Ab initio molecular orbital study of Se nS 4− nN 4 ( n = 0−4)

Abstract

We report an ab initio study of Se n S 4− n N 4 ( n = 0−4). The full geometry optimization for each molecule was performed at the Hartree-Fock level of theory involving the MIDI-4 ∗ basis sets for atomic orbitals. The correction for electron correlation was carried out for optimized geometries by utilizing the second-order Møller-Plesset (MP2) perturbation theory. The fundamental vibrations calculated for all molecular species verified that all molecules lie at the local minima. All molecules showed cage structures similar to those observed experimentally for S 4N 4 and Se 4N 4. The calculated bond parameters of S 4N 4 and Se 4N 4 were in good agreement with the experimental values. All chalcogen-nitrogen bond lengths were significantly shorter than those expected for the single bonds. It was also observed that in all molecules the chalcogen … chalcogen contacts were significantly shorter than the corresponding sums of the van der Waals' radii, indicating interaction between the chalcogen atoms. The total binding energies at the MP2 level of theory indicate that all molecular species are stable with respect to their constituent atoms. The present basis set explains about 75% of the experimental binding energy in S 4N 4 and Se 4N 4. The stability of the Se n S 4− n N 4 molecules decreases as expected with increasing selenium content. The calculated fundamental vibrations show a reasonable agreement with the observed Raman spectra of S 4N 4 and Se 4N 4 and imply that the calculated wavenumbers carry some predictive power for the mixed sulfur-selenium species. Our calculations indicate that it should be possible to synthesize mixed Se n S 4− n N 4 ( n = 0−4) molecules possessing a cage structure similar to that in S 4N 4 and Se 4N 4.