The structure of diphosphaallenic radical cations as evidenced by EPR experiments and ab initio calculations

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=C-13=PAr) in tetrahydrofuran (THF), The two P-31 constants as well as the C-13 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 45 degrees and 135 degrees. These two structures are compatible with the Jahn-Teller distortion of allene, The calculated hyperfine constants support the EPR results.