Open chain versus cyclic 14-electron triatomics: molecular structures and vibrational frequencies of P 2Si, P 2C, SiN 2 and Si 2S

Abstract

Since the studies of Walsh in the 1950s, triatomic molecules have provided a field of investigation for the fundamentals of bonding. A general consensus has been that rings are higher in energy than open chains due to the strain associated with narrow bond angles. The 14-valence-electron triatomics P 2Si, P 2C, SiN 2, and Si 2S were investigated using ab initio molecular quantum mechanical methods. Double zeta plus polarization (DZP) and triple zeta plus double polarization (TZ2P) basis sets have been used in conjunction with self-consistent field (SCF), single and double excitation configuration interaction (CISD) and coupled cluster (CCSD) methods. The all third period P 2Si and Si 2S were found to have rings as their lowest-energy geometries. The ring geometry of P 2C is high in energy because it necessitates formation of a carbene. The ring geometry of SiN 2 is only about 5 kcal/mol above the linear SiNN isomer.