Contrast between electron attachment to CH$_{\mathrm{3}}$SCN and CH$_{\mathrm{3}}$NCS.*

We have made measurements of electron attachment to CH$_{\mathrm{3}}$SCN (methyl thiocyanate) and CH$_{\mathrm{3}}$NCS (methyl isothiocyanate) over the temperature range 300-1000 K in a flowing afterglow Langmuir probe apparatus. Both attachment processes yield mainly the SCN$^{\mathrm{-}}$ pseudohalide anion product. Both molecules are inefficient at attaching electrons, with the rate coefficient k$_{\mathrm{a}}$ for CH$_{\mathrm{3}}$SCN near 2 x 10$^{\mathrm{-10}}$ cm$^{\mathrm{3}}$ s$^{\mathrm{-1}}$ at 300 K and that for CH$_{\mathrm{3}}$NCS orders of magnitude lower and unmeasurable. Both rate coefficients increase strongly with temperature. The k$_{\mathrm{a}}$ for CH$_{\mathrm{3}}$SCN is 100 times larger at 1000 K, while k$_{\mathrm{a}}$ for CH$_{\mathrm{3}}$NCS reaches 4 x 10$^{\mathrm{-9}}$ cm$^{\mathrm{3}}$ s$^{\mathrm{-1}}$ at 1000 K. Calculations of potential energy surfaces imply that the electron attachment mechanisms are completely different. Attachment to CH$_{\mathrm{3}}$SCN requires vibrational excitation, but then dissociation of the parent anion readily follows. Formation of the transient CH$_{\mathrm{3}}$NCS$^{\mathrm{-}}$ anion appears to be more facile, but the dissociative surface has a rate-limiting barrier. The formation of a CN$^{\mathrm{-}}$ anion product is calculated to be 0.5 eV endothermic from CH$_{\mathrm{3}}$SCN or 0.7 eV from CH$_{\mathrm{3}}$NCS at 0 K. CN$^{\mathrm{-}}$ appears faintly in the product mass spectra for CH$_{\mathrm{3}}$NCS, which could be due to impurities, but appears more strongly for CH$_{\mathrm{3}}$SCN as temperature increases, which is less easily explained. We also have results for C$_{\mathrm{2}}$H$_{\mathrm{5}}$SCN, which attaches electrons similarly to CH$_{\mathrm{3}}$SCN at 300 K, but is 3 times less efficient at 1000 K.