Gas Phase Hydration and Deprotonation of the Cyclic C 3 H 3 + Cation. Solvation by Acetonitrile, and Comparison with the Benzene Radical Cation Ridha Mabrouki, Yehia Ibrahim, Enli Xie, Michael Meot-Ner (Mautner), and M. Samy El-Shall* Department of Chemistry, Virginia Commonwealth UniVersity, Richmond, Virginia 23284-2006 ReceiVed: January 18, 2006; In Final Form: April 3, 2006 The binding energies of the first 5 H 2 O molecules to c-C 3 H 3 + were determined by equilibrium measurements. The measured binding energies of the hydrated clusters of 9-12 kcal/mol are typical of carbon-based CH + ‚ ‚‚X hydrogen bonds. The ion solvation with the more polar CH 3 CN molecules results in stronger bonds consistent with the increased ion-dipole interaction. Ab initio calculations show that the lowest energy isomer of the c-C 3 H 3 + (H 2 O) 4 cluster consists of a cyclic water tetramer interacting with the c-C 3 H 3 + ion, which suggests the presence of orientational restraint of the water molecules consistent with the observed large entropy loss. The c-C 3 H 3 + ion is deprotonated by 3 or more H 2 O molecules, driven energetically by the association of the solvent molecules to form strongly hydrogen bonded (H 2 O) n H + clusters. The kinetics of the associative proton transfer (APT) reaction C 3 H 3 + + nH 2 O f (H 2 O) n H + + C 3 H 2 • exhibits an unusually steep negative temperature coefficient of k ) cT -63(4 (or activation energy of -37 ( 1 kcal mol -1 ). The behavior of the C 3 H 3 + /water system is exactly analogous to the benzene +• /water system, suggesting that the mechanism, kinetics and large negative temperature coefficients may be general to multibody APT reactions. These reactions can become fast at low temperatures, allowing ionized polycyclic aromatics to initiate ice formation in cold astrochemical environments. I. Introduction The chemistry of C 3 H 3 + has received considerable attention and continues to be an active area of research. 1-5 This is due to the important roles of the ion chemistry of C 3 H 3 + in flames and combustion processes particularly for the mechanisms of soot formation, 6,7 and in interstellar clouds particularly for the origin of larger hydrocarbon and other complicated molecular species observed in interstellar medium. 8-11 The C 3 H 3 + ions are also likely to be present in the hydrocarbon-containing ionospheres of Jovian planets, 12 Titan, 13 and in interstellar clouds. 8-11 There are two low energy isomers of the C 3 H 3 + ion: the acyclic propargyl ion, H 2 CCCH + , and the cyclopropenyl ion, c-C 3 H 3 + which is more stable than the acyclic isomer by 24.9 kcal/mol. 14 The cyclic isomer c-C 3 H 3 + is, in fact, the smallest cyclic aromatic species. Correspondingly, c-C 3 H 3 + is stable and unreactive. For example, it is well-known that the reactivities of the H 2 CCCH + ion with alkenes, alkynes, aromatic hydro- carbons, and alcohols are significantly higher than those of c-C 3 H 3 + . 15,16 The interactions of aromatic cations such as c-C 3 H 3 + with water and other polar solvent molecules are important in many chemical, astrochemical, physical and biological processes including, for example, solvation shells, hydrophobic hydration, clathrate formation, and proteins conformations. 17-19 Detailed information on these interactions can be provided by gas-phase studies where, for example, the binding energies of the solvent molecules in the inner shell of the hydrocarbon ions can be measured using gas-phase clustering equilibria. 20-22 Recently, we investigated the interactions of ionized aromatics with solvent molecules, including a detailed study of the benzene +• /water system, where we measured binding energies with up to eight water molecules bound to the benzene radical cation (C 6 H 6 +• ). 20,21 In addition to building up clusters, the water molecules can also react with the core ions by extracting a proton. 20,21,23-26 Extraction of protons from ionized aromatics by solvent molecules may have important implications to reaction mech- anisms, inhibition and termination of polymerization and to astrochemical processes. 25-29 For example, we observed recently the deprotonation of benzene +• by several H 2 O molecules, where the rate coefficients of the deprotonation reaction displayed an unprecedented large negative temperature coefficient of k ) cT -67(4 (or an activation energy of -34 ( 1 kcal/mol). 20,21 The deprotonation reaction is driven energetically by the formation of protonated water clusters, (H 2 O) n H + , that contain a core hydronium ion (H 3 O + ) and exhibit strong ionic hydrogen bonds (IHBs). Proton-transfer driven by the association of several molecules through exothermic bond formation may be called associative proton transfer (APT) reactions. These APT reactions can become very efficient under low-temperature astrochemical conditions where various polar molecules, which have proton affinities higher than H 2 O, can co-condense on the hydrocarbon ions and further facilitate the deprotonation of the ions. In this paper we shall study the solvation of c-C 3 H 3 + ions and their deprotonation by water through APT reactions. In comparison with the benzene radical cation C 6 H 6 +• , the c-C 3 H 3 + ions present two interesting features. First, the size of the ion is significantly smaller than that of C 6 H 6 +• and, therefore, the charge density on c-C 3 H 3 + is significantly higher than in C 6 H 6 +• . Second, the proton affinity of the C 3 H 2 • radical (227 kcal/mol) is significantly higher than that of the phenyl radical C 6 H 5 • (212 kcal/mol). 30 These features could provide valuable insights on * Corresponding author. E-mail: selshall@hsc.vcu.edu. 7334 J. Phys. Chem. A 2006, 110, 7334-7344 10.1021/jp0603684 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/20/2006