A theoretical study of the lowest-energy singlet and triplet electronic states p-iminophosphaalkyne and nitrilimine

The equilibrium geometries and fundamental vibrational frequencies of the two energetically lowest-lying electronic states of p-iminophosphaalkyne (HCPNH) and nitrilimine (HCNNH) were determined using a split-valence basis set with polarization functions on the heavy atoms. The most extensively correlated functions used in the geometry optimizations were of the Complete Active Space Self-Consistent Field (CASSCF) variety and included eight electrons distributed among seven active orbitals for HCPNH and six active orbitals for HCNNH. The effects on predicted geometry of the size of the CASSCF active space were investigated for HCPNH and are reported. Multi-Reference Configuration Interaction with Single and Double excitations calculations have been performed at the equilibrium geometries using larger basis sets to determine more accurately the relative energetics of the electronic states. It was found that nitrilimine has the expected singlet ground state, with the triplet lying 38.5 kcal mol⁻¹ higher in energy; however, p-iminophosphaalkyne has a triplet ground state, lying 9.7 kcal mol⁻¹ below the singlet.