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

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 a to both a C-P and a C-0 bond. The resulting hyperfine coupling tensors are compared with those obtained from ab initio calculations on H3P-CH-CH-CHO (1) and are interpreted in terms of spin delocalization on the succinic anhydride ring. The radical stabilization energies have been calculated for H3P=CH-CH-CHO and H2P-CH2-CH-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.