Contrast between the mechanisms for dissociative electron attachment to CH 3 SCN and CH 3 NCS.

The kinetics of thermal electron attachment to methyl thiocyanate (CH3 SCN), methyl isothiocyanate (CH3 NCS), and ethyl thiocyanate (C2 H5 SCN) were measured using flowing afterglow-Langmuir probe apparatuses at temperatures between 300 and 1000 K. CH3 SCN and C2 H5 SCN 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 (CH3 SCN) and 0.14 eV (C2 H5 SCN). CN- product is formed at <1% branching at 300 K, increasing to ∼30% branching at 1000 K. Attachment to CH3 NCS 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 CH3 SCN proceeds through a dissociative state of CH3 SCN- , while attachment to CH3 NCS initially forms a weakly bound transient anion CH3 NCS-* 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 CH3 NCS data only if dissociation through the transient anion is considered.