Acid Base Chemistry at the Ice Surface: Reverse Correlation Between Intrinsic Basicity and Proton-Transfer Efficiency to Ammonia and Methyl Amines

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1 DOI: /cphc Acid Base Chemistry at the Ice Surface: Reverse Correlation Between Intrinsic Basicity and Proton-Transfer Efficiency to Ammonia and Methyl Amines Seong-Chan Park, Joon-Ki Kim, Chang-Woo Lee, Eui-Seong Moon, and Heon Kang* [a] Proton transfer from the hydronium ion to NH 3, CH 3 NH 2, and (CH 3 ) 2 NH is examined at the surface of ice films at 60 K. The reactants and products are quantitatively monitored by the techniques of Cs + reactive-ion scattering and low-energy sputtering. The proton-transfer reactions at the ice surface proceed only to a limited extent. The proton-transfer efficiency exhibits the order NH 3 >(CH 3 )NH 2 =(CH 3 ) 2 NH, which opposes the basicity order of the amines in the gas phase or aqueous solution. Thermochemical analysis suggests that the energetics of the proton-transfer reaction is greatly altered at the ice surface from that in liquid water due to limited hydration. Water molecules constrained at the ice surface amplify the methyl substitution effect on the hydration efficiency of the amines and reverse the order of their proton-accepting abilities. Introduction The basicity of a molecule is a fundamental characteristic that determines the equilibrium of the proton-transfer reaction. Basicity is influenced not only by the intrinsic properties of a molecule, but also by the medium in which the molecule is situated. The basicities of methyl amines (including ammonia) change greatly upon going from the gas phase to aqueous solution, which is a textbook example of the medium effect. [1, 2] Intrinsic basicity, also called gas-phase basicity (GB), is defined as the negative free energy for protonation of a molecule (B) in the gas phase [Eq. (1)]: BðgÞþH þ ðgþ! BH þ ðgþ GBðBÞ ¼ DG ð1þ GB increases with methyl substitution for amines, owing to the methyl group effect that stabilizes the positive charge in the protonated amines (including the ammonium ion): ACHTUNGTRENUNG(CH 3 ) 3 N (918.0 kj mol 1 ) >(CH 3 ) 2 NH (896.6 kj mol 1 ) >CH 3 NH 2 (864.4 kj mol 1 ) >NH 3 (818.8 kj mol 1 ). [3] However, when these species are dissolved in aqueous solution, their basicities dramatically decrease, becoming closer to each other within a range of 9 kj mol 1, and the order of basicity among these species is rearranged: (CH 3 ) 2 NH (61.5 kj mol 1 ) >CH 3 NH 2 (60.8 kj mol 1 ) >(CH 3 ) 3 N (56.1 kj mol 1 ) > NH 3 (52.8 kj mol 1 ). [4] The change in basicity of alkyl amines from the gas to the aqueous phase has been a subject of extensive experimental and theoretical study. [5,6] Detailed analyses of molecular and thermodynamic factors suggest that the aqueous basicity of amines is controlled by subtle balance and competition between the opposite effects of methyl substitution; methyl substitution enhances the intrinsic basicity (base-strengthening effect), while it lowers the electrostatic solvation enthalpy for protonated amines and raises the entropy (base-weakening effects). [6] Another interesting medium for the study of amine basicity is the ice surface, which offers a reaction environment much different from that formed by liquid water, gas, or even bulk ice. Ice surfaces have received increasing attention in recent years owing to their important roles in many low-temperature chemical phenomena in nature, including the atmospheric reactions of stratospheric ice particles and the formation of molecules in the interstellar medium. [7] Although water molecules are extremely immobile inside the ice lattice, they are relatively loosely bound at the ice surface and have some mobility depending on the temperature, [8, 9] which makes the ice surface a unique reaction environment. One of the facile reactions at ice surfaces is proton transfer involving water molecules. The resonant proton transfer between water molecules readily occurs at the surface and interior of ice, through a proton-hopping relay (Grçtthuss mechanism) across a short distance [10] and by the combined motions of proton hopping and defect migration at elevated temperatures. [11] The heterogeneous proton transfer between different donor and acceptor molecules has [a] Dr. S.-C. Park, J.-K. Kim, C.-W. Lee, E.-S. Moon, Prof. Dr. H. Kang Department of Chemistry Seoul National University Gwanak-gu, Seoul (Republic of Korea) Fax: (+ 82) surfion@snu.ac.kr Supporting information for this article is available on the WWW under or from the author Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2007, 8,

2 Proton Transfer to Amines at the Ice Surface been investigated only scarcely at ice surfaces [12 14] compared to its intense study in liquid water. [15] Herein, we study the proton-transfer reactions from the hydronium ion to a series of amine molecules [NH 3,CH 3 NH 2, and (CH 3 ) 2 NH] atice surfaces. By examining the proton-transfer efficiencies to these amines, we aim to improve understanding of the effects of water solvation and methyl-group substitution on the proton-transfer reactions at ice surfaces. Experimental Section In an ultrahigh vacuum chamber, [16] ice films were deposited on a RuACHTUNGTRENUNG(0001) surface at 60 K to a thickness of four to five bilayers (BLs) by condensing D 2 O (Aldrich, % isotope purity) vapor at a partial pressure of Torr. The ice film was exposed to 0.3 L (1 L= Torr s) of HCl gas (Aldrich) at60 K and then warmed up to 140 K at a heating rate of 70 K min 1. Previous studies [10,17] have shown that HCl completely ionizes to hydronium and chloride ions at the ice surface at 140 K, and the resulting hydronium ions mainly reside at the surface. The ice film containing hydronium ions was cooled to 60 K, and then exposed to NH 3,CH 3 NH 2,or (CH 3 ) 2 NH gases (Aldrich) ata pressure of Torr, as measured by an ionization gauge. The ionization gauge reading of amine pressure was calibrated against N 2, with relative sensitivity factors of 1.3 for NH 3, 2.4 for CH 3 NH 2, and 3.5 for (CH 3 ) 2 NH. [18] The neutral and ionic species existing at the surface of ice films were analyzed by the techniques of Cs + reactive-ion scattering (RIS) and low-energy sputtering (LES), respectively. The principles of these methods have been described in detail in previous reports. [9,19] In these experiments, a Cs + beam collided with an ice film surface atthe incidentenergy of 30 ev and beam flux of 3nAcm 2. In RIS, neutral species (X) atthe surface are picked up by the scattering Cs + projectiles to form Cs + neutral clusters (CsX + ). In LES, ionic species (Y + ) at the surface are ejected by Cs + impact. The RIS products (CsX + ) and LES ions (Y + ) are simultaneously detected by a quadrupole mass spectrometer with its ionizer filament switched off, from which the masses of neutral and ionic constituents (X and Y + ) of the surface are identified. The RIS and LES signals measured under these conditions represent the species preexisting at the surface, with negligible contribution from the secondary reactions induced by Cs + beams. [9] Surface analysis was performed typically 5 min after the start of the proton-transfer reaction, unless specified otherwise. Results Proton-transfer reactions between the hydronium ion and amines at the ice surface are examined by measuring the reactant and product species in both neutral and protonated forms [Eq. (2)]: H 3 O þ ðsþþbðsþ!h 2 OðsÞþBH þ ðsþ ½B ¼ðCH 3 Þ x NH 3 x ðx ¼ 0 2ÞŠ Figure 1 a shows LES and RIS mass spectra obtained from a D 2 O ice-film surface exposed to 0.3 L of HCl at 140 K. The LES signals appearing at m/z = amu/charge representhydronium ions (H x D 3 x O +, x =0 3), and their monohydrated species are seen at m/z = The strongest peak at m/z =133 is the reflected Cs + ions. Atthe higher masses, RIS peaks appear at ð2þ Figure 1. a) Mass spectrum obtained from a D 2 O ice film containing hydronium ions H x D 3 x O + (x = 0 3), which were prepared by adding 0.3 L of HCl to a five-bl-thick D 2 O ice film at 60 K and then annealing it up to 140 K. Mass spectra following the addition of 0.13 L of CH 3 NH 2 (b), 0.14 L of (CH 3 ) 2 NH (c), and 0.23 L of NH 3 (d) to the hydronium ion-containing ice films at 60 K. Measurements were made at 60 K in all cases. The Cs + beam energy was 30 ev. m/z = and , which represent the RIS products of one (CsH x D 2 x O +, x = 0 2) and two water molecules (CsH x D 4 x O + 2, x =0 4), respectively. The spectrum shows no RIS signal for molecular HCl at m/z =169, thus confirming that HCl completely ionizes upon warming the ice film to 140 K. [17,20] D 3 O + has the highest population among the hydronium ion isotopomers (H x D 3 x O +, x =0 3), which indicates the occurrence of facile H/D exchange between hydronium ions and D 2 O molecules at this temperature. Figure 1 b shows the result obtained after addition of 0.13 L of CH 3 NH 2 to the ice film surface containing hydronium ions, prepared as for Figure 1 a. The ice film temperature is maintained at 60 K during the addition of CH 3 NH 2. In the LES region, methylammonium ion (protonated methylamine) signals (CH 3 NH x D + 3 x, x =0 3) appear strongly at m/z = At the same time, the hydronium ion signal is greatly reduced in intensity. These spectral changes indicate the occurrence of proton transfer from the hydronium ion to methylamine at the surface. In the RIS region, the signal due to adsorbed methylamine molecules appears at m/z =164 (CsCH 3 NH + 2 ), and the water signal (CsH x D 2 x O +, x =0 2) is slightly attenuated. CH 3 NDH + 2 is the dominant isotopomer observed for the LES signals of methylammonium ions, which indicates that the H/D exchange reaction of the methylammonium ion is inefficient at the surface, once it is formed by proton transfer from the hydronium ion to methylamine. Figures 1 c and d show the spectra following the addition of (CH 3 ) 2 NH and NH 3, respectively, at 60 K to the hydronium-containing D 2 O films prepared as for Figure 1 a. In both spectra, strong LES signals of protonated amines arise while the hydro- ChemPhysChem 2007, 8, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

3 H. Kang et al. nium ion signals greatly decrease. These spectra for proton transfer to dimethylamine and ammonia show qualitatively similar features to those in Figure 1 b. The results of quantitative measurements for the proton transfers are presented in Figures 2 and 3. Figure 2 a shows the [(CH 3 ) 2 NH], and s LES ACHTUNGTRENUNG(H 3 O + )/s LES ACHTUNGTRENUNG(BH + ) = (NH 3 ), (CH 3 NH 2 ), and [(CH 3 ) 2 NH]. Figure 2 a shows that the surface concentration of water molecules decreases as ammonia exposure increases. About half of the film surface is effectively covered with ammonia molecules when its exposure reaches about 0.3 L. Noticeably, the surface concentration of ammonia remains very low until its exposure reaches 0.05 L. In this low-exposure region, the concentration of ammonium ion rapidly increases (Figure 2 b), which indicates very efficient conversion of the ammonia molecule to ammonium ion. The concentration of ammonium ion reaches a maximum at the exposure of about 0.1 L and then declines with further increase in the exposure. The hydronium ion concentration continuously decreases as the ammonia exposure increases. The concurrent decrease of protonated species (hydronium and ammonium ions) athigh exposures can be explained by the gradual formation of an ammonia overlayer, which prevents the detection of these ions by LES which samples only the outermost surface species. We evaluate the extent to which proton transfer occurs from the hydronium ion to amines by using the reaction quotient Q, which is expressed by Equation (3): Q ¼ Cs ðbh þ ÞC s ðh 2 OÞ C s ðbþc s ðh 3 O þ Þ ð3þ Figure 2. a) Variations in relative surface concentrations of water (&) and ammonia molecules (*) with increasing ammonia exposure. b) Relative surface concentrations of hydronium (&) and ammonium ions (*). The surface concentrations are shown in the scale normalized to the initial concentrations of water molecules in (a) and hydronium ions in (b). Ammonia gas was exposed to the hydronium ion-containing ice surface at 60 K, and the measurements were made after 5 min of exposure to each gas. relative surface concentrations of neutral water and ammonia molecules measured as a function of ammonia exposure on the hydronium-containing surface at 60 K, and Figure 2b shows those for the hydronium and ammonium ions. In Figure 2 a, the relative surface concentrations shown are normalized to that of water molecules before ammonia adsorption. Likewise, the relative ion concentrations in Figure 2 b are normalized to that of hydronium ions at the initial surface. The concentrations of all isotopomers are added in these plots, though for the sake of simplicity, each curve is labeled with its lightest isotopomer. The surface concentration (C s ) is calculated using the relationship, I =sc s, where I is the intensity of LES and RIS signals for ions and neutrals, respectively, and s is the detection efficiency for the LES and RIS processes. The detection efficiency for each species is determined by the calibration experiment and procedure described in the Supporting Information. Through this procedure we obtain the relative magnitude of RIS detection efficiency for amine and water molecules [s RIS (B)/s RIS ACHTUNGTRENUNG(H 2 O)], and the relative LES detection efficiency for hydronium and protonated amine ions [s LES ACHTUNGTRENUNG(H 3 O + )/s LES ACHTUNGTRENUNG(BH + )]. The relative detection efficiencies thus estimated are: s RIS (B)/s RIS ACHTUNGTRENUNG(H 2 O) = (NH 3 ), (CH 3 NH 2 ), and where C s represents the surface concentration deduced from the LES or RIS measurement as mentioned above. Figure 3 displays Q values for three acid base pairs measured as a function of base exposure. The three amine molecules have almost the same sticking probability on the surface according to temperature-programmed desorption measurements. At low exposures (< 0.05 L), the Q value for ammonia is significantly larger than those for methylamine and dimethylamine. For example, Q for ammonia atthe smallestexposure of 0.02 L is , whereas those for methylamine and dimethylamine are 2513 and 128, respectively, at the same exposure. Q for ammonia dramatically decreases with increase in ammonia exposure, and it Figure 3. Reaction quotient Q for proton transfer from the hydronium ion to NH 3 (*), CH 3 NH 2 (&), and (CH 3 ) 2 NH (^) atthe ice surface measured as a function of amine exposure. The exposure of each amine is calibrated for its pressure reading by an ionization gauge. The error bars include the experimental fluctuations and uncertainties in the determination of RIS and LES cross sections. The experimental conditions are the same as for Figure Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2007, 8,

4 Proton Transfer to Amines at the Ice Surface becomes close to that for methyl amines at high exposures ACHTUNGTRENUNG(>0.1 L). We have extended the reaction time between amine adsorption and Q measurement up to 40 min, the longest time that a reliable measurement can be made without adsorption of residual water vapor on the film surface. The time delay produced no significantchange in the Q values for the three amines. To examine the effect of proton concentration on the surface, the measurement for ammonia shown in Figure 2 was repeated for HCl exposures of 0.1 and 0.5 L. The experiments found no noticeable effect of proton concentration on the Q value. Discussion As mentioned in the Introduction, the basicity strength of amines in the gas phase is in the order of (CH 3 ) 2 NH > (CH 3 )NH 2 >NH 3. This order is also maintained in the aqueous phase, though only by a slight margin. Therefore, it is surprising that the proton-transfer efficiency at the ice surface exhibits a reverse order for these species. In the following text, we will discuss this observation based on thermochemical analysis of the proton-transfer reactions. In this analysis it should be kept in mind that equilibrium thermochemistry is strictly applicable only to the aqueous-phase reactions, and it is used only as a hypothetical reference for the ice surface reactions. However, the thermochemical method offers a plain approach to analyze the difference in the proton-transfer behaviors in two reaction media. The Q values shown in Figure 3 are much smaller than the equilibrium constants of proton-transfer reactions in the aqueous phase: K eq = for (CH 3 ) 2 NH, for CH 3 NH 2, and for NH 3 at298 K. [4] As shown in previous studies, [14,21] such a behavior arises because reactions at ice surfaces often stop at intermediate states along the reaction coordinate rather than proceeding all the way to the thermodynamically equilibrated states. In this respect, we define the apparent free energy for proton transfer at the ice surface according to the equation DG* ice = RTlnQ, in analogy to the relationship between free energy and equilibrium constant. DG* ice here represents the free energy of reaction to the metastable states trapped at the ice surface, not to the final states that would be reached in liquid water. DG* ice is calculated using the Q value at the infinitely diluted concentration of amine, which is obtained by extrapolation of the Q curve in Figure 3. Ata low amine concentration, the formation of amine clusters that prohibit the proton-transfer process is negligible, as will be discussed later. DG* ice at T =60 K thus estimated has slightly negative values (between 1.3 and 3.3 kj mol 1, see Table 1) for the three reactions. The free energies of proton-transfer reactions in the aqueous phase (DG8 aq ) are also listed in Table 1, and they have substantially large negative values. To see the effect of temperature on these reactions, DG aq for T = 60 K is deduced from the relationship DG =DH TDS by assuming invariance of DH and DS with the temperature and phase change. The extension of the liquid water values to 60 K is unrealistic; however, this assumption may have some relevance for amorphous solid water in that its certain structural and thermodynamic aspects resemble those of liquid water. [26] Even considering such uncertainty in the temperature extrapolation, it is clear that DG aq ACHTUNGTRENUNG(60 K) is more negative than DG* ice by a large gap ( 50 kj mol 1 ). This result suggests that the low temperature is not a main thermodynamic factor that reduces the proton-transfer efficiency observed atthe ice surfaces. We considered the effect of water solvation on proton transfer at the ice surface. Water molecules undergo only restricted motions at the ice surface, solvating reactants and products to a limited extent compared to the fully solvating situation in liquid water. If we consider that limited hydration occurs uniformly for reactants and products, the basicity of molecules at the ice surface will be somewhere between those in the gas and aqueous phases, and the basicity order of (CH 3 ) 2 NH > CH 3 NH 2 >NH 3 will be unchanged. However, observation con- Table 1. Thermodynamic quantities for proton transfers from the hydronium ion to amines in various environments. DG* ice ACHTUNGTRENUNG(60 K) [a] DG8 aq [b] DG aq ACHTUNGTRENUNG(60 K) [c] NH CH 3 NH ACHTUNGTRENUNG(CH 3 ) 2 NH DH8 cl [d] DS8 cl [e] [J K 1 mol 1 ] DG8 cl [f] DG cl ACHTUNGTRENUNG(60 K) [g] m= m= m= m= m= m= m= m= m= m= [a] Apparent free energy for proton transfer at the ice surface estimated by extrapolating the present results to infinitely diluted amine concentrations at 60 K; see the text. [b] Data from ref. [4]. [c] Calculated by DG aq ACHTUNGTRENUNG(60 K) = DH8 aq TDS8 aq, where T = 60 K, and DH8 aq and DS8 aq values are from ref. [4]. [d g] The values for intercluster reactions are for n = 5: H 3 OACHTUNGTRENUNG(H 2 O) BACHTUNGTRENUNG(H 2 O) m!achtungtrenung(h 2 O) 6 + BHACHTUNGTRENUNG(H 2 O) + m. [d] Calculated by Equation (5). PA and GB values from ref. [3]. DH8 hyd values for H 2 O, NH 3, and methyl amines from refs. [22 24], respectively. DH8 hyd values for protonated species from ref. [25]. [e] Calculated by DS8 cl = (DH8 cl DG8 cl )/T, where T = 298 K. [f] Calculated by Equation (6). DG8 hyd values for H 2 O, NH 3, and methyl amines from refs. [22 24], respectively. DG8 hyd values for protonated species from ref. [25]. [g] Calculated by DG cl ACHTUNGTRENUNG(60 K) = DH8 cl TDS8 cl, where T = 60 K. ChemPhysChem 2007, 8, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

5 H. Kang et al. tradicts this expectation, and suggests that the degree of hydration may vary with different species at the ice surface. To examine the possibility of different partial hydrations more closely, we calculated the energetics of proton-transfer reactions that would occur under the conditions of varying degrees of hydration. We used a cluster model, in which a proton transfers from a hydronium ion hydrated by n water molecules to an amine molecule hydrated by m water molecules [Eq. (4)]: H 3 O þ ðh 2 OÞ n þ BðH 2 OÞ m! H 2 OðH 2 OÞ n þ BH þ ðh 2 OÞ m As Scheme 1 depicts, a thermodynamic cycle can be constructed to calculate the reaction enthalpy and free energy of Scheme 1. Calculation of thermodynamic functions for proton transfer from the hydronium ion to amine (B) in hydrated forms in the gas phase. these intercluster proton transfers using literature values of relevant thermodynamic quantities; [3, 22 25] DH8 hyd and DG8 hyd are the hydration enthalpy and free energy for each species, respectively. Proton affinity (PA) is the negative enthalpy change associated with the gas-phase protonation reaction [Eq. (1)] and GB is the corresponding negative free energy change. DH cl ¼ DH hydðh 3 O þ Þ DH hydðbþþpaðh 2 OÞ PAðBÞþ DH hydðh 2 OÞþDH hydðbh þ Þ DG cl ¼ DG hydðh 3 O þ Þ DH hydðbþþgbðh 2 OÞ GBðBÞþ DG hydðh 2 OÞþDG hydðbh þ Þ ð4þ ð5þ ð6þ Table 1 lists standard enthalpies (DH8 cl ), entropies (DS8 cl ), and free energies (DG8 cl ) for the intercluster proton-transfer reactions calculated from Equations (5) and (6). The value of DG cl for T =60 K is also shown for comparison. Detailed thermodynamic quantities used for these calculations are provided as Supporting Information. Since hydronium ions are generated by HCl ionization at 140 K in the present study, these ions are efficiently solvated by water molecules, which are quite mobile at this relatively high temperature. [17,20] On the other hand, amine molecules deposited on the surface after cooling it to 60 K will be surrounded by a smaller number of water molecules, for both neutral and protonated forms. Thus, the most probable situation is n = m. For simplicity, Table 1 shows thermodynamic functions calculated for n = 5 and m =0 3. More comprehensive analysis for n = 3 7 reveals that the relative difference between n and m, rather than the absolute magnitude of n or m, dominates the enthalpy and free energy of the overall proton-transfer reactions. Also, it is revealed that a key factor controlling the reaction energetics is the hydration of ions rather than neutral species, because the ion hydration energies are 3 8 times greater than the neutral hydration energies (Supporting Information). In Table 1, DG cl ACHTUNGTRENUNG(60 K) becomes close to DG* ice ACHTUNGTRENUNG(60 K) when the difference between hydration numbers (n m) is 3 to 4 for the reaction with ammonia and 4 to 5 for the reactions with methyl- and dimethylamine. This supports the idea that proton transfer at the ice surface occurs from hydronium ions, strongly hydrated on the average, to amine (or ammonia) molecules that are surrounded by a relatively smaller number of water molecules. That is, the incomplete hydration causes the proton-transfer reactions to reach only metastable states. The cluster model assumes that the hydration geometries are optimized, but this is an unlikely situation at the ice surface at low temperatures due to structural constraints [27] and low molecular mobility. [28] In this respect, the (n m) value estimated here should be interpreted as the difference in effective hydration numbers of these species, rather than the number of actually binding water molecules. According to the partial hydration model, the reverse order in the proton-transfer yields can be explained by a smaller hydration number for methyl amines than that for ammonia. The presence of methyl group(s) hinders efficient hydration of the protonated and neutral amines by water molecules through hydrogen bonding. In liquid water, this negative effect of the methyl group on hydration may not be large enough to overcome its positive effect for increasing the intrinsic basicity of amines, because highly mobile molecules in the liquid phase can efficiently hydrate these species regardless of methyl substitution. At the ice surface, however, the limited hydration can amplify the difference in the hydration efficiencies of ammonia and methyl amines. We also consider the possibility that the presence of the methyl group affects the proton-transfer paths through water molecules at the ice surface. The structural matching between the ice lattice and the embedded foreign molecules for proper hydrogen bonding is crucial for a successful proton relay via a Grçtthuss-like mechanism. If the methyl group of amines hinders the formation of such optimal structures for proton transfer, it will significantly lower the proton-transfer efficiency. This kinetic effect may act in addition to the partial hydration effect to reduce a Q value atthe ice surface. The final part of this discussion concerns the fact that Q drops rapidly with an increase in amine exposure in the region below 0.1 L (Figure 3). This decline in Q occurs while a substantial population of hydronium ions still exists at the surface, as exemplified in Figure 2 b. Also, the surface coverage by amine molecules is insignificant at this stage. Thus, it cannot be explained by a shortage of hydronium ions available for proton transfer or by the overlayer shielding effect that attenuates the ammonium ion signal. We suspect that the drop in Q is due to the clustering of amine molecules around a protonated amine by surface migration, which forms a stable ion molecule cluster. At a surface coverage of 0.1 monolayers, amine molecules can encounter protonated amines by migrating only one to two intermolecular distances on the surface. Amine molecules contained in the ion molecule cluster may have difficulty in accepting another proton from the hydronium ion, partly due Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2007, 8,

6 Proton Transfer to Amines at the Ice Surface to the charge repulsion that would occur between two closely located protons and the inefficient proton transfer through amine molecules that form the cluster. As a result, the clustering reaction impedes proton transfer to the amine. The smaller decrease in Q observed for methyl amines than for ammonia is probably related to less efficient cluster formation for the former, due to a smaller number of available hydrogen bonds. Conclusions Through RIS and LES measurements of reactants and products in both neutral and protonated forms, we have quantified the efficiency of proton transfers from the hydronium ion to amines at the ice surface. The proton-transfer efficiency defined by reaction quotient Q exhibits the order NH 3 > (CH 3 )NH 2 =(CH 3 ) 2 NH, which opposes the trend of intrinsic basicity of amines or their basicity in aqueous solution. Thermochemical analysis suggests that incomplete solvation of reactant and product species at the ice surface reduces the proton-transfer efficiency and reverses the order of the protonaccepting abilities of amines. Water molecules confined at the ice surface only partially hydrate amines, and this situation magnifies the steric effect of the methyl group compared to that observed in the fully hydrating environment of liquid water. The conclusion that partial hydration significantly alters the energetics and efficiency of proton-transfer reaction at the ice surface might be applicable to reactions in other confined geometries, such as nanoscopic pores and enzymatic active sites. Acknowledgements This work was supported by the Korea Science & Engineering Foundation (R ). Keywords: amines basicity ice proton transfer surface chemistry [1] T. H. Lowry, K. S. Richardson, Mechanism and Theory in Organic Chemistry, 3rd ed., Harper and Row, New York, [2] J. E. Huheey, E. A. Keiter, R. L. 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