Stepwise Hydration of Ionized Aromatics. Energies and Structures of the Hydrated Benzene Cation, and the Mechanism of Deprotonation Reactions
Journal of the American Chemical Society
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.
Stepwise Hydration of Ionized Aromatics. Energies and Structures of the Hydrated Benzene Cation, and the Mechanism of Deprotonation Reactions Y. Ibrahim, M. Meot-Ner, E. H. Alshraeh, M. S. El-Shall, S. Scheiner J. Am. Chem. Soc. 2005 127 (19), 7053-7064