Equilibrium geometry of the HCCP triplet ground state: phospho-carbene, phospho-allene or phosphorene? A combined density functional and ab initio study

The equilibrium geometry of the ground-state HCCP radical was determined by ab initio SCF, CASSCF, MP2, CI and density functional B3LYP calculations using different basis sets. The HCCP radical is predicted to be linear at all levels of calculation considered, in agreement with the experimental conclusion. The calculated molecular orbitals show that the HCCP ground state is of phosphorene form that has been modified by a phospho-allene, which is to some extent different from the experimental result. The calculated rotational constant is close to the experimental value, a larger discrepancy exists in the harmonic vibrational frequencies predicted at the different computational levels.