Open chain versus cyclic 14-electron triatomics: Molecular structures and vibrational frequencies of P2Si, P2C, SiN2 and Si2S

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 P2Si, P2C, SiN2, and Si2S 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 P2Si and Si2S were found to have rings as their lowest-energy geometries. The ring geometry of P2C is high in energy because it necessitates formation of a carbene. The ring geometry of SiN2 is only about 5 kcal/mol above the linear Si-N=N isomer.