Mechanistic study of proton transfer and HD exchange in ice films at low temperatures (100-140 K)

We have examined the elementary molecular processes responsible for proton transfer and HD exchange in thin ice films for the temperature range of 100-140 K. The ice films are made to have a structure of a bottom D(2)O layer and an upper H(2)O layer, with excess protons generated from HCl ionization trapped at the D(2)OH(2)O interface. The transport behavior of excess protons from the interfacial layer to the ice film surface and the progress of the HD exchange reaction in water molecules are examined with the techniques of low energy sputtering and Cs(+) reactive ion scattering. Three major processes are identified: the proton hopping relay, the hop-and-turn process, and molecular diffusion. The proton hopping relay can occur even at low temperatures (<120 K), and it transports a specific portion of embedded protons to the surface. The hop-and-turn mechanism, which involves the coupling of proton hopping and molecule reorientation, increases the proton transfer rate and causes the HD exchange of water molecules. The hop-and-turn mechanism is activated at temperatures above 125 K in the surface region. Diffusional mixing of H(2)O and D(2)O molecules additionally contributes to the HD exchange reaction at temperatures above 130 K. The hop-and-turn and molecular diffusion processes are activated at higher temperatures in the deeper region of ice films. The relative speeds of these processes are in the following order: hopping relay>hop and turn>molecule diffusion.     

Segregation of hydroxide ions to an ice surface

Hydroxide ions that are initially buried within an ice film segregate to the ice film surface at elevated temperatures. This process was observed by conducting experiments with an ice film constructed with a bottom H(2)O layer and an upper D(2)O layer, with an excess of hydroxide ions trapped at the H(2)O/D(2)O interface as they were generated by Na hydrolysis. The transport of hydroxide ions from the interfacial layer to the surface was examined as a function of time using a low energy sputtering method. The progress of the H/D exchange reaction in surface water molecules was also monitored with the Cs(+) reactive ion scattering technique. At 90 K, only a small portion of buried hydroxide ions moved to the surface in the form of OD(-) species. This was due to hydroxide transport via proton hopping through a D(2)O layer, 3 BL thick, in the surface region. At 135 K, at which point water self-diffusion is active in the ice film, the majority of the buried hydroxide ions segregated to the surface after ～1 h. Both OH(-) and OD(-) species were produced at the surface, at an OH(-)/OD(-) population ratio ≥1. Based on kinetic measurements for the transport of OH(-) and OD(-) species and the H/D exchange of surface water molecules, we concluded that the major transport channel for hydroxide ions in this regime is the migration of molecular hydroxide species. H/D exchange reactions also occur between surface hydroxide ions and water molecules. No evidence was observed for the occurrence of the hop-and-turn process at 135 K, although it is known as an important mechanism of proton transport in ice.     

Vertical diffusion of water molecules near the surface of ice

We studied diffusion of water molecules in the direction perpendicular to the surface of an ice film. Amorphous ice films of H(2)O were deposited on Ru(0001) at temperature of 100-140 K for thickness of 1-5 bilayer (BL) in vacuum, and a fractional coverage of D(2)O was added onto the surface. Vertical migration of surface D(2)O molecules to the underlying H(2)O multilayer and the reverse migration of H(2)O resulted in change of their surface concentrations. Temporal variation of the H(2)O and D(2)O surface concentrations was monitored by the technique of Cs(+) reactive ion scattering to reveal kinetics of the vertical diffusion in depth resolution of 1 BL. The first-order rate coefficient for the migration of surface water molecules ranged from k(1)=5.7(+/-0.6) x 10(-4) s(-1) at T=100 K to k(1)=6.7(+/-2.0) x 10(-2) s(-1) at 140 K, with an activation energy of 13.7+/-1.7 kJ mol(-1). The equivalent surface diffusion coefficients were D(s)=7 x 10(-19) cm(2) s(-1) at 100 K and D(s)=8 x 10(-17) cm(2) s(-1) at 140 K. The measured activation energy was close to interstitial migration energy (15 kJ mol(-1)) and was much lower than diffusion activation energy in bulk ice (52-70 kJ mol(-1)). The result suggested that water molecules diffused via the interstitial mechanism near the surface where defect concentrations were very high.     

Diffusion of HDO in pure and acid-doped ice films

In these experiments, a few bilayers of D(2)O were vapor-deposited on a pure crystalline H(2)O ice film or an ice film doped with a small amount of HCl. Upon deposition, H/D isotopic exchange quickly converted the D(2)O layer into an HDO-rich mixture layer. Infrared absorption spectroscopy followed the changes of the HDO from the initial HDO mixture layer to HDO isolated in the H(2)O ice film. This was possible because isolated HDO in H(2)O ice has a unique, sharp peak in the O-D stretch region that can be distinguished from the broad peak due to the initial HDO mixture layer. The absorbance of isolated HDO displayed first-order kinetics and was attributed to diffusion of HDO from the HDO-rich mixture layer into the underlying H(2)O ice film. While negligible diffusion was observed for pure ice films and for ice films with HCl concentrations up to 1 x 10(-4) mole fraction, diffusion of HDO occurred for higher concentrations of (2-20) x 10(-4) mole fraction HCl with a concentration-independent rate constant. The diffusion under these conditions followed Arrhenius behavior for T = 135-145 K yielding E(a) = 25 +/- 5 kJ/mol. The mechanism for the HDO diffusion involves either (i) molecular self-diffusion or (ii) long-range H/D diffusion by a series of multiple proton hop and orientational turn steps. While these spectroscopic results compare favorably with recent studies of molecular self-diffusion in low-temperature ice films, the diffusion results from all the ice film studies at low temperatures (ca. T < 170 K) differ from earlier bulk ice studies at higher temperatures (ca. T > 220 K). A comparison and discussion of the various diffusion studies are included in this report.     

Vacuum ultraviolet photodissociation and surface morphology change of water ice films dosed with hydrogen chloride

Time-of-flight (TOF) spectra of photofragment H atoms from the photodissociation of water ice films at 193 nm were measured for amorphous and polycrystalline water ice films with and without dosing of hydrogen chloride at 100-145 K. The TOF spectrum is sensitive to the surface morphology of the water ice film because the origin of the H atom is the photodissociation of dimerlike water molecules attached to the ice film surfaces. Adsorption of HCl on a polycrystalline ice film was found to induce formation of disorder regions on the ice film surface at 100-140 K, while the microstructure of the ice surface stayed of polycrystalline at 145 K with adsorption of HCl. The TOF spectra of photofragment Cl atoms from the 157 nm photodissociation of neutral HCl adsorbed on water ice films at 100-140 K were measured. These results suggest partial dissolution of HCl on the ice film surface at 100-140 K.     

Temperature, composition, and hydrogen isotope effect in the hydrogenation of CO on amorphous ice surface at 10-20 K

An experiment on the addition reaction of a D atom (deuteration) to CO on a cold ice surface is performed by deuterium atom exposure of three types of samples (pure solid CO, CO-capped H2O ice, and CO-H2O mixed ice) at 10-20 K. The variation of IR absorption spectra for the samples was measured by a Fourier transform infrared spectrometer during exposure to deuterium atoms. Reactions on pure solid CO were observed only at 10 K, while reactions on CO-capped H2O ice and CO-H2O mixed ice were observed to proceed even at 20 K. This indicates that the coexistence of H2O at the surface raises the reactive temperature. In addition, the experiment on H atom exposure was also carried out at 15 K to compare the reaction rate constant between the H and D atoms. The ratio of reaction rate constant kD/kH obtained is about 0.08 at 15 K. The authors provide information on the potential energy for the H+CO reaction at the surface by using the ratio kD/kH and by a model calculation of the potential tunneling with the asymmetric Eckart potential.     

Fast thermal desorption spectroscopy study of H/D isotopic exchange reaction in polycrystalline ice near its melting point

Using fast thermal desorption spectroscopy, a novel technique developed in our laboratory, we investigated the kinetics of HD isotopic exchange in 3 microm thick polycrystalline H2O ice films containing D2O layers at thicknesses ranging from 10 to 300 nm at a temperature of -2.0+/-1.5 degrees C. According to our results over the duration of a typical fast thermal desorption experiment (3-4 ms), the isotopic exchange is confined to a 50+/-10 nm wide reaction zone located at the boundary between polycrystalline H2O and D2O ice. Combining these data with a theoretical analysis of the diffusion in polycrystalline medium, we establish the range of possible values for water self-diffusion coefficients and the grain boundary widths characteristic of our ice samples. Our analysis shows that for the grain boundary width on the order of a few nanometers, the diffusivity of D2O along the grain boundaries must be at least two orders of magnitude lower than that in bulk water at the same temperature. Based on these results, we argue that, in the limit of low concentrations of impurities, polycrystalline ice does not undergo grain boundary premelting at temperatures up to -2 degrees C.     

Ionization and solvation of HCl adsorbed on the D2O-ice surface

The interaction of HCl with the D(2)O-ice surface has been investigated in the temperature range 15-200 K by utilizing time-of-flight secondary ion mass spectroscopy, temperature-programmed desorption, and x-ray photoelectron spectroscopy. The intensities of sputtered H(+)(D(2)O) and Cl(-) ions (the H(+) ions) are increased (decreased) markedly above 40 K due to the hydrogen bond formation between the HCl and D(2)O molecules. The HCl molecules which form ionic hydrates undergo H/D exchange at 110-140 K and a considerable fraction of them dissolves into the bulk above 140 K. The neutral hydrates of HCl should coexist as evidenced by the desorption of HCl above 170 K. They are incorporated completely in the D(2)O layer up to 140 K. The HCl molecules embedded in the thick D(2)O layer dissolve into the bulk, and the ionic hydrate tends to segregate to the surface above 150 K.     

Proton mobility in thin ice films: a revisit

We have examined proton transport through an ice film in the temperature range 73-140 K by initially adding hydronium ions into the interior of the film and then monitoring the build-up of hydronium ion population at the film surface. The result confirms that the proton exhibits limited mobility in the ice film at low temperature, but it becomes highly mobile at temperature above 130 K. Based on this result we suggest an explanation of the anomalous experimental observations in the literature for the proton mobility in ice films.     

Influence of monolayer amounts of HNO3 on the evaporation rate of H2O over ice in the range 179 to 208 K: a quartz crystal microbalance study

The evaporation flux J(ev) of H2O from thin H2O ice films containing between 0.5 and 7 monolayers of HNO3 has been measured in the range 179 to 208 K under both molecular and stirred flow conditions in isothermal experiments. FTIR absorption of the HNO3/H2O condensate revealed the formation of metastable alpha-NAT (HNO(3).3H2O) converting to stable beta-NAT at 205 K. After deposition of HNO3 for 16-80 s on a 1 mum thick pure ice film at a deposition rate in the range (6-60) x 10(12) molecules s(-1) the initial evaporative flux J(ev)(H2O) was always that of pure ice. J(ev)(H2O) gradually decreased with the evaporation of H2O and the concomitant increase of the average mole fraction of HNO3, chi(HNO3), indicating the presence of an amorphous mixture of H2O/HNO3 that is called complexed or (c)-ice whose vapor pressure is that of pure ice. The final value of J(ev) was smaller by factors varying from 2.7 to 65 relative to pure ice. Depending on the doping conditions and temperature of the ice film the pure ice thickness d(D) of the ice film for which J(ev) < 0.85J(ev)(pure ice) varied between 130 and 700 nm compared to the 1000 nm thick original ice film at 208 and 191 K, respectively, in what seems to be an inverse temperature dependence. There exist three different types of H2O molecules under the present experimental conditions, namely (a) free H2O corresponding to pure ice, (b) complexed H2O or c-ice, and (c) H2O molecules originating from the breakup of NAT or amorphous H2O/HNO3 mixtures. The significant decrease of J(ev)(H2O) with increasing chi(HNO3) leads to an increase of the evaporative lifetime of atmospheric ice particles in the presence of HNO3 and may help explain the occurrence of persistent and/or large contaminated ice particles at certain atmospheric conditions.     

H2O and D2 mixtures under pressure: spectroscopy and proton exchange kinetics

We have investigated the pressure-induced spectral changes and the proton exchange reactions of D(2)-H(2)O mixtures to 64 GPa using micro-Raman spectroscopy. The results show the profound difference in the rotational and vibrational Raman spectra of hydrogen isotopes from those of the pure samples, showing the vibrational modes at higher frequencies and continuing to increase with pressure without apparent turnover. This indicates the repulsive nature of D(2)-H(2)O interaction without hydrogen bonds between the two and, thus, interstitial fillings of D(2) molecules into the bcc-like ice lattice. The spectral analysis using the Morse potential yields a hydrogen bond distance of 0.734 ? at 6 GPa--slightly shorter than that in pure--attributed to the repulsive interaction. The pressure-dependent spectral changes suggest that the proton-ordering transition in the ice lattice occurs over a large pressure range between 28 and 50 GPa, which is substantially lower than that of pure ice (40-80 GPa). This again indicates the presence of high internal pressure arising from the repulsive interaction. The Raman spectra show evidences that the proton exchange occurs in various phases including in solid D(2) and H(2)O mixtures. Based on the time-dependent spectral changes, we obtained the proton exchange rates of k ~ 0.085 h(-1) at 0.2 GPa in fluid D(2) and water mixtures, k ~ 0.03 h(-1) and 0.003 h(-1) at 2 GPa and 4 GPa, respectively, in fluid D(2)-ice mixtures, and k ~ 10(-3) h(-1) at 8 GPa in solid D(2) and ice mixtures.     

Glass-liquid transition of carbon dioxide and its effect on water segregation

The interactions between CO(2) and D(2)O molecules have been investigated by using time-of-flight secondary ion mass spectrometry in the temperature rage 13-120 K. The monolayer of CO(2) tends to wet or intermix with the D(2)O film below 40 K and dewets the surface above 60 K. The water nanoclusters deposited on the CO(2) multilayers also start to segregate at 50-60 K and are finally incorporated in the bulk at 85-90 K, where the morphology of the film changes abruptly together with the desorption rate of the CO(2) molecules. The break at 85 K should be caused by the occurrence of the fluidized film whereas the glass-transition temperature of CO(2), as determined from the onset of translational molecular diffusion, is assigned to 50 K. This behavior may be related to the ultraviscous nature of the supercooled liquid, arising from the decoupling between the translational molecular diffusion and viscosity. The He(+) irradiation of the mixed CO(2)-D(2)O ice and the D(2)(+) irradiation of the CO(2) ice at 13 K do not yield any surface residues assignable to H(2)CO(3) and its precursors above 100 K. This result may be related to the segregation between the CO(2) and D(2)O molecules.     

Acetone and water on TiO2(110): H/D exchange

Isotopic H/D exchange between coadsorbed acetone and water on the TiO2(110) surface was examined using temperature programmed desorption (TPD) as a function of coverage and two surface pretreatments (O2 oxidation and mild vacuum reduction). Coadsorbed acetone and water interact repulsively on reduced TiO2(110) on the basis of results from the companion paper to this study, with water exerting a greater influence in destabilizing acetone and acetone having only a nominal influence on water. Despite the repulsive interaction between these coadsorbates, about 0.02 monolayers (ML) of a 1 ML d6-acetone on the reduced surface (vacuum annealed at 850 K to a surface oxygen vacancy population of 7%) exhibits H/D exchange with coadsorbed water, with the exchange occurring exclusively in the high-temperature region of the d6-acetone TPD spectrum at approximately 340 K. The effect was confirmed with combinations of d0-acetone and D2O. The extent of exchange decreased on the reduced surface for water coverages above approximately 0.3 ML due to the ability of water to displace coadsorbed acetone from first layer sites to the multilayer. In contrast, the extent of exchange increased by a factor of 3 when surface oxygen vacancies were pre-oxidized with O2 prior to coadsorption. In this case, there was no evidence for the negative influence of high water coverages on the extent of H/D exchange. Comparison of the TPD spectra from the exchange products (either d1- or d5-acetone depending on the coadsorption pairing) suggests that, in addition to the 340 K exchange process seen on the reduced surface, a second exchange process was observed on the oxidized surface at approximately 390 K. In both cases (oxidized and reduced), desorption of the H/D exchange products appeared to be reaction limited and to involve the influence of OH/OD groups (or water formed during recombinative desorption of OH/OD groups) instead of molecularly adsorbed water. The 340 K exchange process is assigned to reaction at step sites, and the 390 K exchange process is attributed to the influence of oxygen adatoms deposited during surface oxidation. The H/D exchange mechanism likely involves an enolate or propenol surface intermediate formed transiently during the desorption of oxygen-stabilized acetone molecules.     

Alkane C-H bond activation in zeolites: evidence for direct protium exchange

The mechanism of alkane C-H bond activation in heterogeneous acid catalysis is unknown. (1)H solid-state NMR techniques have been used to simultaneously detect the reactivity of both catalyst and alkane reactant protons in a true in-situ experimental design. Specifically, the activation of isobutane C-H bonds by the solid acid zeolite HZSM-5 is directly observed, and the rate of proton transfer between the solid catalyst surface and gaseous isobutane is quantitatively measured using isotopic (1)H/(2)H exchange methods. An observable adsorption complex forms between the isobutane and the primary Bronsted acid site of ZSM-5, which leads to proton exchange between the zeolite surface and the isobutane methyl groups at temperatures (273 K) much lower than previously reported. The secondary acid site in ZSM-5 is less accessible to or less reactive with the isobutane molecule. Simultaneous detection of protium loss from the Bronsted acid site and protium gain by perdeuterated isobutane reveals a common rate constant equal to 4.1-4.6 x 10(-4) s(-1) at 298 K, but at lower temperatures, the transition between this and a much slower rate process is resolved. The measured activation energy for isobutane H/D exchange is 57 kJ/mol. In all experiments, the isobutane reagent was purified to eliminate any unsaturated impurities that might serve as initiators for carbenium-ion mechanisms, and the active catalyst was free of any organic contaminants that might serve as a source of unsaturated initiators. In total, our results are consistent with direct proton exchange between the zeolite surface and the methyl groups of isobutane.     

Proton and deuteron position preferences in water clusters: an ab initio study

In order to explore the effect of H-to-D substitution on the zero-point energy (ZPE) of water clusters, Hessians were computed for a database of 53 optimized (H2O)n clusters, 5 < or = n < or = 21, at the B3LYP6-311 + + G** level. The 53 clusters contained 1524 protons, which were sorted into 18 categories according to the type of their donor O and (if not free) acceptor O. Letting deltaZPE[H]* denote the change in ZPE when the proton H* is replaced by D, mean values for deltaZPE[H*] for the H-bonded categories ranged from -2172 cal mol(-1) for H* in a DDAA-DDAA bond to -2118 for H* in a DAA-DDA bond. Mean value for H* free on DAA (respectively, DA) was -2018 (respectively, -1969). For DAA-DDA bonds, and for short H bonds in general, there was a strong inverse correlation between /deltaZPE[H*]/ and the O-H* distance. deltaZPE for multiple H-to-D substitutions was additive, except for a cooperativity effect of -13.7 to -19.7 cal mol(-1) when two substituted protons were in the same H2O unit and a much smaller cooperativity when one proton's donor was the other's acceptor. Implications of these data include a relative preference for D to occupy H bonded rather than free positions in finite water clusters, a value of 3.82 for the disproportionation equilibrium constant of mixed ice at 150 K, increased occupation by H at surface positions of mixed ice, and a larger average coordination number for liquid D2O than for liquid H2O.     

Low-temperature FTIR spectra and hydrogen bonds in polycrystalline adenosine and uridine

FTIR spectra of polycrystalline samples of adenosine and uridine, pure and containing small (<10%) quantity of N(O)H or N(O)D groups, were measured in KBr pellets from 4000 to 400 cm(-1) at temperatures from 300 to 20 K. For the first time, the bands of narrow isotopically decoupled proton stretching vibration nu1 mode of NH- and OH- groups were found and assigned to ordered hydrogen bonds according to crystal structural data for both nucleosides. The FTIR adenosine spectra in the out-of-plane bending proton nu4 mode range (lower than 1000 cm(-1)) of N(O)H groups revealed at low temperature at least twice more bands, than in the nu1 range, which are influenced by isotopic exchange and (or) cooling. Almost all of them have their counterparts in the N(O)D substance spectrum with an isotopic frequency ratio of 1.30-1.40. These bands were assigned to the differently H-bound disordered NH and OH protons, which could not be seen with crystal structural methods. The energy and length of different H-bonds were estimated from peak positions of both mode bands (as the red shift of nu1 or blue shift of nu4 relatively free molecules) with well-established empirical correlations between spectral, thermodynamic and structural parameters of hydrogen bonds. The results were compared with independent experimental data.     

Sodium interacting with amorphous water films at 10 and 100 k

In the present study, we compare the adsorption of Na on amorphous D(2)O ice films, held at 10 and 100 K. OH, D(2)O, and Na are easily distinguished by their characteristic signatures in metastable impact electron spectroscopy (MIES). It is found that at 10 K substrate temperature the donation of 3sNa charge to the ice film, which is regarded as a precursor for water deprotonation, is significantly reduced relative to 100 K. This observation is discussed on the basis of recent theoretical work, suggesting that a rearrangement of the water molecules at the outermost water surface is the prerequisite for hydration/solvation of the 3sNa electron in the water ice bulk. The MIES spectra, showing spectral features from both OH and D(2)O, can be interpreted as reflecting the composition of the Na-water complexes in the near surface region. The relative intensity of the OH and D(2)O features is the same for 10 and 100 K. This finding suggests that two different sites for Na adsorption exist, one on the perfect water network and the other at OH dangling bond sites whereby, at 10 K, only the latter one leads to deprotonation of D(2)O. Finally, charge exchange phenomena observed when applying electron spectroscopies to ice films are discussed.     

Low-energy electron-stimulated luminescence of thin H2O and D2O layers on Pt(111)

The electron-stimulated luminescence (ESL) from amorphous solid water and crystalline ice films deposited on Pt(111) at 100 K is investigated as a function of the film thickness, incident electron energy (5-1000 eV), isotopic composition, and film structure. The ESL emission spectrum has a characteristic double-peaked shape that has been attributed to a transition between a superexcited state (C) and the dissociative, first excited state (A) in water: C --> A. Comparing the electron-stimulated luminescence and O2 electron-stimulated desorption (ESD) yields versus incident electron energy, we find the ESL threshold is approximately 3 eV higher than the O2 ESD threshold, which is close to the center of the emission spectrum near 400 nm and supports the C --> A assignment for the ESL. For thin films, radiative and nonradiative interactions with the substrate tend to quench the luminescence. The luminescence yield increases with coverage since the interactions with the substrate become less important. The ESL yield from D2O is approximately 4-5 times higher than that from H2O. With use of layered films of H2O and D2O, this sizable isotopic effect on the ESL is exploited to spatially profile the luminescence emission within the ASW films. These experiments show that most of the luminescence is emitted from within the penetration depth of the incident electron. However, the results depend on the order of the isotopes in the film and can be modeled by assuming some migration of the electronically excited states within the film. The ESL is very sensitive to defects and structural changes in solid water, and the emission yield is significantly higher from amorphous films than from crystalline ice.     

Glycine-ice nanolayers: morphology and surface energetics

Ultrathin glycine-ice films (nanolayers) have been prepared in ultrahigh vacuum by condensation of H(2)O and glycine at 110 K and 150 K on single crystalline Al(2)O(3) surfaces and have been investigated by temperature programed thermal desorption, x-ray photoelectron spectroscopy, and work function measurements. Various layer architectures have been considered, including glycine-on-ice, ice-on-glycine, and mixed glycine-ice nanolayers. Low coverages of adsorbed glycine molecules on amorphous ice surfaces suppress the amorphous-to-crystalline phase transition in the temperature range 140-160 K in near-surface regions and consequently lead to a lower desorption temperature of H(2)O molecules than from pure ice layers. Thicker glycine overlayers on ice provide a kinetic restriction to H(2)O desorption from the underlying ice layers until the glycine molecules become mobile and develop pathways for water desorption at higher temperature (>170 K). Ice overlayers do not wet glycine film surfaces, but the glycine molecules on ice are sufficiently immobile at 110 K, so that continuous glycine overlayers form. In mixed glycine-ice nanolayers the glycine phase displays hydrophobic behavior and a phase separation takes place, with the accumulation of glycine near the surfaces of the films.     

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.