THE EXOTHERMIC PNC → PCN REACTION

Abstract

PCN and PNC are possible interstellar species that have not been experimentally characterized. With various ab initio methods, including multireference and restricted open-shell single-reference electronic structure theory, the PCN/PNC species and the transition state for the isomerization reaction PCN ↔ PNC have been studied. The Dunning series of correlation-consistent basis sets, cc-pVXZ and aug-cc-pVXZ (X = T and Q), have been used. Geometries, total energies, dipole moments, harmonic vibrational frequencies, infrared intensities, and zero-point vibrational energies are reported for the PCN/PNC isomers and the transition state. Both PCN and PNC are linear with 3Σ- ground states, and linear [Formula: see text] is predicted to lie 13.7 kcal mol-1 (13.5 kcal mol-1 with ZPVE correction) above linear [Formula: see text] at the aug-cc-pVQZ CCSD(T) level of theory. The CN bond distance in [Formula: see text] is predicted to be 1.174 Å, only 0.002 Å longer than the experimental value of 1.172 Å for diatomic CN (X2Σ+, suggesting that CN has triple bond character in [Formula: see text]. The isomerization transition state is found to be cyclic [Formula: see text], with angles θ e ( PCN ) = 82.2°, θ e ( CNP ) = 63.1°, and θ e ( NPC ) = 34.7°. The isomerization barrier is predicted to be 35.7 kcal mol-1 (34.5 kcal mol-1 with ZPVE correction) relative to linear [Formula: see text]. The predicted dipole moments are substantial, 2.79 debye (polarity + PCN -) and 2.51 debye (polarity + PNC -).