Rate constants and product distributions as functions of temperature for the reaction of OH −(H 2O) 0,1,2 with CH 3CN

Abstract

A selected ion flow tube was used to measure the rate constants and product distributions for the reactions of OH − (H 2O) n with CH 3CN 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 − + CH 3CN in the context of the general reaction OH − + CH 3X, 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, CH 2CN has a large electron affinity and CH 2CN − is delocalized. Thus (1) CH 3CN shows a low heat of deprotonation, making proton transfer exothermic for OH − + CH 3CN, but (2) less than 100% efficient since the product ion is delocalized, and (3) endothermic for OH − (H 2O) 3 + CH 3CN because CH 2CN − has a low hydration energy. Solvent switching with CH 3CN and the thermal dissociation of the solvated product ions were observed as secondary reactions in the OH − (H 2O) + CH 3CN system. This study suggests that thermal dissociation can complicate the interpretation of product distributions in flow-tube and comparable experiments.