The Mechanism of Hydrolysis of Aryldiazonium Ions Revisited: Marcus Theory vs. Canonical Variational Transition State Theory

Several models, theoretical levels and computational methods, all based on the canonical variational transition state approximation, have been used to predict both the experimental activation energies (E-exp) and the experimental activation free energies (G(exp)) for the hydrolysis of aryldiazonium ions. It is demonstrated that the computation of activation energies (E), instead of activation free energies (G), agrees better with the corresponding experimental data, showing that the employed computational methods do not afford reliable entropic contributions to the free energy barriers in the case of the studied reaction. However, the most fitted computations of E were not able to clearly differentiate between the mechanisms proposed for this interesting reaction (S(N)1, S(N)2 and water cluster). In contrast, the use of the Marcus theory (hyperbolic-cosine equation) instead of the canonical variational transition state theory leads to excellent agreement between the in-water-computed activation energies (E-wM) and the corresponding E-exp values for the S(N)2 mechanism, but far beyond the limit of error for the S(N)1 process. The validity of the Marcus theory for the studied S(N)1 and S(N)2 reactions is ensured by the fact that both reactions can be described as SET processes. On the other hand, apparently compelling evidence against the S(N)2 mechanism, such as C-13 KIEs and experimental observation of N-2 scrambling, are also discussed and alternative explanations are proposed.