Stepwise hydration of ionized aromatics. Energies, structures of the hydrated benzene cation, and the mechanism of deprotonation reactions

The stepwise binding energies (DeltaHdegree(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, DeltaHdegree(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 extremely 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 > or = 4. Associative intracluster proton transfer Bz(.+)(H2O)(n+1) + H2O-->C6H5.(H2O)nH+ would also be exothermic for n > or = 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.