Adsorptions of Formic and Acetic Acids on Ice Surface: Surface

Feb 7, 2013 - But at the same time it reduces the chance of its deprotonation. Our ice model clearly demonstrated the significant effects of intrinsic...
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Adsorptions of Formic and Acetic Acids on Ice Surface: Surface Binding Configurations and a Possibility of Interfacial Proton Transfer Mahbubul Alam Shoaib and Cheol Ho Choi* Department of Chemistry and Green-Nano Materials Research Center, College of Natural Sciences, Kyungpook National University, Taegu 702-701, South Korea ABSTRACT: Adsorptions of formic (FA) and acetic acids (AA) on Ih ice surface were studied using quantum mechanical/ effective fragment potential (QM/EFP) scheme. Contrasting to the earlier studies in which trans-conformers were found as major surface configurations, our QM/EFP models found various cisand trans-conformers on ice surfaces with the cis-conformers being more stable. The surface binding energies and configurations were largely dependent on surface heterogeneity yielding the various surface conformers. In addition, the overall binding energies of acetic acid are slightly higher as compared to formic acid, implying the stabilization effect of methyl group. Our study also found a feasible deprotonation route of adsorbed transformic acid. In contrast, acetic acid prefers molecular form due to the unfavorable hydrophobic methyl group. Therefore it is interesting to note that the additional methyl group of acetic acid enhances surface binding energies. But at the same time it reduces the chance of its deprotonation. Our ice model clearly demonstrated the significant effects of intrinsic surface heterogeneity on the distributions of surface binding energies and configurations, which cannot be represented by small water clusters. acid adsorption on six water cluster. Zhou et al.12 also studied the general binding structures of formic acid with two water cluster. With the help of semiclassical molecular-dynamics models, adsorption of formic and acetic acid on hexagonal ice surface was studied and found that the two acid molecules are strongly trapped at the ice surface and that the incorporation of formic acid is favored when compared to acetic acid.13 So far, a majority of ice surface cluster models usually comprises of a small number of water molecules, which may not be sufficient to properly represent bulk ice. Methods based on periodic boundary conditions14−16 as well as QM/MM17 have been often utilized to include long-range effects. In order to properly model chemical reactions on ice surface, several characteristics of ice surface needs to be considered.18 As mentioned above, long-range electrostatics by bulk ice should be included. The electrostatic and polarization interactions play a vital role in various condensed phases containing polar and polarizable molecules. Polarization effects of water are particularly large, since its molecular dipole increases from 1.855 D for an isolated molecule to 2.6−3.2 D in condensed state.19 Since the electrostatic interactions are directional, it has been generally recognized that they are critical in the orderings of water clusters. Such long-range

I. INTRODUCTION By the discovery of the Antarctic Ozone Hole1 and the recognition that atmospheric ice particles2,3 can play a dominant role in determining the chemical composition of the atmosphere, the interest in the interactions of gases with ice surface has largely increased. Ice surfaces offer a unique reaction environment that is different from those formed by liquid water, gas, or even bulk ice.4,5 Although the rate of thermal reaction is expected to be much slower, experimental studies show that reactions occur even at substantially low temperatures at the surface of ice.4,5 The existence of small partially oxidized hydrocarbon (POH) such as formic and acetic acid can play a significant role in the chemistry of upper troposphere region, where cirrus clouds are formed.6,7 Cirrus clouds may provide an important mechanism for removal of these trace gases from upper tropospheric air masses. They are also the most abundant organic acids in the atmosphere.8 Both acids have relatively long photochemical lifetimes (of the order of weeks) and may contribute to acidity in precipitation. Furthermore, these acid may also undergo photo-oxidation and generates HOx radicals, which involve the production of ozone and atmospheric loss.9 In order to understand the interactions between these acids and water/ice in molecular level, Wei et al.10 studied the dimerization and double proton transfer of formic acid and calculated formic acid−water clusters up to four water molecules. Allouche et al.11 studied single and dimer formic © 2013 American Chemical Society

Received: January 5, 2013 Revised: February 4, 2013 Published: February 7, 2013 4181

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ice models. Structurally, various surface binding configurations of HOCl including unusual penta-coordination, which could not be observed with limited ice models, were observed. In general, it was found that the ice surface itself as well as HOCl adsorptions are strongly affected by long-range electrostatics, surface heterogeneity, and hydrogen disorders of bulk ice, revealing the unique and diverse characteristics of ice surface as a reacting environment. Full geometry relaxation at least near the adsorption sites should be also allowed. This aspect is especially important, since it significantly affects direct interfacial interactions between adsorbed molecules and ice surface, which are generally considered as the major contribution to the absorptions. In this paper, surface adsorptions of formic and acetic acid on ice were theoretically studied using our QM/EFP models. The energetics, surface binding configurations as well as possible subsequent deprotonations were systematically explored using our models of ice surface. In the section below, we first describe our cluster models. After that, we present and analyze chemical adsorptions of formic and acetic acids. As compared to formic acid, acetic acid has dual functional groups: hydrophilic by its carboxyl group and hydrophobic by its methyl tail. Therefore, comparatively studying the two species could yield new insights into the interfacial hydrophilic and hydrophobic interactions on ice surface.

electrostatic interactions are expected to be important in the formation of ice crystal, where the directional orderings of tetrahedral network are predominant. It is not unreasonable that the same electrostatic interactions can also affect the chemical absorptions on ice surface. In addition, hydrogen disorders of ice crystal should be taken into account. The structure of popular Ih ice is arranged on a hexagonal lattice. Each oxygen atom has four nearest neighbors at the corners of a regular tetrahedron. The hydrogen atoms are covalently bonded to the nearest oxygen to form H2O molecules, and these molecules are linked to one another by hydrogen bonds, each molecule offering its hydrogens to two other molecules and accepting hydrogen bonds from another two. As a result, there is no long-range order in the orientations of the H2O molecules or of the hydrogen bonds. These disorders are summarized by the ice rules.20,21 This fact makes the use of periodic boundary conditions on the ice crystal difficult. Kuo et al.22 also found static distortion even in the oxygen positions from their crystallographic positions. In short, disorders in the orientations of H2O molecules exist within the framework of directional orderings of tetrahedral network of ice. Therefore, delicate yet complex and collective electrostatic potential exist in ice crystal and play a vital role in the formation of its threedimensional ice structures. Because of this, theoretical approaches based on large clusters rather than the methods utilizing periodic boundary condition are preferred. In addition to electrostatic interactions and hydrogen disorders of ice, surface heterogeneity in which oxygen dangling bond (ODB) (Figure 1a) as well as hydrogen dangling bond (HDB) (Figure

II. THE ICE SURFACE MODEL The “full bilayer” termination of bulk-truncated unreconstructed (0001) surface of popular hexagonal crystalline (Ih) ice is mainly considered in our model. Detailed information of ice structure can be found in previous reviews.23,24 In fullbilayer termination, each outermost O atom in the upper layer is hydrogen-bonded to three neighbors of the lower layer. We preferred “full-bilayer” termination rather than “half bilayer” of (0001) surface, because the former is energetically favored due to larger number of hydrogen bonds. Energy difference between the two surfaces is about 30 kJ/mol per unit cell.25 When the (0001) surface is terminated as a full bilayer, the water molecules in outer layer have one dangling bond (either an ODB or an HDB) pointing outward as shown in Figure 1a,b, respectively. Because of their outermost positions on ice surface these two dangling bond sites can be considered as the primary reactive places. Their statistical distributions on ice surface represent the microscopic heterogeneity of ice surface, which can provide various reacting environments including multiple chemical interactions. A. The Reference Ice Crystal. We proposed a systematic strategy of building ice surface models that can satisfy the requirements as listed in the introduction. At first coordinates of a reference ice crystal of 36 hexagons by 15 hexagons and 5 full-bilayers deep are generated. By randomizing hydrogen bonding distributions we ensured that each layer of 36 by 15 will have a (near to) zero dipole moment. As a result, the entire ice model of 5 layers will also have zero dipole moment. The initial coordinates of water molecules in the reference ice crystal were obtained on the condition that oxygen−oxygen distances, O−H distances and O−O−O angles are experimentally found values of 2.75 Å, 1.00 Å, and 109.3°, respectively. B. Cluster Size Effects and QM/EFP Model. We systematically designed various ice models using the reference ice crystal in our previous paper and found that the binding energy of HOCl is strongly affected by the size of ice model.18 Therefore, the 484 waters were used in this study as shown in

Figure 1. Various ice models of (a) ODB with 4 water cluster, (b) HDB with 4 water cluster, and (c) 484 water cluster.

1b) are statistically distributed, needs to be properly modeled. Such microscopic heterogeneity of ice surface is a direct consequence of hydrogen disorders of ice crystal. It can provide diverse reacting environments including multiple interactions. In our previous study of HOCl adsorption on ice surface,18 these collective effects yielded much larger binding energies of HOCl than quantum mechanical estimations with limited size 4182

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Figure 1c. Since full quantum calculations on our large clusters are not practical, combinations of pure quantum mechanical methods and the effective fragment potential (EFP)26,27 method were adopted. Since detailed descriptions of EFP can be found elsewhere,27 the relevant aspects of EFP shall be briefly mentioned here. The EFP is a quantum mechanical polarizable force field with implicit charge transfer and exchange repulsion corrections. The method has been shown to reproduce the correct structure of liquid water28 and has been successfully applied for understanding the solvent-induced shifts in electronic spectra of uracil in water.29 Therefore EFP method contains the necessary terms in the study of electrostatic and polarization effects of ice surface. The EFP as parametrized with RHF theory was used throughout the current calculations. The particular combination of QM and EFP scheme in this paper is a three-layer model (QM relaxed region/QM fixed region/EFP water), where the coordinates in the QM relaxed region are fully relaxed during geometry optimization, while those of QM fixed and EFP regions are frozen (see Figure 2). The basic strategy behind our three-layer

Figure 3. Various ice surface models of (a) H0(6/6/472), (b) H1(6/6/ 472), (c) H2(6/6/472), and (d) H3(6/6/472). (The EFP waters are not presented for clarity).

computations were done without imposing symmetry unless otherwise specified. The GAMESS (general atomic and molecular electronic structure system)30 program was used for all of the computations. Basis set superposition error (BSSE) corrections on our models were performed with counterpoise method.31 Counterpoise (CP) correction including monomer deformations were applied using the following equation,

Figure 2. A diagram of relaxed QM/fixed QM/EFP scheme. The geometries of relaxed QM region are fully optimized, while those of fixed QM and EFP regions are also fixed during optimization.

scheme is the surface water(s) that is interacting with adsorbate and its first neighbor waters should be in QM Relaxed regions. The quantum atoms in QM Fixed region serve as a buffer between QM Relaxed and EFP regions. This model includes both the geometric relaxation effect due to the adsorptions and the long-range electrostatic interactions of ice crystals at the same time. On the basis of our three layer strategy, (6/6/472) models, which are comprised of 6 relaxed QM waters, 6 fixed QM waters, and 472 EFP waters, were designed. C. Hydrogen Disorder of Ice Crystal and Surface Heterogeneity. To take the hydrogen disorder effects of ice crystal into account, we prepared four ice models with different hydrogen disorders using hexagon (H) adsorption site. The corresponding QM regions of these four models are presented in Figure 3a−d, respectively, where the QM relaxed and QM fixed waters are indicated with “ball and stick” and “thick stick” representations, respectively. The hexagon sites are further categorized as HX (X = 0, 1, 2, 3) where X represents the number of HDB within the hexagon. Therefore, our ice surface shall be denoted as H0(6/6/472), H1(6/6/472), H2(6/6/472), and H3(6/6/472). For example, H2 indicates the hexagon site with two HDB at the upper layer of the bilayer.

CP AB AB AB ΔE bind (AB) = [EAB (AB) − EAB (A) − EAB (B)] A B (A) − EAA (A)] + [EAB (B) − E BB(B)] + [EAB

(1)

where the subscripts and superscripts denote the geometry and the basis, respectively. The chemical system considered is denoted by the symbols in the parentheses. Here A and B stand for acid and ice surface, respectively. Since the geometries of acid (A) and ice surface (B) in the complex (AB) are different from their isolated forms, the above equation was utilized to accurately perform the BSSE correction.

IV. RESULTS AND DISCUSSIONS A. Previous Studies on Formic and Acetic Acid Adsorption. Since formic and acetic acids have both carbonyl and OH functional groups, they can have up to two hydrogen bondings with ice surface. These interfacial hydrogen bondings were considered as the major binding interactions. Both formic and acetic acid has cis and trans conformational isomers in gas phase, with the trans-conformer being more stable by 4−6.1 kcal/mol.32,33 Trans-form has the acidic hydrogen aligned toward the carbonyl oxygen. Earlier studies of the binding energies between these acids with water clusters are presented in Table 1. In the case of formic acid, Wei et al.10 reported binding energies of −4.7 to −8.5 kcal/mol after the BSSE corrections. Zhou et al.12 also reported BSSE corrected values

III. COMPUTATIONAL DETAILS For the calculations of QM regions, MP2 theory was used in combination with 6-311++G(d,p) basis sets. All of the 4183

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dynamics38 yielded binding energy of −18.16 kcal/mol. Experimentally, von Hessberg et al.35 and Sokolov and Abbatt39 reported the adsorption enthalpy of 13.1 and 17.5 ± 2.8 kcal/ mol respectively. In general, the binding energy increases as the number of water increases. Also the binding energies of acetic acid appear to be slightly higher than those of formic acid. Furthermore, trans-isomer is highly preferred in both formic and acetic acids. The interfacial bonding distances of both acids are also in the range of 1.6−2.8 Å as summarized in Table 2. Overall, except the slight difference in binding energies, the two formic and acetic acid appear to have quite similar surface binding characteristics in the previous studies.

Table 1. Previous Studies on Adsorption Energies of Formic and Acetic Acid with Water Clusters (in kcal/mol) Formic Acid theoretical studies model cis-HC(O)OH-H2O

trans-HC(O)OHH2O

trans-HC(O) OH·(H2O)2

trans-HC(O) OH·(H2O)3 trans-HC(O) OH·(H2O)6 trans-conformer trans-HC(O) OH·(H2O)2880

method MP2/6-311+ +G(d,p) B3LYP/6-311+ +G(d,p) MP2/6-311+ +G(d,p) B3LYP/6-311+ +G(d,p) PLAP/DZVP MP2/6-311+ +G(d,p) B3LYP/6-311+ +G(d,p) PLAP/DZVP PLAP/DZVP B3LYP/631+G(d,p) MD simulation GCMC

binding energies −4.7 (−3.5) ∼ −9.5 (−7.4)a −3.2 (−2.8) ∼ −10.3 (−9.5)a −4.0 (−3.0) ∼ −10.4 (−8.0)a −4.2 (−4.0) ∼ −9.0 (−8.1)a −9.8 (−6.8)b −8.9 (−7.0) ∼ −22.8 (−19.4)a −7.7 (−7.4) ∼ −22.8 (−22. 7)

Table 2. Previous Theoretical Predictions of Hydrogen Bond Lengths (in Å)

a

Formic Acid

−11.9 (−8.5)b −8.9 (−5.7)b −17.6

model cis-HC(O)OH-H2O trans-HC(O)OH-H2O

c

−14.6d −15.3e trans -HC(O)OH·(H2O)2

experimental studies −12.2 (±1)f Acetic Acid

trans-HC(O)OH·(H2O)3 trans-HC(O)OH·(H2O)6 trans-conformer

theoretical studies model cis-H3CC(O)OHH2O trans-H3CC(O) OH-H2O trans-H3CC(O) OH·(H2O)2 trans-H3CC(O) OH·(H2O)16

method binding energies DFT/6-311+ −4.6 (−4.4) ∼ −7.0 (−6.6)g +G(3df,3pd) DFT/6-311+ −3.0 (−2.8) ∼ −9.5 (−9.1)g +G(3df,3pd) DFT/6-311+ −8.1 (−7.7) ∼ −20.6 (−19.9)g +G(3df,3pd) PBE/PW −16.4h MD simulation −18.16i experimental studies −17.5 ± 2.8j −13.1f

model cis-H3CC(O)OH-H2O trans-H3CC(O)OH-H2O trans-H3CC(O) OH·(H2O)2 trans-H3CC(O) OH·(H2O)16 a e

Basis Set Superposition error (BSSE) corrected value in parentheses. a Reference 12. bReference 10. cReference 11. dReference 13. e Reference 34. fReference 35. gReference 36. hReference 37. i Reference 38. jReference 39.

method

distance of H bond

MP2/6-311++G(d,p) B3LYP/6-311++G(d,p) MP2/6-311++G(d,p) B3LYP/6-311++G(d,p) PLAP/DZVP PLAP/DZVP B3LYP/6-311++G(d,p) PLAP/DZVP B3LYP/6-31+G(d,p) MD simulation Acetic Acid method DFT/6-311+ +G(3df,3pd) DFT/6-311+ +G(3df,3pd) DFT/6-311+ +G(3df,3pd) PBE/PW

1.792−2.204a 1.789−2.189a 1.817−2.167a 1.812−2.051a 1.83b 1.69b 1.663−2.128a 1.67b 1.646, 1.889c 1.9−2.2d

Reference 12. bReference 10. Reference 36. fReference 37.

c

Reference 11.

distance of H bond 1.862−2.469e 1.807−2.683e 1.656−2.776e 1.857f d

Reference 13.

B. Binding Chracteristics of Formic and Acetic Acid on Ice Surface. As discussed in the Introduction, our ice models include long-range electrostatic interactions by bulk ice, surface heterogeneity effects, as well as hydrogen disorders of ice crystal by adopting four types of ice surface (H0, H1, H2, and H3), which could better represent the diversity of ice surface adsorption environments. Unlike earlier cluster studies, our calculations found various cis- and trans-binding conformations. Figure 4 represents the surface binding structures as well as binding energies of cisconformers on the four types of hexagon sites. Figure 4a−d represents the adsorption structures of formic acid (FA) on H0, H1, H2, and H3 surfaces, respectively. The corresponding surface adsorption structures of acetic acid (AA) are also shown in Figure 4e−h. In the case of the H0 site (Figure 4a,e) where no surface HDB exists, cis-conformer is formed by hydrogen bonding of O7−H41 and a secondary weak hydrogen bond of O13−H40 or O1−H44. Because there is no HDB, the carbonyl O38 cannot form hydrogen bonding. According to the BSSE corrected binding energies (numbers in the parentheses), the two acids have nearly identical stabilities. Figure 4b,f shows structures on H1 site. Since both cis-conformers are adsorbed

of −2.8 to −22.7 kcal/mol with up to two water models. With six water clusters, Allouche11 predicted it to be −17.6 kcal/mol with BLYP/6-31+G(d,p). These three quantum mechanical studies showed that the global minimum of the formic acid− water complexes is a cyclic double-hydrogen-bonded structure. Wei et al.10 also reported proton transfer barrier heights of 14.56−20.84 kcal/mol, depending on the complexes. A classical molecular dynamics13 and Grand Canonical Monte Carlo (GCMC) studies34 yielded binding energies of −14.6 and −15.3 kcal/mol, respectively. Experimentally von Hessberg et al.35 reported an adsorption enthalpy of −12.2 ± 1 kcal/mol. In the case of acetic acid, Gao and Leung36 found −2.8 to −9.1 kcal/mol (after BSSE correction) binding energies interacting with one water and −7.7 to −19.9 kcal/mol (after BSSE correction) for two waters using density functional theory. Using periodic PBE/PW calculation, Allouche and Bahr37 reported adsorption enthalpy of 16.4 kcal/mol on sixteen water molecules unit cell. A classical molecular 4184

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Figure 4. Adsorption structures and binding energies of cis-formic acid on (a) H0, (b) H1, (c) H2, and (d) H3 surface absorption sites and cis-acetic acid on (e) H0, (f) H1, (g) H2, and (h) H3 surface absorption sites. The energies are in kcal/mol and the BSSE corrected values are in the parentheses.

Figure 5. Adsorption structures and binding energies of trans-formic acid on (a) H0, (b) H1, (c) H2, and (d) H3 surface absorption sites and transacetic acid on (e) H0, (f) H1, (g) H2, and (h) H3 surface absorption sites. The energies are in kcal/mol and the BSSE corrected values are in the parentheses.

through hydrogen bond of O7−H41 (ODB site) and O38−H2 (HDB site), they have higher binding energies than H0 site by 5.6 (formic acid) and 5.5 (acetic acid) kcal/mol. Our cisconformations are different from the cyclic structures found with small water clusters due to the differences in basic ice models. Even though other cis-conformers on H2 site (Figure 4c,g) bind the surface through two hydrogen bonds of O13−H41

and O38−H9, the formic acid is particularly less stable than Figure 4b, which can be attributed to the steric hindrance between hydrogen 2 and 40. In the case of acetic acid (Figure 4g), the weak interaction of O16−H44 compensates the hindrance. The H3 site (Figure 4d,h), where three HDB exist, cannot form two interfacial hydrogen bonds since there is no available ODB site. Instead, interesting inserted structures were 4185

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found, where cis-conformers are inserted into the surface hydrogen bonding network (O1−H6). As a result, three hydrogen bondings consisting of O1−H41, O39−H6 and O38− H9 are formed. Because of their rather distorted structure, their binding energies are relatively smaller than the other structures. Surface trans-conformation structures were also found and presented in Figure 5. In the case of H0 site (Figure 5a,e), the adsorbed trans-conformers have only one hydrogen bond (O7− H41). As compared to the corresponding cis-conformers (Figure 4a,e), the trans-conformers on H0 site are less stable by 5 (FA) and 6.1 (AA) kcal/mol, which can be attributed to the missing secondary interactions such as O13−H40 (FA, Figure 4a) and O1−H44 (AA, Figure 4e). The adsorption structures of trans-conformers on H1 site yield higher binding energy as compared to H0 site, due to the two hydrogen bondings (O7−H41 and O38−H2). However they are still less stable than the corresponding cis-conformers of Figure 4b,f by 3.6 and 3.5 kcal/mol, respectively, which can be due to the unfavorable distance between H40-O13. These trans-conformers correspond to the structures predicted with small water clusters in previous studies. In the case of H2 site in Figure 5c,g, two hydrogen bondings exist (O13−H41 and O38−H9). Overall, the conformers on H2 site are quite similar with those on H1 site. Since the H3 site does not have surface dangling oxygen to which formic acid hydrogen can be bound, the acid hydrogen is attached to the surface oxygen (O1−H41) with secondary hydrogen bond of O38−H9 as shown in Figure 5d,h. The formation of primary hydrogen bonding of O1−H41 yielded penta-coordinated oxygen site (O1) on ice surface, making the existing hydrogen bond (O1−H20) longer. In short, unlike earlier studies in which trans-conformers were found as major surface configurations, our systematic studies found various cis-and trans-conformers on ice surface with the cis-conformers being more stable. The energetically preferred cis-conformers can be a result of the favorable parallel configurations, which allow additional secondary interactions between the acids and ice surface. The surface binding energies and configurations are largely dependent on the surface heterogeneity, yielding a wide binding energy range of −8.9 to −20.5 kcal/mol with BSSE correction. Our energetics are in good agreements with experimental data35 and MD study.13,34 In addition, it is seen that overall the binding energies of AA are slightly higher than those of FA, implying that the additional methyl group of AA enhances the surface bindings. Because of the particular conformations, the general structures of cis-and trans-conformers tend to be parallel and perpendicular to the surface, respectively. The primary hydrogen bond lengths of cisand trans-conformers are 1.69−1.75 and 1.66−1.67, respectively, showing that trans-conformers have stronger primary hydrogen bonds. C. Interfacial Proton Transfer. The possibility of deprotonation FA and AA on ice surface has not been theoretically suggested so far.11,37 However, our study found a feasible route of proton transfer from trans-formic acid to the surface on H1 site. The overall deprotonation mechanism is shown in Figure 6, where reactant, transition state, and product are represented by Figure 6 panels a, b, and c, respectively. As the transition state (Figure 6b) shows, the deprotonation occurs by forming interfacial O39−H5 hydrogen bond and by breaking the surface O7−H5 hydrogen bond, which subsequently migrates from H41 of formic acid to the surface oxygen O7 in one step. As a result, a deprotonated formic acid is formed in Figure 6c along with a hydronium at O7. The final

Figure 6. Proton transfer reaction mechanism from formic acid to ice surface on H1 absorption site where panels a, b, and c corresponds to the reactant, transition state, and product, respectively. All energies are relative to the unbound system and the energies are in kcal/mol.

product, Figure 6c is still quite stable, further supporting its existence. Since the transition state has an internal barrier of −5.6 kcal/mol, this particular route can be also kinetically accessible. On the other hand, according to our calculations, acetic acid prefers to stay as molecular form, which can be attributed to the hydrophobic methyl group, yielding a large steric hindrance with the surface.

V. CONCLUSIONS With our QM/EFP scheme, the chemical adsorptions of FA and AA on Ih ice surface, were theoretically studied. Unlike earlier studies in which trans-conformers were found as major surface configurations, our systematic studies found diverse cisand trans-conformers on ice surfaces with the cis-conformers being more stable. The energetically preferred cis-conformers can be a result of the favorable surface-parallel configurations, which allow additional secondary interactions between the acid and ice surface. The surface binding energies as well as binding configurations were found to be largely dependent on the surface heterogeneity, yielding a wide binding energy range of −8.9 to −20.5 kcal/mol. Specifically, we showed that the number of surface dangling hydrogens strongly affects the binding configurations and energies of FA and AA on ice surface. In addition, it is seen that overall binding energies of AA are slightly higher than FA, implying that the additional methyl group of AA enhances the surface bindings. Our study also found a feasible deprotonation route of trans-formic acid to the surface. The deprotonation occurs in one step, yielding a deprotonated and yet adsorbed formic acid with a hydronium. The relatively stable deprotonated structure and low reaction barrier strongly suggest a possibility of deprotonation of formic acid. On the other hand, acetic acid prefers molecular form, 4186

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which can be attributed to the unfavorable hydrophobic methyl group. Therefore, it is interesting to note that the additional methyl group of acetic acid enhances the binding energies. However the same methyl group also reduces the possibility of deprotonation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-53-950-5332. Fax: +82-53950-6330. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2007-0056341 and No. 2012-0002540).

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The Journal of Physical Chemistry C

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