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=O bond. The resulting hyperfine coupling tensors are compared with those obtained from ab initio calculations on H,P=CH-CH-CHO(1) and are interpreted in terms of spin delocalization on the succinic anhydr.ide ring. The radical stabilization energies have been calculated for H,P=CH-CH-CHO and H,P-CH,-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.
As shown from the crystal structure, the oxygen atom of Ph 3 P1CH0C(O)CH 3 forms both intra and intermolecular hydrogen bonds. X-irradiation of this compounds produces a room-temperature-stable radical which was studied by single crystal EPR/ENDOR spectroscopy. Comparison of the experimental hyper®ne couplings with those obtained from ab initio calculations shows that the radical cation Ph 3 P + 0CH1C(OH)CH 2 is formed under radiolysis. The principal directions of the hyper®ne tensors indicate that, in this process, some of the hydrogen bonds are broken and that the radical undergoes a drastic reorientation around the Ph 3 P0C bond.
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