Linear versus cyclic sixteen electron triatomics: P2S and SiS2

Ab initio theoretical methods have been used to determine the geometries, energies and vibrational frequencies of linear, cyclic singlet and cyclic triplet isomers of P2S and SiS2. Although it has not been observed, the cyclic singlet structure for P2S has been previously predicted theoretically to be the ground state. In the present study both the cyclic singlet and the cyclic triplet isomers of P2S are found to lie below the experimentally observed linear isomer. The linear SiS2 isomer, however, is the lowest energy isomer, and the cyclic triplet is quite high lying. The reason for this reversal and why the cyclic P2S isomers have not been observed are discussed.