketone group: an electron paramagnetic resonance/electron nuclear double resonance study of their structure and ab initio investigations on the captodative effects

Abstract

Single crystals of 2-(triphenylphosphoranylidene)succinic anhydride have been X-irradiated and studied by electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy after determination of their crystal structure. The trapped radical results from the departure of a hydrogen atom bound to a carbon located α to both a CP and a CO bond. The resulting hyperfine coupling tensors are compared with those obtained from ab initio calculations on H₃PCH—ĊH—CHO (1) and are interpreted in terms of spin delocalization on the succinic anhydride ring. The radical stabilization energies have been calculated for H₃PCH—ĊH—CHO and H₂P—CH₂—ĊH—CHO; they agree with the presence of a captodative effect. Owing to the charge separation caused by the contribution of the mesomeric phosphonium–enolate structure, this effect is more pronounced for the phosphoranylidene compound.