THE RING AND SUPEROXIDE ISOMERS OF SO2

The superoxide isomer of sulfur dioxide (Fig. 1) was first proposed by Myerson, Taylor, and Hanst in 1957 [J. Chem. Phys. 26, 1309 (1957)] as a possible intermediate in the combustion of CS2, COS, and H 2S as well as a possible source of some troublesome ultraviolet absorptions in the spectra associated with those combustion processes. Subsequent experimental work on SO2 has also referred to the hypothesized asymmetric structure. Single reference post Hartree-Fock methods, including configuration interaction with single and double excitations (CISD), coupled cluster singles and doubles (CCSD), and coupled cluster singles and doubles with perturbative triples [CCSD(T)], as well as multireference configuration interaction (MRCISD) performed with CI natural orbitals (CINOs) have been employed in the interest of characterizing the relative energies of the open, ring and superoxide isomers of SO2. The largest basis used was a triple-ζ plus double polarization function set with f-type functions appended to each atom [TZ2P(f)]. The ring and superoxide isomers are predicted to lie approximately 111 and 104 kcal mol-1, respectively, above the open isomer ground state. Based upon these energy separations, it is predicted that neither the ring nor superoxide isomers are responsible for the troublesome UV absorption continuum, as postulated by Myerson et al. Moreover, neither the ring nor the superoxide structure is the source of the spectroscopic features very recently observed below 100 kcal mol-1 by Dai's group. © 1995 American Institute of Physics.