Ion–Molecule Reactions of Negative Ions. I. Negative Ions of Sulfur

Negative ions from carbonyl sulfide and carbon disulfide have been studied in a time‐of‐flight mass spectrometer. The principal ions formed by dissociative resonance electron capture in carbonyl sulfide are S⁻ and O⁻. Those from carbon disulfide are S⁻ and CS⁻. The onset for the formation of S⁻ by a resonance capture process has been found to occur at 1.2 eV from carbonyl sulfide, and the translational energy of the resulting ion has been found to be approximately 0.04 eV. ${\mathrm{\Delta H}}_f{\mathrm{(S}}^{-})$ is thus found to be 20 kcal/mole in agreement with previous values. At pressures of several microns in the source, secondary ions are formed in both carbonyl sulfide and carbon disulfide. In the electron capture region the principal secondary ion is S₂⁻, formed by reaction of S⁻ with the corresponding parent molecule. At higher electron energies where greater ion intensities are observed, ions of the general formula ${\mathrm{S}}_n^{-}$ are formed with $n$ varying from 2 to 6. These must result from sequential reactions of ${\mathrm{S}}_{\text{n−1}}^{-}$ with the parent molecule. An important ion in the high‐pressure spectrum of carbon disulfide is CS₂⁻, presumably formed by charge transfer. The rate constant from the reaction of S⁻ with COS is about 3.0 × 10⁻¹⁰ cc/molecule·sec.