Sulfur clusters: structure, infrared, and Raman spectra of cyclo-S₆ and comparison with the hypothetical cyclo-O₆ molecule

Abstract

Ab initio quantum mechanical methods have been applied to the S₆ and O₆ molecules at their respective D_3d hexagonal chair equilibrium geometries. Double zeta plus polarization (DZ + P) and triple zeta plus double polarization (TZ + 2P) basis sets have been used in conjunction with the self-consistent field (SCF) method and second-order perturbation theory. Equilibrium geometries, harmonic vibrational frequencies, infrared intensities, and Raman intensities have been predicted for the two cyclic molecules. Two previous vibrational difficulties between theory and experiment for S₆ have been resolved. The O₆ molecule appears to be similar to the well-characterized S₆ in several respects. However, its dissociation energy and vibrational frequencies reveal a much flatter potential energy surface for O₆ in the region of the equilibrium geometry. While still predicted to correspond to a genuine potential minimum, O₆ nevertheless lies about 100 kcal mol^-1 above three separated O₂ molecules. From a methodological viewpoint, the single configuration Hartree-Fock approach is found to be qualitatively satisfactory for S₆ but very poor for O₆.