A Theoretical Study of the Structural Isomers of Phosphaazarine

Abstract

The recently formulated second-order state-selective quasidegenerate perturbation theory (QDPT2) is compared with quadratic configuration interaction singles and doubles (QCISD) calculations for the molecules in this study. The results of calculations using nondegenerate perturbation theory (MP2) are also reported. The molecules in this study are of current or potential experimental interest; furthermore, the molecules studied are difficult to characterize theoretically and so are useful examples on which to assess newly developed formalisms. The equilibrium geometries and vibrational frequencies of five structural isomers of phosphaazarine, HNPCH, that have the phosphorus located between the nitrogen and the carbon have been determined using a split valence one-electron basis set with polarization functions on the heavy atoms and a complete active space self-consistent field (CASSCF) description of the active space. Both QDPT2 and QCISD calculations have been performed at the equilibrium geometries using a larger basis set to accurately determine the relative energies of the various structural isomers. The calculations predict that NPCH₂ is the lowest-lying isomer, being between 2.6 and 3.7 kcal/mol lower in energy than the energetically closest isomer, NPHCH.