Electric Field Effect on Condensed-Phase Molecular Systems: V. Acid

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Electric Field Effect on Condensed-Phase Molecular Systems VI. Acid-Base Proton Transfer at the Interface of Molecular Films Sunghwan Shin, Youngwook Park, Hani Kang, and Heon Kang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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The Journal of Physical Chemistry

J. Phys. Chem. C (Revised Version)

Electric Field Effect on Condensed-Phase Molecular Systems VI. Acid-Base Proton Transfer at the Interface of Molecular Films

Sunghwan Shin, Youngwook Park, Hani Kang, and Heon Kang* Department of Chemistry, Seoul National University, 1 Gwanak-ro, Seoul 08826, South Korea *Corresponding author: [email protected], +82-2-875-7471 (Heon Kang)

Abstract In this work, we studied the effect of an applied electric field on the dissociation of acetic acid (AA) at the interface of the acid and ammonia molecular layers using the ice film capacitor method. The dissociation of AA was monitored by reflection absorption infrared spectroscopy, which measured the changes in the intensities of the vibrational modes of AA and the acetate anion. The extent of field-induced dissociation was found to be dependent on the direction of the applied electric field. Interestingly, the extent of acid dissociation was greater when the field was applied in the opposite direction to that of the proton transfer motion. This suggested that the reaction was aided by the reorientation of the reagent molecules rather than by electrostatic stabilization of the charge transfer energy. The effect of molecular size was studied by comparing the results for AA with those of formic acid and propionic acid. The dissociation of smaller acids was more strongly enhanced than that of larger acids, supporting the interpretation that the field-induced dissociation occurred via molecular reorientation.

I. Introduction

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The effect of externally applied electric fields on chemical reactions has been studied for a long time. For example, in the 1920s, Wien1 observed that the dissociation constants of weak electrolytes increased at high electric fields. This phenomenon, known as the second Wien effect, was explained by Onsager2 using a theoretical model that accounts for the probability of the recombination of dissociated ions via diffusion under the influence of an electric field. Simons3 studied the proton transfer between water and ion-exchange membranes and observed a significant enhancement of ionic current in the presence of an external electric field. It was claimed that proton transfer was accelerated by the alignment of water molecules along the direction of the electric field at the membranes and/or the second Wien effect. This observation, however, was not investigated further in the research community of physical chemistry. Recently, Stuve4 studied the emissions of field-ionized hydrated proton clusters [H+(H2O)n] from a Pt tip coated with a water layer. The observations suggested an increased degree of autoprotolysis of water under the extraordinarily strong fields (~1010 V m−1) present at the tip apex. Boxer and coworkers5 studied the vibrational Stark effect of molecules in frozen films under strong electric fields (≤ 1×108 V m−1), and found that electric fields at the active sites of enzymes play a crucial role in a ketosteroid isomerase reaction by stabilizing a transition state complex. Despite many reports to date that electric fields can influence chemical reactions, the operating mechanism of this effect is not clearly understood in many cases. In the case of acid-base reactions, it would be interesting to investigate how an applied electric field affects the proton transfer from the acid to the base. For instance, does the field change the electrostatic energy of the proton displacement from the acid to the base, or does it affect the reaction by some other means? In this paper, we attempted to answer this question by preparing a sample of stacked molecular films of carboxylic acids and ammonia and applying a high electric field (~108 V m–1) across the sample using the ice film capacitor method.6-8 Infrared absorption spectroscopy was used to evaluate the effect of the applied field on the reactions by monitoring the field-induced changes in the reaction yields.

II. Experimental methods All experiments were conducted in an ultra-high-vacuum (UHV) chamber with a 2


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background pressure of below 1×10−10 Torr, which was equipped with instruments for reflection absorption infrared spectroscopy (RAIRS), Kelvin work function measurements, temperature-programmed desorption (TPD) mass spectrometry.6 A molecular film sample was deposited on a Pt(111) single crystal surface maintained at 70 K inside the UHV chamber. The Pt(111) surface was cleaned using sputtering and annealing procedures, and its cleanliness was verified from the TPD profile of the D2O monolayer formed on the surface.9 Acetic acid (CH3COOH, AA), formic acid (HCOOH, FA), propionic acid (CH3CH2COOH, PA), and NH3 vapors were introduced close to the Pt(111) substrate surface by using a tube doser and deposited at rates below 0.1 ML·s−1 (monolayer per second). The D2O film was deposited using a backfilling method at a deposition rate below 0.2 ML·s−1. The thicknesses of the molecular films were estimated from the TPD measurements. The thickness of the D2O film was determined by comparing the intensity of its TPD signal with that of the D2O monolayer on Pt(111).9 For the acid and NH3 films, mole-to-thickness conversion was performed by using the unit cell volumes of their crystals.10-13 The film thicknesses have been expressed in units of ML in this paper, where 1 ML represents 1.1×1015, 4.7×1014, 3.5×1014, 3.1×1014, and 7.8×1014 molecules cm−2 for D2O, FA, AA, PA, and NH3, respectively. Samples composed of multiple stacks of the carboxylic acids, NH3, and D2O were used to prepare an acid-base interface. The acid-base layered film was sandwiched between two amorphous solid D2O films to prevent direct contact with the Pt substrate or cesium ions. In order to prepare the sample, a 41 ML-thick D2O amorphous solid water (ASW) film was grown on a Pt(111) substrate at 70 K. Subsequently, a 12 ML-thick NH3 film was grown over the ASW film at the same temperature, followed by overlaying of a 20 ML-thick acid film on the NH3 film. Finally, a 41 ML-thick D2O film was used to cover the acid film. In this paper, the prepared D2O/acid/NH3/D2O/Pt(111) layered film has been referred to as an “acid (top)/NH3 (bottom)” film. Electric fields were applied across the frozen molecular films using the ice film capacitor method, which has been described in detail in a previous paper.6, 14 To increase the field strength, Cs+ ions with low incidence energy (< 10 eV) were deposited onto the D2O film surface of the ice film capacitor. To decrease the field strength, low energy (5 eV) electrons were sprayed onto the sample surface to neutralize the positive charges.15 For a sample 3



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composed of different dielectric layers, the electric field strength (F) inside a certain layer, for example, the acid layer, can be expressed by Eq. (1).

= ΔCPD/[ + + ]

(Eq. 1)

In the above equation, ∆CPD is the voltage across the whole film, , , and are the thicknesses of the acid, NH3, and D2O films, respectively, and

,

,

and

refer to the corresponding dielectric constants. Since the dielectric constants for frozen acids and NH3 films have not been reported, they were estimated using the Clausius-Mossotti relation (Eq. 2): ! "# !

%$=

&'

(Eq. 2)

()

where * is the polarizability16 and + is the number density of the molecule. The applicability of this relation was checked for solid D2O. Eq. (2) gives εr = 1.6 for amorphous solid D2O with its density of 0.7 g cm−3 at 70 K, which is close to the previously reported value (εr = 2).17 The closeness of these two values suggests that our estimation is acceptable in the case of the frozen films where dipolar reorientation did not occur. The electric field at the interface of the two films was assumed to be equal to the average value of the electric field strengths experienced by the two films. RAIRS measurements were performed at a grazing angle (84°) reflection geometry using a commercial Fourier-transform infrared spectrometer with a mercury-cadmium-telluride detector. The incident IR beam was linearly p-polarized using a wire grid polarizer. The beam path outside the UHV chamber was purged with dry N2 gas to remove other IR active gases. RAIR spectra were averaged 256 times at a spectral resolution of 4 cm−1.

III. Results We prepared various samples containing molecular and ionized forms of AA and recorded their RAIR spectra in order to use them as spectral references for monitoring the acid-base reaction. The AA molecules dissociated into acetate (AA–) and ammonium (NH4+) ions on contact with NH3 molecules. This ionic dissociation of AA was studied by monitoring the 4


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changes in the vibrational modes of AA and AA–, which are displayed in Figure 1. The peak positions of each vibrational mode were consistent with those reported in previous studies.1819

The pure AA film showed a carboxyl double bond stretching mode (ν(C=O)) at ~1727 cm–1

(Figure 1(a)) and a hydroxyl (ν(OH)) stretching mode in the range 3100–3200 cm–1 (outside the displayed range). The existence of the ν(C=O) and ν(OH) modes indicates that the AA molecules existed in their molecular form in the pure AA film. In the case of the 1:20 AANH3 mixture, as shown in Figure 1(b), these ν(C=O) and ν(OH) modes disappeared and a new peak was observed at ~1565 cm–1, which was assigned to the antisymmetric CO stretching (ν(CO)) in AA–.19 These vibrational features indicate that AA molecules dissociated to AA– ions upon the solvation by NH3 molecules in the mixture sample.

(a)

ν(C=O) of AA

δs(CH3) of AA AA (25 ML) δ (CH ) of AA

0.02

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


as

(b)

-

3

δ(COH) of AA

ν(CO) of AA

AA in NH3

(c)

AA (25 ML) ν2 of NH3

1800

1700

NH3(12 ML)

1600

1500

1400

-1

Wavenumber (cm )

Figure 1. RAIR spectra of various AA samples displayed in the range from 1900 to 1350 cm– 1

. Each sample was prepared on a Pt(111) surface at 70 K. (a) Pure AA film (25 ML) (b) AA-

NH3 mixture film with the AA:NH3 molar ratio of 1:20. (c) AA (top)/NH3 (bottom) sample, which has a structure of D2O (41 ML)/AA (25 ML)/NH3 (12 ML)/D2O (41 ML)/Pt(111). 5



The IR spectrum of the AA (top)/NH3 (bottom) layered film is shown in Figure 1(c), which was sandwiched between two D2O space layers. This sample showed a strong ν(C=O) absorption band of AA and a weak ν(CO) absorption band of AA–. The peak at ~1652 cm–1 corresponds to the NHN scissoring mode (ν2) of NH3. The ν(CO) absorption of AA– originated from the AA– species formed at the AA-NH3 interface rather than those at the D2O-AA interface because this band was absent from a D2O/AA/D2O sandwiched film (see Supporting Information, Figure S1). We estimated the thickness of the reactive boundary region of the NH3-AA interface, where the acid-base reaction between AA and NH3 occurred spontaneously. The changes in intensities of the ν(CO) peak in AA– and the ν(C=O) peak in AA were measured as a function of the thickness of the AA film deposited on the NH3 film. In Figure 2(a), the legend “x ML → y ML” difference spectrum indicates the changes in the absorbance when the thickness of the AA film was increased from x ML to y ML.

6


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(a)

6.3 ML → 8.8 ML

Absorbance

5 ML → 6.3 ML ν(C=O) of AA

3.8 ML →

0.003

5 ML

2.5 ML → 3.8 ML 1.3 ML → 2.5 ML ν(CO) of AA

2000

1800

1600

0 ML → 1.3 ML

1400

1200

-1

Wavenumber (cm ) 0.2

(b) ν(C=O) of AA

-1

Peak area (A cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-

ν(CO) of AA

0.1 AA (x ML) NH3 (12 ML)

0.0 0

2 4 6 AA thickness (ML)

8

Figure 2. (a) Difference RAIR spectra of the samples with varying thicknesses of the AA film on the NH3 film. “x ML → y ML” indicates the difference in the absorbance when the AA film thickness was increased from x ML to y ML. (b) Changes in intensity of the ν(C=O) and ν(CO) bands of AA and AA–, respectively, as a function of the AA film thickness.

When the AA film was deposited for a small thickness (1.3 ML) on the NH3 film, the intensity of the ν(CO) band of AA– increased, while that of the ν(C=O) band corresponding to AA did not show any noticeable increase. This indicated that most of the AA molecules present on the NH3 surface were dissociated. As the thickness of the AA film was increased, 7



the intensity of the molecular ν(C=O) band started to increase, showing that some of the AA molecules remained undissociated on the film. These AA molecules could either have been located within the molecular AA film away from the NH3 surface, or existed on the NH3 surface but with unfavorable molecular orientations for proton transfer to NH3. The intensity of the ν(CO) peak of AA– did not further increase beyond the AA film thickness of 5 ML, implying that the NH3 surface was fully covered with AA molecules and that only molecular AA species were present in the film above this thickness. We did a control experiment to measure changes in the intensities of ν(C=O) of AA and ν(CO) of AA– due to interdiffusion in the absence of an applied field. The intensities of these bands did not change for a period of 30 min at 70 K. The observations indicate that the stacked AA and NH3 films had a wellpreserved interfacial structure without significant diffusional mixing of two species at the present experimental temperature of 70 K. From the saturated absorbance of the ν(CO) peak of AA–, the population of the AA– molecules at the interface was estimated to be 1.4×1015 molecules cm–2, which was four times larger than that of the AA monolayer.11 Therefore, we estimate that the thickness of the reactive boundary phase of the interface was about 4 ML. The estimated thickness includes the effect of wrinkled structures of the interface, which may increase the effective interfacial area and thus the interfacial population of AA–. We applied an electric field to the AA-NH3 layered sample by incorporating this sample into the structure of an ice film capacitor,6 as mentioned in Section II. In our experimental setup, the direction of the applied electric field was fixed from the sample/vacuum interface, where Cs+ ions were deposited, to the sample/metal interface, where negative counter charges were drawn. In order to change the direction of the applied field, the stacking sequence of the AA and NH3 films in the sample was reversed. The field direction has been indicated by marking the bias polarity of the samples, for instance, “(+)NH3/AA(–)” represents a charged NH3 (top)/AA (bottom) film and “(+)AA/NH3(–)” represents a charged AA (top)/NH3 (bottom) film. Figure 3(a) shows the RAIR spectrum of an NH3 (top, 12 ML)/AA (bottom, 25 ML) layered film and a difference spectrum at F = 1.2×108 V m–1. The difference spectrum, which is defined as the difference in the absorbances between the field-on and field-off spectra, indicates that the absorbance of the CO stretching mode of AA– increased and that of the ν(C=O) mode of AA decreased on applying an electric field. These changes indicate that the 8


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neutral AA molecules were dissociated into acetate ions by the external electric field at the reactive boundary region. This dissociation was irreversible when the field strength was decreased from 1.2×108 to 4×107 V m–1 (not shown). Relative absorbance changes in the ν(CO) band of AA– at different field strengths are shown in Figure 3(b) for the (+)NH3/AA(–) and (+)AA/NH3(–) films. The change in the AA– band intensity can be well separated from changes in the neutral AA and NH3 band intensities in the field difference spectra (Figure S3 in Supporting Information) and can provide a reliable indicator of the extent of acid dissociation. The entire RAIR spectra and their difference spectra are given in the Supporting Information (Figure S3). Both the NH3 (top)/AA (bottom) and the AA (top)/NH3 (bottom) layered films show that the extent of AA dissociation increased with increasing field strength.

9



(a) RAIR spectrum of NH3/AA layer ν(C=O) of AA

Absorbance

0.01

NH3 (12 ML) Field - AA (25 ML)

ν(CO) of AA

Difference spectrum (x10)

1800

1700

1600

1500

1400

-1

Wavenumber (cm )

Relative absorbance change (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

50 (b)

40

(+)NH3/AA(-) (+)AA/NH3(-)

30 20 10 0 0.0

0.5

1.0 8

-1

Electric field (10 V m )

Figure 3. (a) (Top) RAIR spectrum of the NH3 (top, 12 ML)/AA (bottom, 25 ML) layered film. (Bottom) Field-on minus field-off difference spectrum, which was magnified by a factor of ten. (b) Relative absorbance changes in the ν(CO) of AA– as a function of the applied electric field strength. To change the direction of the applied field, the stacking sequence of the AA and NH3 films was reversed. The field directions are indicated as (+)NH3/AA(–) and (+)AA/NH3(–).

However, the extent of AA dissociation was different depending on the field direction. For example, at the field strength of 1.2×108 V m–1, the relative absorbance of the ν(CO) peak increased by 38% for (+)NH3/AA(–), but only by 13% for (+)AA/NH3(–). The thickness of 10


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the reactive boundary region must be similar for two samples. Indeed, the RAIR spectra were almost identical for the NH3 (top)/AA (bottom) sample (Figure 3(a)) and the AA (top)/NH3 (bottom) sample (Figure 1(c)) at zero field, with their ν(CO) peak absorbances being almost equal and within ±10% of each other. Therefore, it was reasonable to conclude that the different degrees of the field-induced AA dissociation observed for two samples was a result of the direction of the electric field. The vibrational Stark effect (VSE) was also observed for the vibrational bands of AA and NH3 under the external electric field (Supporting Information, Figures S1 and S2). Unlike the effect of the field on the acid dissociation reaction, the VSE was reversible with respect to an increase or decrease in the field. In general, the VSE appears as band broadening owing to the ensemble average of Stark frequency shifts of randomly oriented molecules.20-22 The present molecular film samples exhibited interesting but complex spectral changes as a result of the VSE. Quantitative analysis of these spectral changes is beyond the scope of the current work. Formic acid (FA) and propionic acid (PA) samples were used in place of AA to study the molecular size dependence of the field-induced acid dissociation. Figure 4(a) shows the RAIR spectra of the NH3 (top)/FA (bottom) and NH3 (top)/PA (bottom) layered films, and their difference spectra at an electric field strength of 1.4×108 V m–1. The positions of the ν(C=O) peaks of FA and PA are 1737 and 1721 cm–1, respectively. These carboxylic acids also dissociated at the acid-NH3 interface and produced formate (FA–) and propionate (PA–) ions. The ν(CO) peak positions of FA– and PA– are 1595 cm–1 and 1560 cm–1, respectively.

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(a) NH3/FA -

ν(CO) of FA

Absorbance

ν(C=O) of FA

Difference spectrum (x4) ν(C=O) of PA

NH3/PA

Difference spectrum (x4)

1800

-

ν(CO) of PA

1700

1600 -1

Wavenumber (cm )

Relative absorbance change (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(b)

60

(+)NH3/FA(-) (+)NH3/AA(-) (+)NH3/PA(-)

40

20

0 0.0

0.5

1.0 8

1.5 -1

Electric field (10 V m )

Figure 4. (a) (Top) RAIR and field difference spectra of the NH3 (top, 12 ML)/FA (bottom, 19 ML) layered film at F = 1.3×108 V m–1. (Bottom) RAIR and field difference spectra of the NH3 (top, 12 ML)/PA (bottom, 20 ML) layered film at F = 1.3×108 V m–1. The difference spectra are magnified by a factor of 4. (b) Relative absorbance changes in the ν(CO) peaks of FA–, AA–, and PA– as a function of applied field strength. The data for AA is identical to that shown in Figure 3(b).

When an electric field was applied to these films, field-induced dissociation occurred similarly as observed for the AA-NH3 layered film. Figure 4(b) shows the changes in the 12


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relative absorbance of carboxylate anions at different field strengths in various carboxylic acid-NH3 layered films. For all carboxylic acids, the field-induced dissociation was enhanced by an increase in the field strength. Formic acid showed the largest enhancement of about 63% at 1.3×108 V m–1, whereas propionic acid exhibited the lowest enhancement (about 34%) at a similar field strength. The entire RAIR spectra and their difference spectra are reported in Supporting Information, Figure S4.

IV. Discussion The present experiment showed that an external electric field increased the degree of dissociation of acids in the acid-base interfacial regions. As a plausible explanation for this phenomenon, we may consider that the applied field influences the electrostatic energy of the proton transfer from the acid to the base.23-24 Figure 5(a) illustrates this electrostatic mechanism. When the direction of the H-bond of the AA-NH3 pair is properly oriented along the field, the electrostatic interaction between the partially charged transition state (TS) and the external field will lower the energy barrier of proton transfer and stabilize the product. This direct field-induced mechanism was proposed to explain the molecular isomerization in the active sites of proteins5 (field catalysis mechanism) and the field-induced ionization phenomena at a sharp tip of a scanning tunneling microscope.25 However, this mechanism is not consistent with our experimental results. According to this mechanism, the AA molecules in the (+)AA/NH3(–) sample would dissociate more easily than those in the (+)NH3/AA(–) sample because of the favorable direction of the applied electric field. In contrast, the AA molecules in the (+)NH3/AA(–) sample dissociated more efficiently (Figure 3(b)), which indicates that the electrostatic mechanism is not the major mechanism of the field-induced acid dissociation at the molecular film interfaces.

13



Figure 5. Pictorial illustration of the field-induced acid dissociation mechanisms. (a) Electrostatic mechanism. When the OH…N hydrogen bond of the AA-NH3 pair is properly oriented along the field direction, the field facilitates the proton transfer from AA to NH3 by stabilizing the charged transition state and the product with electrostatic interaction energy (Stark potential, indicated by dotted lines). (b) Reorientation mechanism. The field reorients the AA (or NH3) molecule to generate a favorable geometry of the AA-NH3 pair for spontaneous proton transfer. Note that the favorable field directions are opposite in the cases of electrostatic and reorientation mechanisms. The dipole moment of AA is indicated by -AA.

An alternative possibility is that the acid dissociation occurs via reorientation of molecules under the applied field. Figure 5(b) illustrates the reorientation mechanism. For acid dissociation to occur, the adjacent AA and NH3 molecules should have a suitable orientation such that the OH group of AA points toward the lone electron pair of NH3. However, a substantial portion of the AA-NH3 pairs in the reactive boundary region did not possess such 14


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favorable orientations because the samples were prepared by a random deposition of gas molecules on a cold substrate surface. The activation barrier for proton transfer is very large for typical AA-NH3 pairs under this situation. By applying an external electric field, a certain portion of the AA molecules can be reoriented as a result of the molecular dipole-field interaction, to bring its OH group close to the lone pair of NH3. In this favorable configuration, proton transfer from AA to NH3 is energetically downhill and can occur spontaneously and irreversibly. This mechanism is consistent with our experimental results with regard to the direction of the field. In support of the reorientation mechanism, it has been shown that the reorientation of small polar molecules can occur under external electric field strength of 108 V m–1 in frozen molecular films.14, 26 Without the external electric field, the molecules are frozen in position at 70 K. The significant dependence of the field-induced acid dissociation on the molecular size, as shown in Figure 4(b), is also consistent with the reorientation mechanism. The observed field-induced effect is in the order FA > AA > PA (Figure 4), i.e., smaller acids exhibit a stronger effect because of easier reorientation by the field. If we consider the energy lowering effect by the electrostatic mechanism, an applied field in the order of 108 V m–1 can change the electrostatic energy of ionic bond dissociation by roughly ~1 kJ mol–1. This energy change is very small compared to the whole reaction energy of acid dissociation and will not significantly affect the reaction. It is expected that a much stronger electric field (109–1010 V m–1) would be required to observe the occurrence of the electrostatic mechanism.4-5, 27 A small increase in acid dissociation was observed even when the field-induced reorientation of AA was expected to produce the opposite effect, for example, for the (+)AA/NH3(–) sample. The wrinkled, rather than flat, structure of the AA/NH3 interface may explain such deviation from ideal behavior. As discussed in Figure 2, the effective area of the interface was large, which means that the AA-NH3 pairs were not as perfectly aligned as expected for an ideally flat interface. In this case, a small portion of the pairs may sit in reversed position, and their reactions can be triggered even by the opposite field. In addition, the reorientation of NH3 molecules may also occur and contribute to the acid dissociation. The field-induced reorientation of NH3 will facilitate proton transfer at the interface of the (+)AA/NH3(–) sample, whereas it will suppress proton transfer at the (+)NH3/AA(–) interface. 15



Because a greater extent of AA dissociation was observed for the latter in Figure 3(b), the effect of reorientation of AA must be larger than that of NH3 reorientation, although the latter may also occur to a certain extent. The preferential occurrence of the indirect reorientation mechanism over the direct electrostatic mechanism in the acid dissociation may be discussed from the viewpoint of reaction dynamics. Thermal reorientation of the molecules is a rare event in the molecular films at low temperature. On the other hand, once a favorable acid-base configuration is established by molecule reorientation, proton transfer from the acid to the base occurs instantaneously. Therefore, the molecule reorientation is the rate-determining step of the acid dissociation reaction when the reaction occurs thermally in the absence of external electric field, for instance, by heating the frozen molecular film to activate the molecular motion. This situation is somewhat analogous to electron transfer reactions in solutions, where electron transfer occurs much faster than solvent reorganization. In this case, the solvent motion becomes the rate-determining reaction coordinate and the solvent reorganization energy becomes the overall activation energy, according to Marcus theory.28 For reactions in these situations, the perturbation effect of an applied field, if it occurs at all, would be the most apparent when it occurs for the rate-determining step, which is the molecule reorientation in the present case of the acid dissociation reactions. On the other hand, the effect of the applied field on the proton transfer step would not contribute to the overall reaction efficiency to any significant degree. Our results seem to indicate that the external field increases the efficiency of the reactions in the condensed phase by affecting the kinetics of the slow event, rather than the energetics.

V. Conclusion We studied the effect of an external electric field on the proton transfer between carboxylic acids and ammonia molecules located at the interface of the acid and base layers. The applied field increased the degree of dissociation of the carboxylic acids. The extent of field enhancement of acid dissociation was found to be dependent on the direction of the applied field and the size of the acid molecules. A greater enhancement effect was observed when the direction of applied field was opposite to the direction of proton displacement. Furthermore, a 16


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smaller acid exhibited a larger enhancement effect. These observations indicated that the field-enhanced proton transfer from the acid to the base does not occur via a direct electrostatic mechanism, in which the Stark interaction potential lowers the energy barrier of the proton transfer. Instead, the field enhancement occurs via an indirect mechanism, in which the field reorients the acid and/or base molecules at the interface to trigger a spontaneous proton transfer between them. Therefore, it appears that the applied field affects the kinetics of the slow event of reactions, rather than the energetics, to increase the overall reaction efficiency.

Supporting Information Absorption spectra and field difference spectra of AA, NH3/AA, NH3/FA, NH3/PA samples.

Acknowledgment This work was supported by Samsung Science and Technology Foundation (SSTF-BA130104).

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TOC Graphic

19



Fig. 1 286x199mm (300 x 300 DPI)


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Fig. 2 199x286mm (300 x 300 DPI)



Fig. 3 199x286mm (300 x 300 DPI)


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Fig. 4 199x286mm (300 x 300 DPI)



Fig. 5 254x190mm (96 x 96 DPI)


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TOC Figure 41x41mm (220 x 220 DPI)


Electric Field Effect on Condensed-Phase Molecular Systems: V. Acid

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