STRUCTURES AND ENERGETICS OF NEW NITROGEN AND SILICON MOLECULES - AN AB-INITIO STUDY OF SI2N2

New species with molecular formula Si2N2, not yet observed experimenta lly, are described theoretically for the first time. Nine different st ationary points have been examined and the effects of electronic corre lation on the structural parameters, harmonic frequencies, and relativ e energies are described at increasingly higher levels of correlation treatment (MP2, MP4, CCSD(T)). The global minimum corresponds to a lin ear singlet state ((1) Sigma(g)(+)) SiNNSi. At the CCSD(T) level, the next most stable species (at 15.08 kcal/mol) has a nonclassical tetrah edral-like structure similar to the global minimum of Si2H2. This is f ollowed by another local minimum with a linear structure SiNSiN (at 20 .25 kcal/mol) and by a rhomboidal-type structure (at 21.33 kcal/mol), which is in fact a transition state connecting two equivalent tetrahed ral-like structures. Another nonclassical structure similar to the mon obridged one in the case of Si2H2 was also found to be a local minimum (at 28.18 kcal/mol). An interconversion path from this latter structu re to the linear SiNNSi one is likely to occur via another transition state located at about 38 kcal/mol. With the exception of the linear i somer SiNNSi, the triplet states were found to lie very high energetic ally and to correspond to unstable structures. None of these species e xhibits any appreciable amount of silicon-silicon bonding, and the ana logue of cyanogen (NCCN) NSiSiN is unstable. The nature of the bonding in the most relevant species is also discussed, as well the energetic s of dissociation. An analysis of the energetics and structural simila rities and differences between Si2N2, C2N2, Si2H2, and Si2C2 is also c arried out. Caution must be exercized in generalizing results at a low level of theory since they have not been confirmed by the CCSD(T) cal culations.