Predicting ion rearrangement reactions — The energetics of the internal SN2 reaction in gas-phase proton-bound molecular pairs

Abstract

A common rearrangement reaction for gas-phase proton-bound molecular pairs corresponds to an internal SN2 reaction that results in the loss of a small neutral molecule. For pairs (RCN)(ROH)H+, the energies of the two transition states (TSaand TSb) and the intermediate complex (IC) in the isomerization reaction (relative to the proton-bound pair, in kJ mol–1) can be estimated using the following relationships: E(TSa) = 87 – 9(n) – 0.33(ΔPA), E(IC) = 83 – 9(n) – 0.33(ΔPA), and E(TSb) = 107 – 9(n) – 0.10(ΔPA), where 87, 83, and 107 kJ mol–1are the values for (CH3CN)(CH3OH)H+. Here, n is the number of stablizing alkyl groups on the central SN2 carbon and ΔPA is the difference between the proton affinity of the migrating moiety and that for the base system (in this case, CH3CN). For the analogous pairs (ROH)(R′OH)H+, only the first value in each expression is different (98, 94, and 121 kJ mol–1, respectively, calculated for (CH3OH)2H+).Key words: proton-bound molecular pairs, isomerization, internal SN2 reaction, energetics, metastable ions.