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

Abstract

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.

Investigation of substituted-benzene dopants for charge exchange ionization of nonpolar compounds by atmospheric pressure photoionization

“…Further insight into the deprotonation of benzene photoions, at the molecular level, can be had from a recent, in-depth study of the hydration of ionized aromatics [16]. This study provides details of the mechanism through which water, with an even lower GB (660 kJ mol…”

“…. O ␦Ϫ bond (see Figure 4 in reference [16]). Note that the positive charge on C 6 H 6 ϩ• is distributed among its hydrogens, even though the missing electron is from a ring -orbital.…”

Revisiting benzene cluster cations for the chemical ionization of dimethyl sulfide and select volatile organic compounds

“…For large clusters (n ≥ 4) IR spectra of the clusters suggest features that are nearly identical to protonated water clusters, suggesting exclusion of benzene and the formation of an attached water cluster. For smaller clusters (n ≤ 3), IR spectra were shown to be consistent with a hydrated benzene cation, where benzene retains over 90 % of the charge (Ibrahim et al, 2005). It is likely that the sensitivity dependence we observe in these experiments is related to the aforementioned laboratory studies and depends on the location of the charge (benzene vs. water) and the size of the benzenewater cluster (n), both of which are related to q.…”

“…Expected contributions from carbon-13 isotopes were accounted for in the calculated proton transfer products. Ibrahim et al (2005) show that the stepwise binding energies for attaching a water molecule to C 6 H + 6 are on the order of 8.5 kcal mol −1 , suggesting that the water molecules are removed via collisions in the transfer optics. Laboratory studies have shown previously that benzene-water clusters [C 6 H 6 -(H 2 O) n ] + can be made efficiently (n = 1-23) (Miyazaki et al, 2004).…”

“…In addition, benzene ion chemistry is not exceedingly selective as compared with other ion-molecule chemistries, resulting in the potential for interferences in low mass resolution instruments, particularly in polluted air masses. Finally, while it is not expected that benzene cations will directly ionize water, it has been shown that under high specific humidity, benzene cations may have a series of attached water molecules (Ibrahim et al, 2005;Miyazaki et al, 2004) that may introduce a water dependence in the ion chemistry.…”