AB-INITIO MOLECULAR-ORBITAL STUDY OF SENS4-NN4 (N=O-4)

We report an ab initio study of SenS4-nN4 (n = 0-4). The full geometry optimization for each molecule was performed at the Hartree-Fock leve l of theory involving the MIDI-4 basis sets for atomic orbitals. The correction for electron correlation was carried out for optimized geom etries by utilizing the second-order Moller-Plesset (MP2) perturbation theory. The fundamental vibrations calculated for all molecular speci es verified that all molecules lie at the local minima. All molecules showed cage structures similar to those observed experimentally for S4 N4 and Se4N4. The calculated bond parameters of S4N4 and Se4N4 were in good agreement with the experimental values. All chalcogen-nitrogen b ond lengths were significantly shorter than those expected for the sin gle bonds. It was also observed that in all molecules the chalcogen ch alcogen contacts were significantly shorter than the corresponding sum s of the van der Waals' radii, indicating interaction between the chal cogen atoms. The total binding energies at the MP2 level of theory ind icate that all molecular species are stable with respect to their cons tituent atoms. The present basis set explains about 75% of the experim ental binding energy in S4N4 and Se4N4. The stability of the SenS4-nN4 molecules decreases as expected with increasing selenium content. The calculated fundamental vibrations show a reasonable agreement with th e observed Raman spectra of S4N4 and Se4N4 and imply that the calculat ed wavenumbers carry some predictive power for the mixed sulfur-seleni um species. Our calculations indicate that it should be possible to sy nthesize mixed SenS4-nN4 (n = 0-4) molecules possessing a cage structu re similar to that in S4N4 and Se4N4.