Contrast between the mechanisms for dissociative electron attachment to CH₃SCN and CH₃NCS

The kinetics of thermal electron attachment to methyl thiocyanate (CH₃SCN), methyl isothiocyanate (CH₃NCS), and ethyl thiocyanate (C₂H₅SCN) were measured using flowing afterglow-Langmuir probe apparatuses at temperatures between 300 and 1000 K. CH₃SCN and C₂H₅SCN undergo inefficient dissociative attachment to yield primarily SCN^- at 300 K (k = 2 × 10^-10 cm³ s^-1), with increasing efficiency as temperature increases. The increase is well described by activation energies of 0.17 eV (CH₃SCN) and 0.14 eV (C₂H₅SCN). CN^- product is formed at <1% branching at 300 K, increasing to ∼30% branching at 1000 K. Attachment to CH₃NCS yields exclusively SCN^- ionic product but at a rate at 300 K that is below our detection threshold (k < 10^-12 cm³ s^-1). The rate coefficient increases rapidly with increasing temperature (k = 6 × 10^-11 cm³ s^-1 at 600 K), in a manner well described by an activation energy of 0.51 eV. Calculations at the B3LYP/def2-TZVPPD level suggest that attachment to CH₃SCN proceeds through a dissociative state of CH₃SCN^-, while attachment to CH₃NCS initially forms a weakly bound transient anion CH₃NCS^-* that isomerizes over an energetic barrier to yield SCN^-. Kinetic modeling of the two systems is performed in an attempt to identify a kinetic signature differentiating the two mechanisms. The kinetic modeling reproduces the CH₃NCS data only if dissociation through the transient anion is considered.