Abstract
A selected ion flow tube was used to measure the rate constants and product distributions for the reactions of OH− (H2O)n with CH3CN over the temperature range 240–363 K for the case n = 1 and at 298 K for n = 0 and 2. Proton transfer was the only primary reaction channel observed; this process was found to be fast (efficiency ≅ 70%) for n = 0 and 1 but much slower (efficiency ≅ 4%) for n = 2. Interpreting OH− + CH3CN in the context of the general reaction OH− + CH3X, two features are important. First, CN has an abnormally large electron affinity. This gives CN− a large methyl cation affinity and nucleophilic displacement a large barrier: it is not observed, even though exothermic. Second, CH2CN has a large electron affinity and CH2CN− is delocalized. Thus (1) CH3CN shows a low heat of deprotonation, making proton transfer exothermic for OH− + CH3CN, but (2) less than 100% efficient since the product ion is delocalized, and (3) endothermic for OH− (H2O)3 + CH3CN because CH2CN− has a low hydration energy. Solvent switching with CH3CN and the thermal dissociation of the solvated product ions were observed as secondary reactions in the OH− (H2O) + CH3CN system. This study suggests that thermal dissociation can complicate the interpretation of product distributions in flow-tube and comparable experiments.