Dissociation of Trifluoroacetic Acid in Amorphous Solid Water: Charge

Publication Date (Web): May 30, 2017 ... Such characteristics of excess protons may be responsible for the appearance of the Zundel continuum absorpti...
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Dissociation of Trifluoroacetic Acid in Amorphous Solid Water: Charge-Delocalized Hydroniums and Zundel Continuum Absorption Sunghwan Shin, Youngwook Park, Youngsoon Kim, and Heon Kang* Department of Chemistry, Seoul National University, 1 Gwanak-ro, Seoul 151-747, South Korea S Supporting Information *

ABSTRACT: We studied the ionic dissociation of trifluoroacetic acid (TFA) in D2O amorphous solid water (ASW) using reflection absorption infrared spectroscopy, low-energy sputtering, and H/D isotopic exchange experiment. TFA readily dissociated to hydronium and counterions in ASW at 60 K, which indicates a significant increase in the acidity as compared with that in aqueous solution at room temperature. The acid dissociation produced a Zundel continuum band in the 1000−3000 cm−1 region and an accompanying lowered intensity of the O−D stretching band of D2O. The reduced intensity of D2O was several times larger than that expected for 1:1 stoichiometric proton transfer from TFA to water. Excess protons released from the acid migrated through as many as 20 water layers in ASW. These observations indicate that excess protons are highly mobile and dynamically delocalized in the hydrogen-bonded water chain. Such characteristics of excess protons may be responsible for the appearance of the Zundel continuum absorption.

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INTRODUCTION Ionic dissociation of an acid to hydrated protons and counteranions in an aqueous solution is one of the most fundamental phenomena in chemistry, but its molecular mechanism is not yet fully understood in detail. In general, it is considered that the ionic dissociation of an electrolyte in an aqueous solution occurs via a mechanism that can be divided, with somewhat arbitrary criteria, into several intermediate stages.1 The dissociation of ionic bond initially leads to the formation of a contact ion pair (CIP), in which the emanating ion fragments are still in close contact, without the introduction of solvent molecules into the space between them. CIP is transformed to a solvent-separated ion pair (SSIP) as a solvation layer develops around the ions. The solvation assists further separation of these ions through dielectric screening of opposite charges. Many studies have investigated ion pairing of aprotic electrolytes in dielectric media.1,2 The dissociation of protonic acids in water represents a special case of electrolyte dissociation. The excess proton released from the acid moves via a unique proton-hopping mechanism in aqueous solution.3 Proton motion in solution involves interconversion between various hydrated structures according to theoretical studies.4,5 Zundel and coworkers6 studied the dissociation of carboxylic acids in aqueous solutions using transmittance IR spectroscopy, and they observed the appearance of continuous absorption (“Zundel continuum”) in the IR spectra extending over a wide region (typically 1750− 3000 cm−1). They interpreted the continuum band as signatures of polarizable hydrogen bonds of protonated water and acid−water cluster structures. The dissociation of acids in ice or ASW was also studied.7−16 Such studies have relevance in © 2017 American Chemical Society

investigations of atmospheric heterogeneous reactions of acids occurring in ice particles in the upper atmosphere. Also, the studies may provide useful information regarding corresponding processes in aqueous solutions. For example, Devlin and coworkers10,17 studied the dissociation of strong acids (HCl and HBr) adsorbed on ice nanocrystals with transmittance IR spectroscopy and theoretical calculations, and they reported IR spectroscopic signatures of the solvation and ionization stages of HCl. Also, the effect of temperature on acid dissociation was investigated at ice surfaces. These studies reported a wide range of observations from complete dissociation of HCl8−12 to a substantial portion of molecular HCl adsorption at the surface of ice at low temperature.9−11 Ayotte and coworkers13−16 studied the dissociation of hydrogen halides in ASW with reflection absorption infrared spectroscopy (RAIRS) and theoretical computations and reported13,16 that HF behaves like a strong acid at cryogenic temperatures with its extensive ionic dissociation. It was suggested13 that the strong Zundel absorption is generated by a broad distribution of protonshared complexes and hydrated proton CIP structures. It has now been well established that protons are very mobile in crystalline ice18 and ASW.19,20 The high mobility of protons raises a question about the possibility of forming CIP structures during acid dissociation in ASW, with protons adjacent to the anions. In the present work, we explore this question through investigation of the dissociation of TFA in ASW. RAIRS, lowenergy sputtering (LES), and H/D isotopic exchange experiReceived: April 11, 2017 Revised: May 21, 2017 Published: May 30, 2017 12842

DOI: 10.1021/acs.jpcc.7b03415 J. Phys. Chem. C 2017, 121, 12842−12848

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The Journal of Physical Chemistry C

stretching band of water in the spectra. The behavior of AA in ASW was also examined as a counter example of a nonionizing acid. Figure 1a shows the IR spectra of a TFA-D2O mixture film (top) and a pure TFA film (bottom) measured at 60 K. The

ments were performed to elucidate the acid dissociation mechanism and monitor the behavior of excess protons released from the acid. The results showed efficient dissociation of TFA in ASW and the generation of charge-delocalized hydronium states rather than the formation of CIP structures.

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EXPERIMENTAL METHODS

The experiment was conducted in an ultrahigh vacuum chamber with a base pressure below 1 × 10−10 Torr,21 equipped with instrumentation for RAIRS, LES, and temperature-programmed desorption (TPD) mass spectrometry. The sample was grown on a Pt(111) substrate surface at 60 K. The substrate temperature was variable in the 60−1200 K range using resistive heating and helium cryostat cooling. The temperature was measured using N-type thermocouple wires, which were connected directly to the Pt crystal.21 The Pt surface was cleaned by annealing it at 1200 K and Ar+ sputtering at 2 keV. The cleanliness of the surface was judged from the characteristic shape of the TPD spectra of a D2O monolayer adsorbed onto the surface.22 Trifluoroacetic acid (TFA; CF3COOH), acetic acid (AA; CH3COOH), and D2O liquid samples were purified through three freeze−vacuum−thaw cycles, and their purity was verified by analyzing their vapor in the chamber with a quadrupole mass spectrometer. TFA and AA gases were guided close to the substrate through tube dosers. The stacking rates of these gases on the substrate surface were below 0.05 monolayer per second (ML·s−1). D2O gas was deposited onto the substrate surface by using a backfilling method at a rate slower than 0.1 ML·s−1. Acid−D2O mixture films were prepared by codeposition of the acid and D2O gases, and their mole ratio was controlled by varying the partial pressure of the acid. The TFA or AA concentration in the film was quantified through TPD experiment. The samples were prepared in a sandwich structure, in which the acid−D2O mixture layer (80 ML) was trapped between two pure D2O layers (30 ML) at the bottom and top so as to prevent exposure of the acid at the metal or vacuum interface. RAIRS experiments were performed in a grazing reflection angle of 84° using a commercial Fourier-transform infrared spectrometer with an external mercury−cadmium telluride detector. The IR beam path was purged with dry nitrogen gas during the measurement. Each RAIR spectrum was obtained at a spectral resolution of 4 cm−1 after cooling the sample to a stabilized temperature of 60 K to minimize thermal shifts in the background spectral intensity. LES experiments were conducted to detect ions on the sample surface. A Cs+ ion beam was collided with the sample surface at an incident energy of 30 eV using a low-energy alkali ion gun, and the preexistent ions on the surface were ejected by the Cs+ impact. The emitted ions were detected by a quadrupole mass spectrometer with its ionizer filament switched off. The incident flux of the Cs+ beam was kept low (∼1011 ions cm−2 s−1) to minimize surface contamination by Cs+ ions. Secondary ionization of adsorbates induced by Cs+ collision did not occur under this experimental condition.

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Figure 1. (a) (Top) RAIR spectrum of a TFA-D2O mixture film consisting of 4% TFA and 96% D2O in molar ratio. (Bottom) RAIR spectrum of a pure TFA film. (b) (Top) RAIR spectrum of an AAD2O mixture sample consisting of 4% AA and 96% D2O. (Bottom) RAIR spectrum of a pure AA film. The samples were grown on a Pt(111) substrate at 60 K, and the spectra were measured at 60 K. The spectral intensities for the pure TFA and AA films are shown magnified by two times.

pure TFA film showed the ν(O−H) stretching band of molecular TFA at ∼3100 cm−1. This band disappeared from the spectrum of the TFA−D2O mixture, indicating ionization of the carboxylic group of TFA in the water-solvating environment of ASW. The ν(C = O) stretching band of TFA also changed for the TFA-D2O mixture; the ν(C = O) band of the pure TFA sample appearing at 1800 cm−1 was drastically red-shifted for the TFA−D2O mixture with an accompanying change in spectral shape. These changes are consistent with a weakening of the C = O bond due to ionization of the carboxylic group. The absent ν(O−H) band intensity in the mixture sample indicates that the acid ionization was almost complete. This is a significant increase in the extent of acid ionization in ASW at 60 K as compared with that in aqueous solution at room temperature (α ≈ 0.5 at 4% molar fraction with pKa = −0.25). Figure 1b shows the IR spectra of an AA−D2O mixture film (top) and a pure AA film (bottom) at 60 K. In contrast with the case of TFA, the ν(O−H) stretching band of AA at 3100 cm−1 persisted in the AA−D2O mixture sample, indicating nonionization of AA. Also, the ν(C = O) band appeared in the same position (1800 cm−1) for the AA−D2O mixture and the pure AA films. Figure 2 shows spectral changes for the TFA−D2O mixture samples prepared with different molar portions of TFA. The spectrum of a pure D2O film is also shown to indicate the spectral baseline in the absence of TFA. Against this baseline, a Zundel continuum structure with broad absorption from 1000

RESULTS

We studied the ionization mechanism of TFA in ASW by monitoring changes in the vibrational absorption features of the acid in the RAIRS experiment. D2O was used as an ASW matrix to differentiate the O−H stretching band of TFA and the O−D 12843

DOI: 10.1021/acs.jpcc.7b03415 J. Phys. Chem. C 2017, 121, 12842−12848

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The Journal of Physical Chemistry C

Additional evidence of acid ionization came from the appearance of the ν(O−H) stretching band of water at ∼3300 cm−1, which increased upon heating the TFA−D2O sample, as shown in Figure 3a. This band was well-separated

Figure 2. RAIR spectra of TFA−D2O mixture samples prepared with different molar ratios of TFA: (a) 1.5% TFA, (b) 4% TFA, and (c) 17% TFA. RAIR spectrum of a pure D2O film (dotted red line), which serves as a reference of spectral baseline for the TFA−D2O samples, is shown for comparison. The D2O film was prepared to have the same optical thickness as the TFA−D2O films and deposited at the same temperature (60 K). The spectral measurements were made at 60 K.

to 3000 cm−1 is discernible.6,15 The spectra shown in Figure 2a−c show that a higher TFA content increased the amplitude of Zundel absorption. The Zundel continuum structure has been reported in IR spectroscopic studies of ice particles and ASW films doped with ionized acids.14,15,17 In accordance with these studies, we attribute the observed continuum structure to protons injected into the lattice from the acid. The identification of a Zundel continuum requires careful subtraction of the spectral baseline. The spectral baseline of a thin-film sample smoothly decreases at higher frequencies above ca. 3000 cm−1 because of the optical interference of the film.23 We checked the baseline profile by recording the RAIR spectra of a solid Xe film and a D2O−ASW film, both of which are transparent at wavelengths >3000 cm−1, and we confirmed that the two samples showed identical baseline spectra (not shown). Another reference point of the spectral baseline is the absorbance in the transparent region of the samples near 1000 cm−1. Figure 2 shows that the baseline absorbance of the pure D2O film is equal to that of the TFA−D2O samples at 1000 cm−1 when they are prepared to have the same optical thickness. The increase in the amplitude of the Zundel continuum at higher TFA concentration was accompanied by a decrease in the intensity of the O−D stretching band of D2O around 2500 cm−1. For quantitative analysis of these intensity changes, the ν(O−D) intensity for the TFA−D2O mixture sample was compared with that of a pure D2O sample that was prepared to have the same amount of D 2 O (see the Supporting Information). The decreased intensity of the ν(O−D) stretching in Figure 2 was larger than that expected for the loss of D2O molecules by a 1:1 stoichiometric protonation reaction between excess proton and water: H+ + D2O → HD2O+. For example, in the case of the TFA(17%)− D2O(83%) mixture sample, the decrease in ν(O−D) intensity was about four times greater than that expected for the stoichiometric reaction (Supporting Information).

Figure 3. RAIR spectra of a TFA(4%)−D2O(96%) mixture sample after heating at 60, 100, and 140 K. (a) ν(O−H) stretching band region of HDO. (b) Zundel continuum region. All spectra were measured after cooling the sample at 60 K to prevent thermal shift of the spectral baseline.

from the ν(O−H) stretching position of molecular TFA at 3100 cm−1 (Figure 1a).24 The 3300 cm−1 feature is the O−H stretching band of HDO species. The HDO species is formed in the D2O matrix by H/D exchange between D2O and H+containing hydronium produced by acid ionization.18 The O− H vibration of HDO is decoupled from the D2O lattice vibrations as well as from the O−H vibration of other HDO species within the D2O ice sample at the present concentrations of TFA (≤17% molar ratio), and thus it appears as a singlecomponent peak.25 Figure 3a shows the evolution of the ν(O− H) band as the sample temperature was increased from 60 to 140 K. The increasing ν(O−H) intensity indicates that the population of isolated HDO species increased at higher temperatures. This peak was not observed for a nonionizing AA-D2O sample, as expected. Apart from this spectral change, sample heating resulted in decreased absorbance in a broad region below ∼3200 cm−1. This feature may be related to the morphology change of the ASW sample at the elevated temperature, which accompanied a red shift of the huge ν(O− D) band of D2O.26 The same heating procedure of the sample did not significantly change the Zundel continuum amplitude, as shown in Figure 3b. Apparently, the hydronium density in the sample did not increase with further increasing temperature because TFA was already fully ionized at 60 K. Despite the constant density of hydronium, the population of HDO, indicated by the ν(O−H) intensity, continually increased at higher temperatures. This indicates that hydronium acts as a catalyst in the formation of HDO and this process is thermally activated. In support of this interpretation, Devlin and 12844

DOI: 10.1021/acs.jpcc.7b03415 J. Phys. Chem. C 2017, 121, 12842−12848

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The Journal of Physical Chemistry C coworkers27,28 have shown that an isolated HDO species is formed in an H2O−D2O mixture ice only when the transport of excess proton occurs in conjunction with thermally activated reorientation of water (a “hop-and-turn” mechanism). We examined the transport distance of protons released by acid dissociation in ASW. For this, samples with a TFAsandwich structure were prepared, as shown in Figure 4. First,

substantially large distance and were not bound to the counteranions as CIP.

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DISCUSSION The following mechanism can be proposed for the dissociation of TFA in ASW based on the observations described above. Figure 5 illustrates the proposed mechanism. First, proton

Figure 4. LES measurement of surface NH4+ species as a function of H2O overlayer thickness of acid-sandwich films. The sample structure is depicted in the Figure. The samples were prepared by sequential deposition of corresponding films on a Pt(111) substrate at 70 K. The black, red, and blue data points correspond to the TFA-sandwich film, the AA film, and the pure H2O film, respectively. The lines are visual guides. The NH4+ intensity from the pure H2O film represents a baseline.

an amorphous H2O film (50 ML) was grown on Pt(111). TFA (0.2 ML) was then adsorbed onto the H2O film, and an additional H2O layer was overlaid onto the film for varying thicknesses between 0 and 40 ML. Finally, ammonia molecules were placed on top of the sample surface. If protons released from TFA migrated across the H2O overlayer to reach the sample surface, then they would meet the proton acceptor (NH3) and form NH4+ ions. On the contrary, if the released protons remained close the counteranions as CIP, then NH4+ would not form. Figure 4 displays the results of the LES measurement of the NH4+ (m/z = 19) signal at the sample surface for varying thicknesses of H2O overlayer. The results of control experiments performed with AA-sandwich films and pure H2O films, which did not have excess protons, are also shown for comparison. For the AA-sandwich films, the NH4+ signal intensity was relatively small and sharply decreased with an increase in H2O overlayer thickness, vanishing completely above the thickness of 5 ML. Note that AA does not dissociate in ASW (Figure 1). Therefore, the NH4+ intensity for these samples at small thicknesses of H2O overlayer must be produced by direct proton transfer from AA to NH3 molecules that are in physical contact. When the H2O film thickness was increased above ∼5 ML, all AA molecules were buried under the H2O overlayer, and thus proton transfer from AA to NH3 was blocked. In contrast, for TFA-sandwich films, the NH4+ signal was larger and persisted even at an H2O overlayer thickness of ∼20 ML. Apparently, TFA and NH3 were not in direct contact at this film thickness, as compared with the AAsandwich film. Therefore, the NH4+ signal indicates proton transfer through the H2O overlayer. In turn, this result indicates that protons released from TFA could migrate over a

Figure 5. Illustration of the dissociation path of TFA in ASW. The 3D network structure of ASW is represented by the so-called square ice in two dimensions for convenience. Hydrogen bonding is shown by the dotted lines. (a) Molecular acid (HA) and water (D2O) connected with hydrogen bond. (b) The acid releases a proton to the adjacent D2O to form HD2O+ (structure I). The excess proton migrates along the water chain via D+ hopping and forms, for example, structure II. Structures I and II are rapidly interconvertible via the concerted hopping motion of protons (D+ or H+) in the water chain; that is, the charge of excess proton is dynamically delocalized in the extended network structure. (c) Thermal reorientation of water (HDO marked by a dotted circle) is necessary for H+ to escape from the protonshuttling water chain. This leads to the formation of isolated HDO species (marked by a dotted ellipse).

transfer from TFA to adjacent D2O [path from (a) to (b)] would produce structure I, which is a CIP structure of hydronium (HD2O+) and conjugate base anion. However, owing to the high mobility of excess protons in the lattice via the Grotthuss mechanism, the hydronium structure can quickly migrate along the water chain, for example, to form structure II. The hydronium migration can occur over a substantially long distance, up to distance of ∼20 water layers, as demonstrated in Figure 4 by NH4+ formation at the surface of the acid-sandwich sample. The hydronium migration may occur back and forth along the water chain if there is no proton trap.7,18,29 Also, the Grotthuss mechanism is known to occur via concerted proton hopping along the water chain.30 Therefore, the proton oscillation may be viewed as dynamic delocalization of protons 12845

DOI: 10.1021/acs.jpcc.7b03415 J. Phys. Chem. C 2017, 121, 12842−12848

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When the dissociation mechanism of TFA in ASW and that of aprotic electrolytes in aqueous solution are compared, we may consider that a certain resemblance exists between the formation of isolated HDO species in the former and SSIP in the latter. Their common features are that dissociated cations and anions are physically separated by solvent molecules, and this process is assisted by the thermal motion of solvent molecules. On the contrary, acid dissociation does not form an analogous CIP of aprotic electrolyte dissociation. A rapid interconversion between structures I and II prevents the formation of a discrete CIP structure during the acid dissociation. Rather, the hydronium state is characterized by the dynamic delocalization of excess proton in an extended water cluster unit, which is called the charge-delocalized hydronium (CDH) state. The CDH state may be responsible for the appearance of Zundel continuum absorption. The molecular origin of the Zundel continuum has been an interesting question since its initial discovery in acidic aqueous solutions.6 In early studies,6,33 the continuum band was attributed to the high polarizability of hydrogen bonds in protonated water units or between molecular acid and water. More recently, theoretical studies have proposed that the solvation dynamics of excess proton in water should be considered to explain the continuum band.34−38 For example, the dynamic oscillation of Eigen (H9O4+) structures may facilitate delocalization of the charge of excess proton in the extended water cluster.36,37 These dynamically resonating distorted Eigen structures may produce the continuous broad absorption in the IR spectrum.38 However, continuum absorption also appears for solid water samples doped with acids,13−15,17 although an excess proton can hardly form a Zundel or Eigen structure in solid water and the rapid rearrangement of hydrated proton structure is not possible.30 For this reason, different molecular interpretations must be considered for the appearance of the continuum band in solid water. Iftimie et al.13 studied cryogenic mixtures of water and HF with RAIRS and theoretical computations, and they suggested that the continuum absorption can be reproduced by an ensemble of various intermediate structures between molecular (HF−water complex) and dissociated (CIP) forms of the acid. Confusingly, ASW samples doped with HF and DF showed basically identical Zundel spectra, indicating almost complete acid ionization and isotopic scrambling in the samples.16 In the case of full ionization, the continuum band must originate from protonated water clusters only, but it is difficult to form a broad distribution of protonated water cluster structures in a crystalline ice lattice because of the strong geometric constraints. Nevertheless, the continuum band appears for both crystalline ice and ASW.8,10,13−16 Here we suggest the possibility that CDH produces the continuum band without large rearrangement of the hydronium molecular structure in the lattice. As discussed above, an excess proton is dynamically delocalized in the CDH state as a result of the concerted oscillatory motion of protons in the water chain, rather than it moving along the chain in a stepwise fashion.30 Under this situation, it is likely that the charge of the excess proton is delocalized over water molecules in the chain rather than localized at the position of a migrating hydronium. Also, the concerted motion of protons in the water chain may create a condition such that the extent of charge transfer at an instant moment differs for each hydrogen bond and varies progressively along the length of the water chain. The ensemble of molecular systems with such varying degrees

in an extended network of proton-transmitting water cluster. In this case, water molecules of the cluster may have certain hydronium characteristics. This explains why the loss of the ν(O−D) intensity of D2O, shown in Figure 2, was several times greater than that expected from the 1:1 stoichiometric reaction between excess proton and D2O molecule (see the Supporting Information). Heating a sample activates the reorientational motion of water and results in the formation of isolated HDO species in the sample. Figure 5c illustrates one plausible mechanistic path of this process. The reorientation of water (HDO marked by a dotted circle) coupled to proton transfer relocates H+ from the original proton-transmitting chain to a new one.27,28 Recurrence of the hop-and-turn process eventually leads to the formation of isolated HDO species (marked by a dotted ellipse) that is separated from the proton-transmitting chain.27,28 To check this interpretation, we examined the ν(O−H) intensity of isolated HDO species as a function of sample temperature, and the results are shown in Figure 6. At 60−100

Figure 6. Change of ν(O−H) band intensity (3300 cm−1) of vibrationally decoupled HDO species as a function of the sample temperature. The sample composition was 4% TFA and 96% D2O.

K, the ν(O−H) intensity was weak and exhibited a low growth rate. This small ν(O−H) signal may be due to impurity HDO species in the sample. Control experiments showed that the adsorption of residual HDO gas on the sample surface could account for 70% of the observed small ν(O−H) intensity. Besides, the exothermic energy of acid ionization (local heating effect) could generate some HDO species in the sample. Isothermal kinetic measurement showed that the ν(O−H) intensity did not noticeably increase with time at the low temperatures. Increase in the ν(O−H) intensity became large above 120 K. This behavior supports the interpretation that HDO formation is related to water reorientation, the onset temperature of which is known to be ∼115 K at the ASW surface and ∼135 K in the bulk.27,31,32 We mention in passing that the HDO unit in structure II (Figure 5) is not the isolated HDO species. Being a part of the charge-delocalized extended structure, this HDO unit may have a certain degree of hydronium (HD2O+) character. Accordingly, its ν(O−H) frequency is expected to appear in the Zundel continuum region, as will be discussed shortly, rather than at the position of isolated HDO species (∼3300 cm−1). Indeed, the 3300 cm−1 band intensity is very small at 60 K, although TFA fully ionizes to structure II at this temperature. 12846

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(3) Eigen, M. Proton Transfer Acid-Base Catalysis + Enzymatic Hydrolysis.I. Elementary Processes. Angew. Chem., Int. Ed. Engl. 1964, 3, 1−19. (4) Voth, G. A. Computer Simulation of Proton Solvation and Transport in Aqueous and Biomolecular Systems. Acc. Chem. Res. 2006, 39, 143−150. (5) Stoyanov, E. S.; Stoyanova, I. V.; Reed, C. A. The Structure of the Hydrogen Ion (H-Aq(+)) in Water. J. Am. Chem. Soc. 2010, 132, 1484−1485. (6) Leuchs, M.; Zundel, G. Polarizable Acid−Water Hydrogen Bonds with Aqueous Solutions of Carboxylic Acids. J. Chem. Soc., Faraday Trans. 2 1980, 76, 14−25. (7) Lee, D. H.; Bang, J.; Kang, H. Surface Charge Layer of Amorphous Solid Water with Adsorbed Acid or Base: Asymmetric Depth Distributions of H+ and OH− Ions. J. Phys. Chem. C 2016, 120, 12051−12058. (8) Parent, P.; Lasne, J.; Marcotte, G.; Laffon, C. HCl Adsorption on Ice at Low Temperature: A Combined X-Ray Absorption, Photoemission and Infrared Study. Phys. Chem. Chem. Phys. 2011, 13, 7142. (9) Kang, H.; Shin, T.; Park, S.; Kim, I.; Han, S. Acidity of Hydrogen Chloride on Ice. J. Am. Chem. Soc. 2000, 122, 9842−9843. (10) Devlin, J. P.; Uras, N.; Sadlej, J.; Buch, V. Discrete Stages in the Solvation and Ionization of Hydrogen Chloride Adsorbed on Ice Particles. Nature 2002, 417, 269−271. (11) Devlin, J. P.; Kang, H. Comment on″ HCl Adsorption on Ice at Low Temperature: A Combined X-Ray Absorption, Photoemission and Infrared Study″ by P. Parent, J. Lasne, G. Marcotte and C. Laffon. Phys. Chem. Chem. Phys. 2012, 14, 1048; Phys. Chem. Chem. Phys. 2012, 14, 1048−1049. (12) Parent, P.; Lasne, J.; Marcotte, G.; Laffon, C. Reply to the ‘Comment on “HCl Adsorption on Ice at Low Temperature: A Combined X-Ray Absorption, Photoemission and Infrared Study”’by Jp Devlin and H. Kang. Phys. Chem. Chem. Phys. 2012, 14, 1050; Phys. Chem. Chem. Phys. 2012, 14, 1050−1053. (13) Iftimie, R.; Thomas, V.; Plessis, S.; Marchand, P.; Ayotte, P. Spectral Signatures and Molecular Origin of Acid Dissociation Intermediates. J. Am. Chem. Soc. 2008, 130, 5901−5907. (14) Marchand, P.; Marcotte, G.; Ayotte, P. Spectroscopic Study of HNO3 Dissociation on Ice. J. Phys. Chem. A 2012, 116, 12112−12122. (15) Ayotte, P.; Marchand, P.; Daschbach, J. L.; Smith, R. S.; Kay, B. D. HCl Adsorption and Ionization on Amorphous and Crystalline H2O Films Below 50 K. J. Phys. Chem. A 2011, 115, 6002−6014. (16) Ayotte, P.; Rafiei, Z.; Porzio, F.; Marchand, P. Dissociative Adsorption of Hydrogen Fluoride onto Amorphous Solid Water. J. Chem. Phys. 2009, 131, 124517. (17) Buch, V.; Sadlej, J.; Aytemiz-Uras, N.; Devlin, J. P. Solvation and Ionization Stages of Hci on Ice Nanocrystals. J. Phys. Chem. A 2002, 106, 9374−9389. (18) Petrenko, V. F.; Whitworth, R. W. Physics of Ice; Oxford University: Oxford, England, 1999; pp 73−78. (19) Moon, E.-s.; Kim, Y.; Shin, S.; Kang, H. Asymmetric Transport Efficiencies of Positive and Negative Ion Defects in Amorphous Ice. Phys. Rev. Lett. 2012, 108, 226103. (20) Moon, E. S.; Lee, C. W.; Kang, H. Proton Mobility in Thin Ice Films: A Revisit. Phys. Chem. Chem. Phys. 2008, 10, 4814−4816. (21) Shin, S.; Kim, Y.; Kang, H.; Kang, H. Effect of Electric Field on Condensed-Phase Molecular Systems. I. Dipolar Polarization of Amorphous Solid Acetone. J. Phys. Chem. C 2015, 119, 15588−15595. (22) Haq, S.; Harnett, J.; Hodgson, A. Growth of Thin Crystalline Ice Films on Pt(111). Surf. Sci. 2002, 505, 171−182. (23) Cholette, F.; Zubkov, T.; Smith, R. S.; Dohnálek, Z.; Kay, B. D.; Ayotte, P. Infrared Spectroscopy and Optical Constants of Porous Amorphous Solid Water. J. Phys. Chem. B 2009, 113, 4131−4140. (24) Shin, S.; Kang, H.; Cho, D.; Lee, J. Y.; Kang, H. Effect of Electric Field on Condensed-Phase Molecular Systems. II. Stark Effect on the Hydroxyl Stretch Vibration of Ice. J. Phys. Chem. C 2015, 119, 15596− 15603.

of polarized hydrogen bonds may be able to produce the continuous IR absorption.

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CONCLUSIONS (i) TFA dissociates almost completely in ASW at cryogenic temperatures, at which the thermal diffusion or the reorientational motion of water is prohibited. This indicates a significant increase in the acidity of TFA in ASW as compared with that in aqueous solution at room temperature. Acetic acid does not dissociate in ASW under similar conditions. (ii) The excess protons released from TFA are mobile in the lattice at low temperatures via the Grotthuss mechanism. This leads to the formation of a CDH state, which is characterized by charge delocalization in the water chain as a result of the concerted oscillatory motion of protons of water molecules in the chain. We propose that this CDH state may be responsible for the Zundel continuum absorption. (iii) Water reorientation is activated at temperatures above 120 K, which breaks the proton-transfer water bridge between hydronium and conjugate ions. The hydronium and conjugate ions are separated by the recurring hop-and-turn mechanism. This process gives rise to a vibrationally isolated HDO species in the D2O lattice, which is detected by the ν(O−H) stretching band at ∼3300 cm−1. (iv) The present study highlights the difference between the dissociation mechanisms of acids and aprotic electrolytes. In the case of the acid dissociation in ASW, the energy barrier for conversion between CIP and SSIP is almost wiped out because of the dynamic delocalization of an excess proton in the lattice, although this barrier is significant in the dissociation dynamics of aprotic electrolytes in aqueous solutions. Because rapid proton transfer via the Grotthuss mechanism is a common feature for both ASW and liquid water, we consider that certain features of the acid dissociation mechanism discussed in this work for ASW may be applicable to aqueous solutions as well.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03415. Absorbance of the ν(O−D) band of D2O in TFA−D2O mixture and pure D2O samples. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: surfi[email protected]. Tel: +82-2-875-7471. ORCID

Heon Kang: 0000-0002-7530-4100 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Samsung Science and Technology Foundation (SSTF-BA1301-04). REFERENCES

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DOI: 10.1021/acs.jpcc.7b03415 J. Phys. Chem. C 2017, 121, 12842−12848