An ab initio study of many isomers of S 2O 2. A combined theoretical and experimental analysis of the harmonic force field and molecular structure of cis-planar OSSO

Abstract

Thirteen singlet and six triplet isomers of S 2O 2 have been studied by ab initio methods; a DZ(P) basis was used for most calculations, but a larger TZ(2)P basis was used for a few final results. All geometries were fully optimized. The nine most stable singlet isomers were all found to be true local minima at the SCF level. Vibrational frequencies, IR intensities and MP3 energies (at the SCF geometries) are reported. Although a triplet form of S 2O 2 is the most stable at the SCF level, a singlet isomer is the global minimum when MP3 energies are included; this isomer is a thio species related to SO 3, and at the TZ(2)P/CCD level it is some 70 kJ mol −1 more stable than the cis-planar OSSO isomer which has been identified by microwave spectroscopy in an SO 2 discharge by Lovas et al. cis- and trans-planar isomers are found to be almost degenerate. Our best estimates for the equilibrium structure of the lowest energy isomer are r (SS) = 1.881 (10) Å, r(SO) = 1.431 (5) Å and < SSO = 120.4 (0.5)°, and for the ground-state rotational constants are A 0 = 10327(160), B 0 = 4355(50) and C 0 = 3063(25) MHz; its binding energy compared to 2 SO is about 100 kJ mol −1 at the TZ(2)P/CCD level. The strikingly long formal SS double bond in cis-planar OSSO cannot be understood using the orbital (SCF) approximation, but its length is satisfactorily predicted by correlated methods which show the importance of the HOMO→LUMO double excitation to be substantial. For the global minimum structure, however, the SCF description is a reasonably good approximation. Factors determining the importance of correlation in the description of SS linkages are discussed. The potential function linking cis- and trans-planar OSSO isomers has been studied by SCF, MP3 and CISD methods; two electronic configurations were found to cross near a dihedral angle of 90°, so all these single-references methods failed to give physically realistic results. The DZ(P)/SCF harmonic force fields for cis- and trans-planar OSSO isomers have been scaled using factors derived from parallel calculations on S 2O. Several of the scaled force constants for cis-planar OSSO have been refined, using all the experimental data available on centrifugal distortion constants and vibrational frequencies. The SS force constant is only 2.2(4) aJ Å −2, in good agreement with the DZ(P)/MP3 value of 2.18 aJ Å −2, and the torsional barrier to internal rotation is about 100 kJ mol −1. r z structural parameters for the cis-planar isomer are r(SS) = 2.023(7) Å, r(SO) = 1.467(3) Å and < SSO = 112.8(3)°.