A THEORETICAL-STUDY OF THE LOWEST-ENERGY SINGLET AND TRIPLET ELECTRONIC STATES OF P-IMINOPHOSPHAALKYNE AND NITRILIMINE

The equilibrium geometries and fundamental vibrational frequencies of the two energetically lowest-lying electronic states of p-iminophospha alkyne (HCPNH) and nitrilimine (HCNNH) were determined using a split-v alence basis set with polarization functions on the heavy atoms. The m ost extensively correlated functions used in the geometry optimization s were of the Complete Active Space Self-Consistent Field (CASSCF) var iety and included eight electrons distributed among seven active orbit als for HCPNH and six active orbitals for HCNNH. The effects on predic ted 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 fo und that nitrilimine has the expected singlet ground state, with the t riplet lying 38.5 kcal mol(-1) higher in energy; however, p-iminophosp haalkyne has a triplet ground state, lying 9.7 kcal mol(-1) below the singlet.