AB-INITIO STUDIES OF SILICON AND NITROGEN CLUSTERS - CYCLIC OR LINEARSI2N

Theoretical studies are carried out on the doublet and quartet states of three isomeric forms of the species Si2N. Correlation effects on th e structural parameters, harmonic frequencies, and relative energies a re investigated at increasingly higher levels of theory (MP2, MP4, and CCSD(T)) and basis sets (DZP, cc-pVTZ-f, and cc-pVTZ). At the highest level of theory (CCSD(T)/cc-pVTZ) all three isomers are found to be t hermodynamically stable species with the symmetric linear structure (S iNSi) as the global minimum; a symmetric cyclic structure (93.1 degree s) lies only 4.90 kcal/mol higher in energy, while the asymmetric line ar isomer (SiSiN) is much higher located (85.23 kcal/mol). Dissociatio n of the most stable isomer into the channels SiN + Si and Si-2 + N wo uld require 123 and 148 kcal/mol, respectively, including the zero-poi nt energies. Chemical bonding as reflected in bond distances indicates a SiN bond character intermediate between that of a single and a doub le bonds in the linear SiNSi isomer (1.644 Angstrom), changing then to a single bond in the cyclic structure (1.695 Angstrom) and a double b ond in SiSiN (1.608 Angstrom). The energetics involved in various diss ociation channels is also analyzed, as well as the strength of the vib ronic interaction in the linear isomer estimated by the computation of the Renner parameter. A comparison with the molecules C2N, Si2C, Si2C -, and Si2O clearly shows structural and stability trends among these triatomics.