Theoretical Study on the Structure and Cation− Anion Interaction of

Aug 25, 2010 - The stable geometries of the cation, anion, ion pair, as well as the ion-pair dimer [Pro]2+[NO3]2− with no imaginary frequencies were...
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J. Phys. Chem. A 2010, 114, 10243–10252

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Theoretical Study on the Structure and Cation-Anion Interaction of Amino Acid Cation Based Amino Acid Ionic Liquid [Pro]+[NO3]Haiyan Gao, Ying Zhang, Hai-Jun Wang,* Jianhua Liu, and Jianming Chen School of Chemical and Material Engineering, Jiangnan UniVersity, Wuxi, Jiangsu, 214122, People’s Republic of China ReceiVed: May 25, 2010; ReVised Manuscript ReceiVed: July 26, 2010

The proline cation based amino acid ionic liquid [Pro]+[NO3]- was systematically studied by density functional theory at the B3LYP/6-311++G** level. The stable geometries of the cation, anion, ion pair, as well as the ion-pair dimer [Pro]2+[NO3]2- with no imaginary frequencies were obtained and characterized. In the single [Pro]+[NO3]- unit, proton transfer from [Pro]+ to [NO3]- can be observed in some of the configurations and the corresponding proton-transferred products (neutral pairs) are strongly hydrogen bonded. While in the ion-pair dimer [Pro]2+[NO3]2-, proton transfer does not occur and the components are stabilized by ionic interaction and hydrogen bonding interaction jointly. The proton transfer reaction between the cation and the anion in the single ion-pair unit has been investigated, and the role that the proton transfer reaction may play in the physicochemical property change of the ionic liquids has also been discussed. The interactions in the single ion-pair unit [Pro]+[NO3]- and in the ion-pair dimer [Pro]2+[NO3]2- were both carefully studied. The hydrogen bonds (H bonds) between the various fragments were analyzed by the atoms in molecules theory and harmonic vibrational frequency. 1. Introduction Ionic liquids (ILs) have attracted increasing interest in the chemical community as green alternatives to classical environmentally damaging media for synthesis, catalysis, separation, and other various chemical tasks.1-3 This is mainly due to their unique physical and chemical properties which include a negligibly small vapor pressure (avoid the volatile organic compounds (VOCs) emission, a major source of environmental pollution), high thermal stability, high ionic conductivity, and wide range of solubility.1,4-7 However, commonly used ILs are synthetic chemicals and are therefore not as green as desired. So in recent years, much attention has been paid to the environmental concern for the direction of development of ILs. Discovery and synthesis of environmentally benign ILs are thus of great importance.8-11 Development of amino acid ionic liquids (AAILs) provides a great opportunity for novel green ILs which are promising alternatives to commonly used ILs.12-14 To a certain extent amino acid based AAILs are completely green, not only for its naturally occurring raw materials but also for its biodegradable final products AAILs. Additionally, these AAILs are nontoxic, pharmaceutically acceptable.15 These appealing features have attracted considerable attention in recent years. In 2005, Kou and Ohno et al. have reported a new kind of IL that is directly derived from naturally occurring amino acids.16-18 Extensive experimental studies have been performed on AAILs. Fang et al. study the physicochemical properties of AAILs [Cnmim]+[Ala]- (n ) 2, 3, 4, 5, 6), Yang et al. study the properties of AAIL [Emim]+[Gly]-.19-21 However, amino acid based ILs have rarely been studied at the theoretical level compared to the imidazole based ILs.22-24 The design process of ILs and AAILs is still in a way “try-and-error” but not “task specific”, because the features that control the physicochemical properties of ILs remain poorly understood. To a large extent * Corresponding author, [email protected].

the process of design remains a random event.25,26 Thus, an understanding of the intermolecular forces and the structure of AAILs is crucial for the development of special and tunable properties of AAILs. Meanwhile, there is ample evidence that our current theory is not only able to complement experiment and reveal some properties that are so far not accessible but also proven to be able to provide elegant explanations for many contradicting behaviors of the ILs considered.27 So in this paper the amino acid cation based AAIL [Pro]+ [NO3]- was chosen as a representative to study this series of AAILs at the theoretical level, which is the only room temperature free-flowing liquid in the family of amino acid cation based AAILs.14 We hope that the present results could provide some useful information, such as geometrical characteristics, electronic properties, and interaction mechanism between the cations and the anions, which are essentially important for the continuous exploitation and application of AAILs. The next section describes the calculation methods used. This is followed by the results part where first the single ion-pair unit was analyzed, for each configuration, only one cation and one anion were considered. After this, ion-pair dimers were introduced. Then we proceed in the results part with a systematic assessment of interactions in clusters with double [Pro]+[NO3]units. Finally, conclusions end this article. 2. Calculation Methods DFT with many currently available functionals may not be able to offer sound and reliable predictive capabilities when it comes to describing systems dominated by very weak van der Waals forces (because the dispersion forces have not been taken into account for the DFT calculation).28 However, as for the studied [Pro]+[NO3]- system, the interaction energy is dominated by electrostatic attractions,22,29,30 here the contribution from dispersion forces is not so important. Moreover, the DFT

10.1021/jp104775z  2010 American Chemical Society Published on Web 08/25/2010

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Figure 1. The two stable conformers (R and β) of [Pro]+, and the potential energy surface scan of the dihedral angle D(N5-C4-C6-O7) for [Pro]+: step size, 5.0°; total steps, 72 (B3LYP/6-311++G**).

associates accuracy with low computational cost in such a way that it constitutes an attractive alternative to other postHartree-Fock (HF) procedures.31-34Thus the popular DFT with and the Becke’s three-parameter hybrid functional (B3LYP)35,36 with 6-311++G** basis set29,37-39 was employed for the geometry optimization. For interactions between the protontransferred products (neutral pairs) in a single ion-pair unit, the corresponding interaction energies are very small and these weak interactions are mainly van der Waals and H bond interactions, so interactions between the proton-transferred products were also calculated using the MP2/6-31+G* basis set,40 because MP2 is relatively more accurate than DFT to predicte these weak interactions. For the sake of simplicity, the following discussions are based on the results that were obtained at the B3LYP/6311++G** level of theory if not noted otherwise. No restrictions were imposed on the initial structure. Therefore, all the geometry optimizations for the minima configuration were with all degrees of freedom, except that isolated [NO3]is D3h symmetry. Frequency analyses (unscaled) of all stationary points have been performed to guarantee the stable geometry of the configurations to be found. Interaction energy (∆E) is defined as the difference between the energy of the appointed fragment and the sum of the energies of its free fragments (components), e.g., for the single ion-pair unit it can be evaluated in eq 1

∆E ) E[Pro]+[NO3]- - E[Pro]+ + E[NO3]-

(1)

where E[Pro]+[NO3]- is the energy of the ion-pair [Pro]+[NO3]-, E[Pro]+ is the energy of [Pro]+, and E[NO3]- is the energy of [NO3]-. The interaction energies are corrected by the basis set superposition errors (BSSE) correction,41 with the counterpoise procedure method advanced by Boys and Bernardi.42 Zero-point energy (ZPE) correction is also used for the interaction energy calculations of a single ion-pair unit and the total interaction energy calculations of the ion-pair dimer [Pro]2+[NO3]2-. The bond characteristics for the relevant configurations were illustrated based on the atom-in-molecule theory (AIM) analysis,43 and the harmonic vibrational frequency analysis was done to provide complementary information on the H bond interaction. All calculations have been performed using the GAUSSIAN03 program,44 except that the topological properties were performed by using the AIM2000 package45 with the wave functions generated from the B3LYP 6-311++G** results. 3. Results and Discussion 3.1. Single Ion-Pair Unit. In general, the overall energy of the single molecular is mainly affected by the geometric distortion and the intramolecular hydrogen bonds. Here, as [Pro]+ is concerned, the intramolecular hydrogen bonds may play the decisive role in stabilizing the cation. In order to find the stable conformer of [Pro]+, the dihedral angle D(N5-C4C6-O7), which is closely related to the intramolecular hydrogen bond, was scanned. Geometry optimization of [Pro]+ was carried out by the following two steps.46 First, all the degrees of freedom were relaxed except that the dihedral angle of D(N5-C4-

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Figure 2. The stable configurations for [Pro]+[NO3]- ion pair at B3LYP/6-311++G** level. Hydrogen bonds are indicated by dotted lines and the corresponding lengths (Å) and angles (deg) (in parentheses) are given.

C6-O7) increased with a step of 5.0°, and a total of 72 steps were scanned (360.0°) (Figure 1). Second, two initial geometry guesses for R and β were obtained in the scan curve (two lowest points in Figure 1) and reoptimized. No symmetry constraints were applied during the geometry optimization. Ultimately two stable conformers were obtained (Figure 1R, β). The total energy difference of the two stable geometries is 19.42 kJ/mol, and the energy barrier for the C4-C6 bond rotation is only 38.67 kJ/mol. So the two stable conformers R and β may coexist at room temperature, and both of them can be an integral part of the ion-pair system. Thus, both R and β should be taken into account for the initial geometry design of the ion-pair system. As for the anion, the optimized structure of the isolated [NO3]- is D3h symmetry, the three O atoms are equivalent, bond lengths of the three N-O bonds are 1.260 Å, the bond angles

O-N-O are 120.00°. The whole structure exhibits a coplanar characteristic (dihedral angle of D(O20-O21-N19-O22) is 179.99°). 3.1.1. Geometric Structures. The anion was arranged around the cation in “chemically intuitive” positions where interaction with hydrogen atoms of the cation is a possibility. Finally seven stable configurations for [Pro]+[NO3]- named as a, b, c, d, e, and f with no imaginary frequencies were located (Figure 2). As can be seen from the final optimized structures of the single ion-pair unit, the interactions between the two fragments are charaterized by both single or multiple H bonds which mainly formed between the electronegative O atoms of the anion and the active carboxyl, amino group of the cation. Anion mainly scatters above or below the tetrahydropyrrole ring.

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TABLE 1: The Selected Bond Distances (Å), Bond Angles (deg), and the Dihedral Angles (deg) of the Located Configurations of a Single Ion-Pair Unit parameters

a

b

c

d

e

f

C1-C2 C2-C4 C4-N5 N5-C3 C3-C1 C1-H10 C3-H14 C4-H15 N5-H16 N5-H18 C6-O7 C6-O8 O8-H17 ∠C2-C1-C3 ∠C4-N5-C3 DC2-C4-N5-C3 D(C4-N5-C3-C1) N19-O20 N19-O21 N19-O22 ∠O20-N19-O21 ∠O21-N19-O22 ∠O22-N19-O20 D(O20-N19-O22-O21)

1.537 1.531 1.528 1.512 1.530 1.093 1.088 1.090 1.017 1.057 1.217 1.282 1.490 103.54 108.85 -6.50 -17.97 1.204 1.354 1.232 115.76 118.03 126.21 -179.82

1.539 1.534 1.533 1.507 1.527 1.093 1.089 1.091 1.018 1.074 1.203 1.325 1.031 103.35 107.99 6.97 -29.09 1.215 1.271 1.287 121.06 119.03 119.91 179.53

1.554 1.538 1.487 1.481 1.537 1.091 1.092 1.098 1.016 1.658 1.201 1.350 0.969 104.77 104.56 40.34 -43.50 1.203 1.220 1.373 127.86 116.87 115.25 -179.86

1.554 1.545 1.490 1.489 1.539 1.091 1.091 1.094 1.019 1.583 1.203 1.340 0.977 105.12 103.93 43.19 -42.44 1.235 1.198 1.356 126.64 116.44 116.92 -179.85

1.555 1.544 1.510 1.503 1.535 1.090 1.089 1.090 1.020 1.101 1.206 1.322 1.001 105.76 104.89 38.34 -40.70 1.274 1.211 1.288 122.07 120.79 117.11 -179.16

1.535 1.556 1.487 1.494 1.531 1.094 1.091 1.092 1.542 1.020 1.208 1.347 0.970 102.28 108.14 -0.78 -22.27 1.360 1.205 1.227 115.89 126.98 117.12 179.93

The equilibrium distances between the proton and proton acceptor atom, referred to as the intrinsically preferred H bond length, along with the corresponding angle are all marked out in Figure 2. A H bond will be indicated if the H · · · X distance is less than the van der Waals H · · · X distance and the corresponding XsH · · · Y (X ) O, N, or C, Y ) O, N) angle is greater than 90°.47 Other selected geometrical parameters of these configurations are listed and compared in Table 1. From the H bond lengths shown in Figure 2 and the parameters listed in Table 1, it can be found that the carboxylic hydrogen or amino hydrogen (H17 or H18) of the cation in configurations a, c, d, and f all have been taken away by the electronegative anion, when the cation combines with the anion. The corresponding ion pairs have turned to neutral pairs (the corresponding O · · · H or N · · · H distances are shown in boldface in Table 1). Let us take the configuration a for example; the O8 · · · H17 distance has changed from 0.972 Å (in conformer β) to 1.490 Å, which is far beyond the length of a normal O-H covalent bond, while the O21 · · · H17 bond length becomes a normal O-H covalent bond when the cation combines with the anion. It indicates that the proton H17 has transferred from cation to anion, when going from isolated [Pro]+ and [NO3]to configuration a. Most of the H bond lengths between fragments in configurations are shorter than the common H bond. Especially in configuration a, the bond length of O8-H17 is only 1.490 Å, which indicates that these H bonds are very strong. For the nonproton transfer configurations b and e, the hydrogens on the nitrogen atom and the oxygen atom of the cation are both hydrogen bonded to the anion and the proton transfer does not occur. It may be that the electronegativity of the anion is dispersed by the two electronegative hydrogen atoms (H17 and H18) that formed two H bonds with the anion simultaneously. Further investigations are focused on the geometry changes of the cation and the anion when they go from isolated [Pro]+ and [NO3]- to configurations. As for [Pro]+, no obvious geometry changes can be observed, except that the tetrahydropyrrole ring has some changes (distorted out of planarity) when going from isolated [Pro]+ to configurations (Table 1). For [NO3]-,

the bond lengths of N-O in configurations are lengthened, when they are involved in the formation of the N-O · · · H bonds, especially the N-O bonds that are connected to the newly formed O-H covalent bond (by H transfer), while the other N-O bond lengths are shortened in contrast with the corresponding bond lengths in isolated [NO3]- (Table 1). This may be assigned to the formation of the intermolecular H bonds N-O · · · H and the O-H covalent bond. The formation of these bonds gives rise to lengthening of N-O bond lengths, and shortening of the other N-O bond in accordance with the bond order conservation principle.48 Furthermore, as shown in Table 1, the corresponding N-O bond lengths in configurations a, c, d, and f are similar to the optimized neutral HNO3 molecular that the three N-O bonds are 1.417, 1.194, and 1.209 Å, respectively. Moreover, due to the destroying of D3h symmetry of [NO3]-, the bond angles of ∠O-N-O in configurations are changed significantly in comparison with isolated [NO3]-. But the coplanar characteristic of the anion is still maintained after the formation of the single ion-pair unit (Table 1). 3.1.2. AIM Analysis. The bond properties between each pair of atoms were systematically analyzed using atoms in molecules (AIM) theory,43 which is based on the topological analysis of electron density (Fc) and its Laplacian (32Fc) at the bond critical points (BCP). The Laplacian 32Fc at the BCP is the sum of the three curvatures of the density at the critical point, the two perpendicular to the bond path, λ1 and λ2, being negative (by convention, λ1 > λ2) whereas the third, λ3, lying along the bond path, is positive. The negative curvatures measure the extent to which the density is concentrated along the bond path and the positive curvature measures the extent to which it is depleted in the region of the interatomic surface and concentrated in the individual atomic basins. In covalent bonding the two negative curvatures are dominant and 32Fc < 0; in contrast, in closedshell bonding, for example, ionic, hydrogen bonding, or van der Waals interactions, the interaction is characterized by a depletion of density in the region of contact of the two atoms and 32Fc > 0. Fc is used to describe the strength of a bond, a stronger bond associated with a larger Fc value. In general, Fc

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TABLE 2: Properties of the Electron Density of Bond Critical Point (bcp) for the Atom Pairs of Configurations a-f Calculated at the B3LYP/6-311++G** Level (au) conformers

atom pairs

λ1

λ2

λ3

FBCP

∆2FBCP

a

O21-H17 H17-O8 O22-H18 O22-H14 O21-H17 H17-O8 O22-H18 H18-N5 O22-H18 H18-N5 O20-H17 O22-H18 H18-N5 H17-O8 O20-H17 O22-H18 N5-H18 H17-O8 O20-H11 O20-H16 H16-N5 O22-H15

-1.4129 -0.1542 -0.0119 -0.0073 -0.1533 -1.3493 -0.1195 -1.1121 -1.3984 -0.1156 -0.0453 -1.249 -0.1487 -1.7558 -0.0991 -0.1751 -0.9881 -1.5835 -0.0070 -1.1947 -0.1733 -0.0070

-1.3801 -0.1507 -0.0067 -0.0056 -0.1485 -1.3265 -0.1157 -1.1098 -1.3618 -0.1140 -0.0434 -1.2161 -0.1471 -1.7297 -0.0944 -0.1666 -0.9862 -1.5633 -0.0064 -1.1619 -0.1720 -0.0047

0.9183 0.4709 0.0612 0.0442 0.4493 0.8862 0.3880 0.7619 0.9019 0.3192 0.1973 0.8638 0.3735 1.0412 0.3367 0.4796 0.7651 0.9847 0.0449 0.8628 0.4152 0.0391

0.2870 0.0728 0.0118 0.0082 0.0723 0.2821 0.0620 0.2882 0.2923 0.0628 0.0301 0.2730 0.0754 0.3468 0.0534 0.0803 0.2657 0.3166 0.0091 0.2658 0.0833 0.0079

-1.8748 0.1660 0.0427 0.0312 0.1475 -1.7896 0.1527 -1.4600 -1.8583 0.0896 0.1086 -1.6013 0.0777 -2.4443 0.1432 0.1379 -1.2092 -2.1621 0.0315 -1.4938 0.0698 0.0275

b

c d

e

f

is greater than 0.20 au in shared (covalent) bonding and less than 0.10 au in a closed-shell interaction. From the values listed in Table 2, it can be concluded that the interactions between the cation and the anion which are marked by the dotted line are all closed shell systems (H bonding interaction). For values of the 32Fc, all fall in the range of a normal H bond (0.020-0.139 au), and Fc is no more than 0.20 au. While for the H17-O8 in b, d, and e, O20-H17 in a, etc., all are 32Fc < 0, and the corresponding Fc are all greater than 0.20 au and are characterized as covalent bonds.49 Above all, just as the geometry analysis mentioned above, the corresponding protons in configurations a, c, d, and f really have been taken away by the electronegative anion (Figure 2). The H bonds are formed between the neutral nitric acid molecule and the proline molecule. As for the configurations b and e, the anion formed two H bonds simultaneously with two electronegative H (H atoms on O8 and N5), also confirmed by AIM analysis (Table 2). 3.1.2. The H-Transfer Reaction between the Cation and the Anion. It is well-known that proton transfer in the gas phase may be a critical step when ILs evaporate, sublimate, or chemically decompose.50 Probably, the balance between the cation and anion should be destroyed through the proton transfer between [Pro]+ and [NO3]-. To investigate this point, the protontransfer mechanisms have been studied. As a result, for configuration f the designed proton-transferred products have been optimized into the corresponding reactants spontaneously, implying the infeasibility of the proton transfer. On the other hand, as displayed in Figure 3, four transition states corresponding to the proton transfer reaction in configurations a-e have been located, where their unique imaginary frequencies are 314.87i, 210.57i, 643.44i, and 675.41i cm-1, respectively. Further intrinsic reaction coordinate (IRC) analysis suggests that those transition states really connect the corresponding reactants and products. The energy changes along the IRC are presented in Figure 4. The IRC analysis has also revealed the inherent relationship between the six locally stable configurations. The results show that the energy barrier of the H-transfer reaction between the ions is small (TSab only 1-2 kJ/mol, TSbc 6 kJ/mol, TSde5 kJ/mol, and TSae about 13-15 kJ/mol, Figure 4) compared with the amide-, triazolium-, tetrazolium-, and pentazole-based ionic liquids,51-54 which means the H-transfer

Figure 3. Four transition states corresponding to the proton transfer reactions in configurations a-e at the B3LYP/6-311++G** level of theory (TSab means transition state linking configurations a and b).

reaction between the ions here is relatively easier. Furthermore, for TSab and TSde the energy barrier heights of a f TSab and d f TSde are smaller than their counterparts b f TSab and e f TSde, which means the proton H17 in configuration a and H18 in configuration d are more inclined to transfer from HNO3 to Pro than from [Pro]+ to [NO3]-. Thus, the proton-transferred products (neutral pairs) should convert to corresponding ion pairs instantaneously even if the proton transfer occurs; just as its name implies, the ionic liquid considered here is composed of the cation and anion. While for the transition states TSbc and TSae, their IRC curves are sharp and high symmetry; the forward energy barrier heights are similar to those of the reverse results. It indicates that the corresponding neutral pairs and ion pairs are balanced in the H-transfer reaction, or the result is consistent with TSab and TSde that the H-transfer reaction is more inclined to change the system to exist as ion pairs b and e rather than neutral pairs c and a. Finally, as for TSae, the calculated H-transfer reaction energy barrier heights are relatively larger than other three transition states, which means the less likely of the corresponding H-transfer reaction occurs, compared to other H-transfer reactions. 3.1.3. Binding Energies between Cation and the Anion. The interaction energies of the configurations and their corrected values of BSSE and ZPE at B3LYP/6-311++G** and MP2/ 6-31+G* level are shown in Table 3.41 As can be seen, the interaction energies that between the ion pair (b and e) are much more larger than that between the neutral pairs (a, c, d, and f). It is due to the existence of strong electrostatic forces in the ion pair. This comparison also indicates that the electrostatic forces play an important role in the cation-anion interaction of the ILs. The proton-transferred product f possesses the lowest total energy among the six configurations, with a value of -682.112836 au. Henceforth, this structure is taken as the zero energy structure against which all other energies of the remaining structures are compared. The relative total energy (∆EZPE*) of the configurations are listed in the Table 3. ∆EZPE* of configurations b and

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Figure 4. The IRC analysis of the proton transfer reaction and the transition states linking the ionic pairs to the neutral dimers (step size 0.1 (amu)1/2 bohr).

TABLE 3: The Total Interaction Energies (∆E), the Interaction Energies Corrected by BSSE and ZPE (∆EBSSE+ZPE), the Relative Energy (∆EZPE*), and the Corrected Values of BSSE and ZPE at the B3LYP/ 6-311++G** Level (kJ/mol)a conformers ∆BSSE ∆ZPE a b c d e f

5.46 4.90 6.48 7.70 6.95 6.39

∆E

∆EBSSE+ZPE

4.40 -100.56 -90.69 (-45.21) -0.56 -514.44 -510.10 2.45 -57.86 -51.38 (-13.89) 5.76 -71.52 -63.82 (-24.09) 0.34 -545.60 -538.65 3.97 -90.17 -79.80 (-58.52)

relative EZPE* 22.13 21.07 21.81 24.99 19.20 0

a Configuration f with a total energy of -682.112836 au. Note, as shown in the table, here the interactions in conformers a, c, d, and f (neutral pairs) have also been calculated at the MP2/6-31+G* level (in parentheses).

e which have the relatively larger interaction energy values are not the configuration that has the lowest total energy. This result is not consistent with the former research which is generally the higher the binding energy the lower the total energy. This may be due to influence of the H-transfer reaction which changes the components of the system from ion pairs to neutral pairs.55 According to the calculated results that the small energy barriers for H-transfer reaction (Figure 4) and the big difference of the interaction energy in the ion pair and the neutral pairs

(Table 3), one possible process can be inferred, which may play the role in the changing of physicochemical properties of the ILs. That is the overall interaction intensity between the fragments in the bulk AAILs will significantly be weakened when the system is changed from ion pair to neutral pair through H-transfer reaction. The weakening of the interaction intensity between the ions may lead to the change of physicochemical properties of the ILs. As the former research mentioned that the proton transfer may be a critical step when ionic liquids change in physical and chemical properties. Here, the weakening of the interaction intensity in the bulk AAILs, which is caused by an H-transfer reaction may play the role. This result may throw some light on the fact that AAILs [Pro]+[NO3]- namely is the only room temperature free-flowing liquid in the family of amino acid cation based AAILs and it is easy to decompose; the decomposition temperature is only 138 °C.12,14 3.2. Ion-Pair Dimer. In many cases these calculations have been restricted to a single ion or ion pair (consisting of one cation and one anion, and only one cation and one anion have been considered) in the past research.22,23 Moreover, the description of the cation-anion interactions of the ILs, which was solely based on the single ion-pair unit is certainly not sufficient enough, because it was idealized, a key point is that the cooperative effect between the ions and between the ion pairs have been neglected during the calculation.56 For example, structures with free H bonding sites that are higher in energy

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B

C

D +

-

Figure 5. The stable [Pro]2 [NO3]2 clusters optimized at the B3LYP/6-311++G** level of theory. The corresponding parts are defined and marked out for the discussion of the interaction energies list in Table 8.

may actually be more important in the bulk IL, because the free H bonding sites have the potential ability to combine with other fragments. So in order to give a more reasonable description of the AAIL system, studies of larger clusters containing up to four component units have also been attempted. The ionpair dimer [Pro]2+[NO3]2- has been introduced, which is composed of two cations and two anions, because this double ion-pair coexisting system is more similar to the real situation that multi-ion-pairs coexist in the air phase or condensed phase. 3.2.1. Optimized Geometries. Various chemically reasonable structures have been optimized at the B3LYP/6-311++G** level, and all the possible cation-anion interaction modes have been taken into account during the initial geometry design for the ion-pair dimer. Finally, four stable geometries with no imaginary frequency were obtained (Figure 5). The active sites for the cation-anion interaction in the ion-pair dimer are also mainly the N-H and O-H, which are similar to the single ionpair unit. The cations in the configurations are connected by the anions, and the while configurations all show a ringlike structure. The main difference between the configurations of the ion-pair dimer and the single ion-pair unit is that the protontransferred products (neutral pairs) have been located in a single ion-pair unit, but they have not been obtained in the ion-pair dimer (Figure 2 and Figure 5). The structures of ion-pair dimer are all ionic, H-transfer reaction does not occur, while some structures of the single ion-pair unit are neutral pairs and the corresponding fragments are connected by the normal H bonds (Figure 2 and Figure 5). 3.2.2. AIM and Vibrational Frequency Analysis. The AIM analysis was also performed to analyze the interactions between the various fragments in the two ion-pairs coexisting system. From the figures listed in Table 4, a conclusion can be draw; it is that the interactions marked by the dot line between the cations and the anions in the ion-pair dimer are all closed shell systems, and they can be classified as classical H bonds.49,59 The properties of the hydrogen bonds were further investigated by vibrational frequency analysis. As presented in Figure

O42-H17 O43-H39 O20-H16 O22-H15 O22-H38 O20-H16 O21-H34 O21-H37 O22-H38 O22-H15 O42-H18 O44-H40 O43-H39 O42-H17 O42-H18 O29-H18 O21-H38 O20-H12 O22-H16 H40-O43 H38-O22 H37-O21 H16-O20 H15-O22 O42-H18

λ1

λ2

λ3

FBCP

∆2FBCP

-0.1478 -0.2493 -0.0840 -0.0099 -0.1031 -0.0771 -0.0062 -0.0059 -0.0811 -0.0062 -0.0795 -0.0728 -0.1805 -0.0871 -0.0388 -0.0056 -0.1072 -0.0058 -0.0770 -0.0833 -0.0767 -0.0092 -0.0768 -0.0064 -0.0892

-0.1410 -0.2409 -0.0796 -0.0067 -0.0980 -0.0727 -0.0051 -0.0044 -0.0764 -0.0048 -0.0741 -0.0683 -0.1743 -0.0819 -0.0380 -0.0037 -0.1017 -0.0053 -0.0733 -0.0786 -0.0715 -0.0086 -0.0725 -0.0041 -0.0836

0.4429 0.6131 0.2899 0.0547 0.3459 0.2768 0.0341 0.0347 0.2864 0.0338 0.2825 0.2647 0.5007 0.3083 0.1892 0.0347 0.3435 0.0310 0.2828 0.2892 0.2773 0.0478 0.2743 0.0351 0.3057

0.0695 0.0988 0.0473 0.0107 0.0550 0.0447 0.0070 0.0071 0.0462 0.0069 0.0456 0.0429 0.0796 0.0488 0.0287 0.0068 0.0566 0.0065 0.0445 0.0471 0.0443 0.0096 0.0446 0.0073 0.0495

0.1541 0.1229 0.1263 0.0381 0.1449 0.1270 0.0229 0.0245 0.1289 0.0228 0.1289 0.1236 0.1459 0.1393 0.1123 0.0253 0.1346 0.0199 0.1326 0.1273 0.1291 0.03 0.125 0.0246 0.1328

6, the X-H (X ) O, N, C) bonds that are involved in the formation of the XH · · · O H bonds all have frequency (stretching vibrations) changes in going from the isolated ions to the hydrogen-bonded structures. It can be observed that these O-H and N-H bonds all have a red shift after the formation of the ion-pair dimer, and their corresponding H bonds are red-shifted hydrogen bonds. However, the C-H bonds are all shifted to the direction of blue; their corresponding H bonds are blueshifted hydrogen bonds.49 This feature is consistent with the character of O-H, N-H, and C-H bond elongation in forming an ion-pair dimer (O-H and N-H bonds are elongated, while C-H bonds are shortened, Table 5). The magnitudes of the frequency changes for these X-H bonds are generally in agreement with the trend of the magnitudes of their elongation. The O30-H39 bond in configuration A is the largest in magnitude of bond elongation among all the configurations (Table 5), correspondingly the O30-H39 bond in configuration A has the largest value of frequency changes (Figure 6). All these features indicate the formation of the true H bonds between the various fragments. 3.2.3. Binding Energy between Ions. In this part, the interaction between the various fragments of the ion-pair dimer was studied systematically, including the total interaction between the four ions (Table 6), the interaction between one cation and one anion within the cluster, [Pro]+ vs [NO3]- (Table 7), and interactions between the two ion pairs, [Pro]+[NO3]vs [Pro]+[NO3]- (Table 8). The corresponding fragments are defined and marked out in Figure 5. In order to get a more reasonable result, when we calculated the interaction energies in Table 7, the ghost atoms were set. Detailed information is given along with tables. Tables 6, 7, and 8 give the interaction energies between the various parts of the two ion-pairs coexisting system, and it can support a systematic analysis of this system. Table 6 shows that configuration C with the largest interaction energy may be the most favorable configuration. From the list in Tables 7 and 8, it can be found that the interaction intensity between the cation

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Figure 6. Histograms of the selected stretching vibration frequency of the X-H bond for each configuration. The heights of the bar are the vibrational frequency of the X-H bond in the monomer and X-H (H-bond) associated with XH · · · Y (Y ) O, N) contact.

TABLE 5: The Bond Length Changes (Å) in the Ion-Pair Dimer Compared to the Original Free Fragment [Pro]+ and [NO3]A

B

C

D

bonds

∆r

bonds

∆r

bonds

∆r

bonds

∆r

O8-H17 O30-H39 C4-H15 N27-H38 N5-H16

0.058 0.098 -0.003 0.035 0.033

N5-H16 N5-H18 C4-H15 C24-H34 C26-H37 N27-H38 N27-H40

0.027 0.015 -0.002 -0.001 -0.004 0.029 0.023

O7-H17 N5-H18 N5-H16 O30-H39 C2-H12 N27-H38

0.024 0.005 0.028 0.071 -0.001 0.043

N27-H40 N27-H38 C26-H37 N5-H18 N5-H16 C4-H15

0.018 0.027 -0.001 0.020 0.028 -0.001

TABLE 6: The Total Interaction Energies (∆E), the Total Interaction Energies That Are Corrected by BSSE and ZPE (∆EBSSE+ZPE), the Relative Energy (∆EZPE*), and the Corrected Values of BSSE and ZPE for the Ion-Pair Dimers at the B3LYP/6-311++G** Level (kJ/mol)a conformers ∆BSSE ∆ZPE A B C D

15.70 16.65 17.46 16.81

4.21 9.68 9.64 8.24

∆E -1056.27 -1049.43 -1143.57 -1042.26

∆EBSSE+ZPE relative EZPE* -1036.36 -1023.09 -1116.47 -1017.21

0 7.17 1.42 6.15

a Counterpoise ) 4, P1 (fragment 1), P2 (fragment 2), P3 (fragment 3), P4 (fragment 4). Configuration A with a total energy of -1364.598545au.

and anion (Table 7 an average of -422.77 kJ/mol) is stronger than that between the two ion pairs (Table 8 an average of -211.14 kJ/mol). It means that the weakest interaction intensity between the various fragments is between the two neutral ion pairs but not between the cation and anion, as far as the two ion pair coexisting system is concerned. Thus, we can speculate that the physicochemical properties of ILs, which may relate to the so-called binding energy, will be determined by the

binding energy between neutral ion pairs rather than the interaction between the anion and cation. Some experiments show that the thermal transfer of ILs into the gaseous phase occurs only via neutral ion pairs. Free ions and larger charged or uncharged ion clusters are not relevant in the gas phase.58,59 Here, the result will further give hints on the IL vaporization process at the molecular level. That is, for [Pro]+[NO3]-, it is very likely to evaporate from the condensed phase to the gas phase as the form of neutral ion pairs. So, in this way, some of the physicochemical property of the ILs, such as the viscosity, melting point, vapor pressure, thermal stability, etc., which may have a relationship with the interaction energy will be more reasonable to be evaluated by the interaction energy between the neutral ion pairs than the interaction energies between the cation and anions. It is not enough to explore the relationship between the physicochemical property and interaction within the ILs based solely on the cation-anion interaction, the interaction between the different fragments of the system should be studied systematically, and the relationship between them needs to be further explored.

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TABLE 7: The Interaction Energies (∆E), the Interaction Energies That Are Corrected by BSSE (∆EBSSE), and the Corrected Values of BSSE for the Ion-Pair Dimers at the B3LYP/6-311++G** Level (kJ/mol)a conformers

interaction model

∆BSSE

∆E

∆EBSSE

A

P1 vs P2 P1 vs P4 P2 vs P3 P3 vs P4 P1 vs P2 P1 vs P4 P2 vs P3 P3 vs P4 P1 vs P2 P1 vs P4 P2 vs P3 P3 vs P4 P1 vs P2 P1 vs P4 P2 vs P3 P3 vs P4

3.85 2.83 2.75 3.17 3.00 3.01 3.39 3.25 3.70 4.32 3.67 3.33 3.11 3.30 3.93 3.11

-430.45 -350.81 -431.07 -404.12 -442.62 -417.11 -438.22 -424.71 -448.31 -486.05 -448.07 -378.42 -446.01 -412.96 -433.26 -425.85

-426.61 -347.98 -428.32 -400.94 -439.62 -414.10 -434.83 -421.46 -444.61 -481.73 -444.39 -375.09 -442.90 -409.65 -429.33 -422.74

B

C

D

a P1 vs P2 means the interaction between P1 (fragment 1) and P2 (fragment 2) in corresponding configuration, when the atoms in the remaining part of the configuration (P3and P4) are set as ghost atoms. The remaining symbols also express similar meaning.

TABLE 8: The Interaction Energies (∆E), the Interaction Energies That Are Corrected by BSSE (∆EBSSE), and the Corrected Values of BSSE for the Ion-Pair Dimers at the B3LYP/6-311++G** level (kJ/mol)a interaction models

∆BSSE

∆E

∆EBSSE

A (P1+P2) vs (P3+P4) B (P1+P2) vs (P3+P4) C (P1+P2) vs (P3+P4) D (P1+P2) vs (P3+P4) A (P1+P4) vs (P2+P3) B (P1+P4) vs (P2+P3) C (P1+P4) vs (P2+P3) D (P1+P4) vs (P2+P3)

8.17 10.21 11.18 10.04 9.74 10.08 9.47 9.76

-221.69 -182.10 -316.84 -170.39 -274.38 -196.86 -209.45 -196. 04

-213.52 -171.89 -305.66 -160.35 -264.64 -186.78 -199.98 -186.28

a (P1+P2) vs (P3+P4) means the interaction between P1+P2 (take P1and P2 as fragment 1) and the P3+P4 (take P3 and P4 as fragment 2) in configuration A. The remaining symbols also express similar meaning.

4. Conclusions In this work, amino acid ionic liquid [Pro]+[NO3]- has been systematically studied at the electronic or molecular levels. The main conclusions that can be reached are as follows: (1) In the single [Pro]+[NO3]- unit, proton transfer between [Pro]+ and [NO3]- has been found in some of the configurations and the corresponding proton-transferred products are strongly hydrogen bonded. However, in the ion-pair dimer [Pro]2+ [NO3]2-, proton transfer does not occur and the components are all stabilized by ionic interactions and H bond interaction jointly. This may be because the electronegativity or the proton absorption capacity of the anion is dispersed by the multiinteractions from the cations. The electronegativity or the proton absorption capacity of the anion is thus not sufficient enough to have the proton transfer reaction occur. (2) The possible H-transfer reaction path between the cation and the anion in a single ion-pair unit demonstrates that the ionic liquids considered here tend to exist in the form of ion pairs rather than neutral pairs, just as its name implies, composed of the cation and anion. However, due to the low energy barrier of H-transfer reaction, the balance between the cation and anion should be destroyed through the proton transfer, which changes

thesystemfromthecation-anioninteractiontothemolecule-molecule interaction. Moreover, the significantly declined bonding energy, when going from ion-pair interaction to neutral-pair interaction, may play an important role in the physicochemical property changes of ILs. (3) In the ion-pair dimer [Pro]2+[NO3]2-, the interaction intensity between [Pro]+ and [NO3]- is stronger than the interaction between the two ion pairs ([Pro]+[NO3]- vs [Pro]+[NO3]-). This result will enrich the investigations that try to relate ion-pair association energy, a derived “connectivity index”, and the diversity of structures with the physicochemical property of the ILs. (4) However, all these calculated results and discussions were only limited in the gas phase, the salvation effect on the calculated results and discussions was not taken into consideration. Especially the common water molecules, it may also play a role in the proton transfer reaction in the real bulk ILs, the salvation effect on the H-transfer reaction between the cation, and the anion should be cleared and the salvation effect on the ILs need to be further studied in the future work. Certainly, more extensive studies on the larger cluster of the ion pairs and the salvation effect are in progress in our laboratory. Above all, these conclusions will be helpful for understanding of these AAILs and will contribute to the design and synthesis of AAILs in a “task specific” way. References and Notes (1) Welton, T. Chem. ReV. 1999, 99, 2071–2083. (2) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792–793. (3) Ionic liquids: industrial applications for green chemistry; Rogers, R. D., Seddon, K. R., Eds.; ACS Symposium Series 818; American Chemical Society: Washington, DC, 2002. (4) Wassercheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772– 3789. (5) Plechkova, N. V.; Seddon, K. R. Chem. Soc. ReV. 2008, 37, 123– 150. (6) Welton, T. Coord. Chem. ReV. 2004, 248, 2459–2477. (7) Horvath, I. T. Green Chem. 2008, 10, 1024–1028. (8) Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Green Chem. 2003, 5, 361–363. (9) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351–356. (10) Swatloski, R. P.; Spear, S. K.; Holbery, J. D. J. Am. Chem. Soc. 2002, 124, 4974–4795. (11) Handy, S. T. Chem.sEur. J. 2003, 9, 2938–2944. (12) Tao, G. H.; He, L.; Sun, N.; Kou, Y. Chem. Commun. 2005, 28, 3562–3564. (13) Plaquevent, J. C.; Levillain, J.; Guillen, F.; Malhiac, C.; Gaumont, A. C. Chem. ReV. 2008, 108, 5035–5060. (14) Chen, X.; Li, X.; Hu, A.; Wang, F. Tetrahedron: Asymmetry. 2008, 19, 1–14. (15) Pretti, C.; Chiappe, C.; Pieraccini, D.; Gregori, M.; Abramo, F.; Monnia, G.; Intorre, L. Green Chem. 2006, 8, 238–240. (16) Ohno, H.; Fukumoto, K. Acc. Chem. Res. 2007, 40, 1122–1129. (17) Fukumoto, K.; Yoshizawa, M.; Ohno, H. J. Am. Chem. Soc. 2005, 127, 2398–2399. (18) Tao, G.; He, L.; Liu, W.; Xu, L.; Xiong, W.; Wang, T.; Kou, Y. Green Chem. 2006, 8, 639–646. (19) Fang, D. W.; Guan, W.; Tong, J.; Wang, Z. W.; Yang, J. Z. J. Phys. Chem. B 2008, 112, 7499–7505. (20) Yang, J. Z.; Zhang, Q. G.; Wang, B.; Tong, J. J. Phys. Chem. B 2006, 110, 22521–22524. (21) Guan, W.; Xue, W. F.; Li, N.; Tong, J. J. Chem. Eng. Data 2008, 53, 1401–1403. (22) Mou, Z.; Li, P.; Bu, Y.; Wang, W.; Shi, J.; Song, R. J. Phys. Chem. B 2008, 112, 5088–5097. (23) Rong, H.; Li, W.; Chen, Z.; Wu, X. J. Phys. Chem. B 2008, 112, 1451–1455. (24) Turner, E. A.; Pye, C. C.; Singer, R. D. J. Phys. Chem. A 2003, 107, 2277–2288. (25) Zhang, S. J.; Lv, X. M. Ionic liquidssfrom fundamentals to applications; Scientific Publish, Ltd.: Beijing, China, 2006. (26) Zhang, S. J.; Sun, N.; Zhang, X. P. Sci. China, Ser. B: Chem. 2006, 49, 103–115. (27) Kirchner, B. Top. Curr. Chem. 2009, 290, 213–262.

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