Stepwise Hydration of Ionized Aromatics. Energies and Structures of the Hydrated Benzene Cation, and the Mechanism of Deprotonation Reactions

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Journal of the American Chemical Society

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American Chemical Society




The stepwise binding energies (ΔH°n-1,n) of 1−8 water molecules to benzene•+ [Bz•+(H2O)n] were determined by equilibrium measurements using an ion mobility cell. The stepwise hydration energies, ΔH°n-1,n, are nearly constant at 8.5 ± 1 kcal mol-1 from n = 1−6. Calculations show that in the n = 1−4 clusters, the benzene•+ ion retains over 90% of the charge, and it is externally solvated, that is, hydrogen bonded to an (H2O)n cluster. The binding energies and entropies are larger in the n = 7 and 8 clusters, suggesting cyclic or cage-like water structures. The concentration of the n = 3 cluster is always small, suggesting that deprotonation depletes this ion, consistent with the thermochemistry since associative deprotonation Bz•+(H2O)n-1 + H2O → C6H5• + (H2O)nH+ is thermoneutral or exothermic for n ≥ 4. Associative intracluster proton transfer Bz•+(H2O)n-1 + H2O → C6H5•(H2O)nH+ would be also exothermic for n ≥ 4, but lack of H/D exchange with D2O shows that the proton remains on C6H6•+ in the observed Bz•+(H2O)n clusters. This suggests a barrier to intracluster proton transfer, and as a result, the [Bz•+(H2O)n]* activated complexes either undergo dissociative proton transfer, resulting in deprotonation and generation of (H2O)nH+, or become stabilized. The rate constant for the deprotonation reaction shows a uniquely large negative temperature coefficient of k = cT-67±4 (or activation energy of −34 ± 1 kcal mol-1), caused by a multibody mechanism in which five or more components need to be assembled for the reaction.


Originally published in the Journal of the American Chemical Society by the American Chemical Society . Publisher’s PDF and HTML fulltext available through remote link. DOI: 10.1021/ja050477g

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