The isotropic hyperfine coupling constants of the diphosphaalkene radical cation have been measured by EPR spectroscopy after electrochemical oxidation of ArP]] C]] PAr (and ArP]] 13C]] PAr) in tetrahydrofuran (THF). The two 31P constants as well as the 13C coupling are close to 90 MHz. Taking HPCPH as a model compound, the structure has been assessed, by extensive ab initio calculations including correlation effects at the MP2 and MCSCF levels of theory. It is found that oxidation of the allenic ]P]] C]] P] structure leads to the formation of two rotamers with HPPH dihedral angles of 458 and 1358. These two structures are compatible with the Jahn–Teller distortion of allene. The calculated hyperfine constants support the EPR results

Liquid phase EPR spectra of a diphosphaallenic radical anion have been recorded after electrochemical reduction of a solution of ArP᎐ ᎐ C᎐ ᎐ PAr in THF at 293 K (Ar = 2,4,6-Bu t 3 C 6 H 2 ). The hyperfine coupling interactions of two 31 P and one 13 C nuclei (in the case of Ar᎐ ᎐ 13 C᎐ ᎐ PAr) are discussed in the light of AM1 calculations carried out on (ArP᎐ ᎐ C᎐ ᎐ PAr) Ϫ , of ab initio calculations performed on the model radical anion (HP᎐ ᎐ C᎐ ᎐ PH) Ϫ at the MP2 and MCSCF levels of theory and of DFT calculations on (HP᎐ ᎐ C᎐ ᎐ PH) Ϫ .The structure of the radical anion is compared with that of the neutral molecule. CalculationsAb initio calculations were performed with GAUSSIAN94 5 and HONDO8.5. 6 Semi-empirical calculations were carried out with the AM1 method as implemented in the Gaussian package. This package was also used for DFT calculations. Results and discussion EPR spectraElectrochemical and chemical reductions of 1 lead to EPR spectra exhibiting the same hyperfine structure. Since the spectrum obtained after electrolysis is more intense than that recorded after reaction with potassium, we decided to produce (ArP᎐ ᎐

Cyclic voltammetry shows that monophosphaallene ArPCC(C6H5)2 (where Ar = C6H2 tBu3-2,4,6), 1a, undergoes irreversible reduction at 2266 mV in THF. The EPR spectra of the reduction products are obtained in liquid and frozen solutions after specific 13C enrichment of the allenic carbon atoms. The resulting hyperfine tensors are compared with those obtained from ab initio MP2, MCSCF, CI, and DFT calculations for the radical anion (HPCCH2)-• and for the monophosphaallylic radical (HP • −CHCH2) ↔ (HPCH− • CH2). The most elaborate treatments of the hyperfine structure (CI and DFT) indicate that the species observed by EPR is the monophosphaallylic radical.

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Cyclic voltammetry of phosphafulvene and dibenzophosphafulvene shows that in DMF these compounds are reduced at -1.200 and -1.349 V, respectively. The EPR spectra of the corresponding radical anions, formed by electrochemical reduction or by reaction on a potassium mirror, are Recorded between 110 K and room temperature. The g and 31P hyperfine tensors are measured and compared to those previously obtained for a phosphaalkene radical anion. Abinitio investigations on model phosphaalkene and phosphafulvene radical anions show that, in accord with the experimental results, the electronic structure of these two species are quite different: whereas the unpaired electron is delocalized on the whole PC(H)R moiety in the phosphaalkenic anion, it is markedly localized on the phosphorus atom in the phosphafulvene anion. Calculated spin densities and charge distributions for phosphafulvene and azafulvene anions are compared