Thiothionyl bromide (SSBr2): The elusive isomer of dibromodisulfane (BrSSBr)

CCSD(T) with a series of correlation consistent basis up to quadruple-zeta is used to investigate the structures, vibrational spectra, relative stability, heats of formation, and barrier to isomerization of SSBr2 and BrSSBr. It represents the most accurate and detailed characterization of these molecules to date. We show that the frequency mode at 302 cm−1, detected in various studies and assigned to impurities by some authors, and to the anti-symmetric SBr stretch in BrSSBr by others, thus in fact corresponds to the anti-symmetric SBr stretch in the elusive SSBr2 species; it thus corroborates and complements an earlier partial IR spectra study attributable to SSBr2. Including corrections for relativistic and core-valence correlation effects, we also predict 26.33 (12.74) kcal/mol for ΔHf (298.15 K) of SSBr2 (BrSSBr). For the SSBr2 → BrSSBr reaction, our best estimates for the Gibbs free energy and the enthalpy of the reaction at 298.15 K are −13.71 and −13.44 kcal/mol, respectively. For a value of ΔG# equal to 23.52 kcal/mol, we estimate a TST rate constant, at 298.15 K, of 3.57 × 10−5 s−1.