Water-trapping of unstable carbocations taking place into the inverted region of the Marcus equation. First experimental and computational evidence

Abstract

The reaction rates of the water-trapping of unstable 1-norbornyl cations in relation to the rates of their rearrangement (ring contraction) have been experimentally determined. The activation barrier of the rearrangement reactions was calculated with the QST2 method, whereas the Marcus theory was applied to predict the activation barrier of the trapping reaction. All computations in gas phase were performed with the DFT B3LYP/6-31G(d) method, while the PCM model was used for the computation of unspecific solvent effects. The established methodology was able to predict the major product formed in the solvolysis of the corresponding triflate precursors of the studied carbocations. The agreement between experiment and theory supports, for the first time, the interesting conclusion that the very exergonic trapping-reaction of unstable 1-norbornyl cations takes place with relatively high barriers of activation and, hence, into the Marcus inverted region. Additionally, a noteworthy relationship between the free energy of activation for the carbocation rearrangement in gas phase and the length of the rearranging bond in the ground state is reported.