Environ. Sci. Technol. 2011, 45, 288–293
Density Functional Theory Study on Aqueous Aluminum-Fluoride Complexes: Exploration of the Intrinsic Relationship between Water-Exchange Rate Constants and Structural Parameters for Monomer Aluminum Complexes X I A O Y A N J I N , † Z H A O S H E N G Q I A N , †,‡ BANGMEI LU,† WENJING YANG,† AND S H U P I N G B I * ,† School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry of China & Key Laboratory of MOE for Life Science, Nanjing University, Nanjing 210093, China, and College of Chemistry and Life Science, Zhejiang Normal University, Jinhua, Zhejiang 321004, China
Received December 18, 2009. Revised manuscript received October 29, 2010. Accepted November 12, 2010.
Density functional theory (DFT) calculation is carried out to investigate the structures, 19F and 27Al NMR chemical shifts of aqueous Al-F complexes and their water-exchange reactions. The following investigations are performed in this paper: (1) the microscopic properties of typical aqueous Al-F complexes are obtained at the level of B3LYP/6-311+G**. AlsOH2 bond lengths increase with F- replacing innersphere H2O progressively, indicating labilizing effect of Fligand. The Al-OH2 distance trans to fluoride is longer than other AlsOH2 distance, accounting for trans effect of Fligand. 19F and 27Al NMR chemical shifts are calculated using GIAO method at the HF/6-311+G** level relative to F(H2O)6and Al(H2O)63+ references, respectively. The results are consistent with available experimental values; (2) the dissociative (D) activated mechanism is observed by modeling water-exchange reaction for [Al(H2O)6-iFi](3-i)+ (i ) 1-4). The activation energy barriers are found to decrease with increasing Fsubstitution, which is in line with experimental rate constants (kex). The log kex of AlF3(H2O)30 and AlF4(H2O)2- are predicted by three ways. The results indicate that the correlation between log kex and AlsO bond length as well as the given transmission coefficient allows experimental rate constants to be predicted, whereas the correlation between log kex and activation free energy is poor; (3) the environmental significance of this work is elucidated by the extension toward three fields, that is, polyaluminum system, monomer Alorganic system and other metal ions system with high chargeto-radius ratio.
* Corresponding author phone: +86-25-8620 5840; fax: +86-258331 7761; e-mail:
[email protected]. † Nanjing University. ‡ Zhejiang Normal University. 288
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1. Introduction Aluminum is the most abundant metal in Earth’s crust and aluminum chemistry investigation is concerned by many researchers due to its great significance in environment and geoscience (1-5). On the one hand, the elevated levels of soluble aluminum in aquatic ecosystems arising from the acidification of surface water have a striking biotic impact (6-8). The impact is sensitive to the aluminum species and the species are influenced directly by the ubiquitous ligands in environment, such as F- or carboxylate (9, 10). For this reason the aqueous Al-F complexes investigation provokes many environmental researchers’ interest. On the other hand, the previous investigations suggest that F- considerably labilizes GaAl12 molecule serving as a model of mineral surface reaction (11) and enhances the dissolution of aluminum mineral (12, 13). To our knowledge, it is commonly difficult to explore the mineral surface reaction mechanism dominating the overall environmental and geochemical process. However, the analogies between reactions at mineral surfaces and in dissolved metal-ligand complexes confirmed provide a possibility of constraining the surface reaction pathway (14). Thus the study on the reaction mechanism and reactivity of aqueous Al-F complexes is helpful to understand the surface reactions. Potentiometry and nuclear magnetic resonance (NMR) are the main methods of investigating aqueous Al-F complexes. Brosset and Orring identified six distinct aqueous Al-F species by potentionmetric method in earlier study (15), that is, [Al(H2O)6-iFi](3-i)+ (i ) 1-6). Since Connick et al. first employed 19F and 27Al NMR to study their species and kinetics (16), NMR method has been widely applied in studying binary and ternary (with OH-) aqueous Al-F complexes (17-19). Although octahedral conformation was admitted for all the Al-F complexes in above studies, theoretical studies can further provide insights into their structures which are difficult to obtain experimentally. The combined experimental and theoretical studies indicate that the quantum chemical calculation of NMR chemical shift is very useful to test the appropriateness of molecular model and previous peak assignments (20, 21). Tossell calculated 19 F and 27Al NMR shieldings for the lower-order aqueous Al-F species (22, 23), but the calculated data is absent for the higher-order species. For the water-exchange reactions on [Al(H2O)6-iFi](3-i)+ (i e 2) 17O NMR was applied to determine the rate constants and dissociative interchange (Id) mechanism was attributed (24, 25). It was suggested that reaction rates increased by a factor of about 102 with Fsubstitution stepwise for H2O in the inner-coordination sphere. However, it is difficult to study the higher-order species (i > 2) due to low concentration in the experimental condition. In this regard the theoretical calculation as a promising approach can unravel microscopic process for water-exchange reactions on Al-F complexes involving the higher-order species. More recently the water-exchange reactions on a series of monomer and polyaluminum species were simulated successfully by our group, which lays foundation for studying water exchange of Al-F complexes (26-29). The present work focuses on the following three aspects: (1) to investigate the structures of Al-F complexes in aqueous solution by DFT method and calculate 19F and 27Al NMR chemical shifts using F(H2O)6- and Al(H2O)63+ references, respectively, followed by comparing them with experimental results; (2) to simulate water-exchange reactions for [Al(H2O)6-iFi](3-i)+ (i ) 1-4) and predict water-exchange rate 10.1021/es102872h
2011 American Chemical Society
Published on Web 12/06/2010
constants of the higher-order species; (3) to reveal the environmental significance of this study by extending the work to three environmentally relevant fields, that is, polyaluminum system, monomer Al-organic system and other metal ions system with high charge-to-radius ratio.
TABLE 1. Calculated 19F NMR and 27Al NMR Shieldings and Shifts (in ppm) with SM-PCM Models for Aqueous Al-F Complexes by GIAO Method at The HF/6-311+G** Levela 19
2. Computational Details According to our previous work, the supermolecule-polarizable continuum model (SM-PCM) is suitable to model aqueous Al complex systems (26-29). Thus all the calculations are based on the SM-PCM model in this study (Details see Section 1 in the Supporting Information (SI)). All the geometry optimizations and frequency calculations were carried out at the level of B3LYP/6-311+G** (30, 31). Zeropoint energy (ZPE) was employed for energy correction. Atomic charges were obtained from natural population analysis (NPA). 19F and 27Al NMR shieldings were calculated using both Gauge-Independent Atomic Orbital (GIAO) and Hartree-Fock (HF) method in combination with 6-311+G** basis set based on the above optimized geometries and the HF method was demonstrated to be the best previously (32). It was indicated that there are six water molecules around F- in aqueous solution by the neutron diffraction experiment (33). Therefore, F(H2O)6- and Al(H2O)63+ were used as the references to obtain 19F and 27Al chemical shifts, respectively and their geometries were optimized by PCM at the level of B3LYP/6-311+G**. In order to simulate a water-exchange reaction it is necessary to reconstruct the second hydration sphere due to the rigid hydrogen bond network without a hole for water exchange in the above supermolecule model (34). Reactants, transition states, and products for waterexchange reaction were fully optimized with reconstructing supermolecule model at the level of B3LYP/6-311+G** and vibrational calculations were carried out to confirm the stable structures and the transition states. Single-point PCM using MP2 method was carried out on the basis of the supermolecule models to obtain more accurate energies. The activation enthalpy (∆H*) for water exchange reactions was obtained by single-point PCM method, and the activation entropy (∆S*) was achieved from frequency calculations. The transition-state rate constant (kTST) was calculated by Eyring equation (35). The relationship of kex and kTST was expressed by the equation of kex ) κ kTST where κ denotes transmission coefficient (36).
3. Results and Discussion 3.1. Structures and NMR. Optimized geometries of a series of [Al(H2O)6-iFi](3-i)+ (i ) 1-6) are displayed in SI Figure S1. The previous experimental results show that the structures are octahedral coordination even if four F- are bound to Al3+, which differ from tetrahedral Al(OH)4- species (19, 37). The corresponding structural parameters and energies are given in SI Table S1. The energetic and structural features on the Al-F complexes are found as follows: (1) the energy differences between various isomers are very little, suggesting that the isomers can coexist in aqueous solution; (2) AlsOH2 bond length greatly increases with the replacement of bound water by F- stepwise (see SI Figure S5A). This is consistent with the rising water-exchange rates as a function of the number of F- ligands (25). One can note that the distance of AlsOH2 bond trans to fluoride is greater than that of other AlsOH2 bonds, which means that trans water molecule is more labile than other water molecules (More details in latter section); (3) according to natural population analysis (NPA), the charge on central Al atom increases markedly, whereas the charge on F atom, oxygen charge and hydrogen charge on bound H2O vary slightly (see SI Figure S5B). The calculated 19 F NMR shifts using SM-PCM models are close to experimental values, as shown in Table 1. Optimized geometry of F(H2O)6- reference is presented in SI Figure S6. Instead of
27
F NMR
complex
σ
δ
δ (expl.)
Al(H2O)5F2+ cis-Al(H2O)4F2+ trans-Al(H2O)4F2+ mer-Al(H2O)3F30 fac-Al(H2O)3F30 cis-Al(H2O)2F4trans-Al(H2O)2F4Al(H2O)F52-
375.0 372.9 372.2 375.1 371.6 372.4 372.3 373.8
-35.9 -33.8 -33.1 -36.0 -32.5 -33.3 -33.2 -34.7
-37.3 -36.9 -36.2 -33.3 -31.7
b
Al NMR
σ
δ
δ (expl.)c
609.7 609.3 608.2 610.4 609.5 610.9 610.4 612.2
-0.4 0 1.1 -1.1 -0.2 -1.6 -1.1 -2.9
0.7 1.3 -
a
“-” means no data available. Reference shieldings of F(H2O)6- and Al(H2O)63+ are 339.1 ppm and 609.3 ppm respectively, which are obtained at the same calculated level. Chemical shift δ ) σ (reference) -σ (complex). We assume 19F shielding depends on the favored F atoms in number with respect to nonequivalent F atoms. b Experimental 19F chemical shifts from ref 19. The values of Al(H2O)2F4- and [Al(H2O)F5]2- are from a solution with 0.6 M TMACl at 278 K. c Experimental 19Al chemical shifts from ref 39.
a common reference CFCl3, F(H2O)6- reference is applied to lessen computational error due to the structural similarity between reference and complexes. AlF63- is not listed because it is hard to obtain 19F NMR signal experimentally, even though [F]/[Al] ratio is up to 120 (38). Table 1 shows that the 27 Al NMR chemical shifts for all the species are near zero arising from the uniform octahedral geometry. Because 27Al nucleus with a large quadrupole moment results in large field gradient and considerable peak broadening, the downfield broad signal (e2 ppm) is commonly assigned to mixed aqueous Al-F complexes in many experiments (17, 39) and only Al(H2O)5F2+ and Al(H2O)4F2+ shifts are confirmed up to now. One can see that the calculated results compare well to the chemical shifts determined experimentally, implying that the SM-PCM models are appropriate to describe the structures of aqueous Al-F complexes. The deviation from the experimental values for the calculated shifts using gas phase (GP), polarizable continuum model (PCM) and supermolecule (SM) models support the point (see SI Tables S5-S7) and the corresponding structures and parameters are given in SI Figures S2-S4 and Tables S2-S4. 3.2. Water-Exchange Reaction for Aqueous Al-F Complexes. The water-exchange reactions on [Al(H2O)6-iFi](3-i)+ (i ) 1-4) are simulated by reconstructing the second hydration sphere for the above-mentioned reason. The following three aspects are discussed: The Process of Water-Exchange Reaction and Activated Mechanism. The number of adequate explicit water molecules in the second hydration sphere is determined by calculating activation energy barriers for water exchange. Evans et al. (34) and Hanauer et al. (40) suggest that the computational energies are variable using supermolecular model, which is interpreted as induced by the absence of the bulk solvent effect and insufficient water molecules (29, 41). Therefore, the energy barriers with different water molecules in SMPCM model are calculated to determine the number of the adequate explicit water molecules. For Al(H2O)5F2+ the energy barriers with four and five explicit water molecules are very close and n ) 5 is regarded as sufficient to represent explicit solvent effect (see SI Table S8). The number is also applied to model other isomers. Figure 1 shows the optimized geometries of reactant (R), transition state (TS), and product (P) with five explicit water molecules for water exchange reaction on Al(H2O)5F2+ and corresponding structural parameters are listed in SI Table S10A. Five explicit water VOL. 45, NO. 1, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Energy Barriers (kJ mol-1) of Water Exchange on [Al(H2O)6-iFi](3-i)+(i = 1-4) and the Favorable Energy Barriers for Isomers Are in Bold Type
FIGURE 1. Optimized structures for water exchange on Al(H2O)5F2+, the leaving water molecule is colored with white to differ from the others. molecules are hydrogen bonded to the coordinated water molecules and F- ligand in reactant with the distance of 2.007 Å between central aluminum atom and the leaving H2O. In transition state the AlsOH2 distance lengthens to 2.978 Å with the formation of hydrogen bond between the leaving H2O and neighboring coordinated H2O. As the distance further lengthens to 4.100 Å, the leaving H2O becomes the part of the second hydration sphere and the pentacoordinated product (i.e., intermediate) appears. SI Table S10A shows the average AlsF and AlsO bond lengths decrease considerably as reaction proceeds due to the variation in coordination number. Dissociative activated mechanism was inferred from pentacoordinated intermediate and the increase of total metal-ligand distances. SI Table S9 shows that the bulk solvent effect increases the activation energy and the explicit solvent effect lowers the energy, which is consistent with the previous studies (26-28). The determination of the number of explicit water molecules, geometries for reactants, transition states and products as well as corresponding structural parameters for Al-F complexes are shown in SI Table S8, Figure S7, and Table S10, respectively. The activation volume (∆V*) is viewed as the strongest criteria to differentiate substitution mechanisms experimentally and the negative values show the trend toward A mechanism, whereas the positive values are the indication toward D mechanism. Hugi-Cleary reported a positive activation volume of 5.7 cm3 mol-1 for Al(H2O6)3+ (aq) and estimated an Id mechanism (42). Previously an Id mechanism was attributed for Al(H2O)5F2+ and Al(H2O)4F2+ (24). The total metal-ligand bond lengths change between the reactant and transition state is considered as reliable theoretical criteria of mechanism attribution (43). In previous study the perfect agreement between two criteria has been reported (44, 45). Obviously the water-exchange reaction follows dissociative mechanism for all Al-F complexes according to the variations of total Al-ligand distances. The dissociative trends of isomers are compared based on the variations, which can not be probed experimentally. In the case of Al(H2O)5F2+ (trans to F), trans-Al(H2O)4F2+ (cis to F), and fac-Al(H2O)4F30 (cis to F), their water exchanges proceed via a more dissociative mechanism than respective isomers (see SI Table S10). This is mainly interpreted as different hydrogen bonds between the leaving water and the neighboring ligands in transition state. It is clear that the transition state where the leaving water forms one hydrogen bond with neighboring ligands has a more dissociative character than those foming two hydrogen bonds. When the number of the hydrogen bonds is equal, the hydrogen bond forming with F- ligand is more dissociative than that forming with H2O ligand. For trans-Al(H2O)2F4- (cis to F), it is difficult to interpret the more dissociative character and further research is necessary. It should be noted that the lifetime of intermediates based on the energy difference between the transition state and the corresponding intermediate (44, 45) is considerably short for AlF(H2O)52+ (see Table 2), implying 290
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complex
leaving H2Oa
nb
TSc
Pc
AlF(H2O)52+ AlF(H2O)52+ trans-AlF2(H2O)4+ cis-AlF2(H2O)4+ cis-AlF2(H2O)4+ fac-AlF3(H2O)3 fac-AlF3(H2O)3 mer-AlF3(H2O)3 trans-AlF4(H2O)2cis-AlF4(H2O)2-
trans to F cis to F cis to F trans to F cis to F trans to F cis to F trans to F cis to F trans to F
5 5 5 5 5 3 3 3 3 3
58.8 71.9 53.1 46.7 61.3 36.2 59.8 40.1 20.7 10.1
56.6 54.2 47.6 38.6 48.9 20.9 10.2 22.7 2.0 1.6
a The position of the leaving H2O. Trans denotes H2O trans to one of the F- ligand and cis denotes H2O cis to all the F- ligands. b The number of explicit water molecules in the second hydration sphere. c The energies are based on the respective reactants.
that Id mechanism might be preferred. As a matter of fact, it is difficult to simulate Id mechanism for Al(III) complexes arising from the limitations of the molecular model although the experimental evidence for Id is clear (46). The Trans Effect and Labilizing Effect of F- Ligand. Table 2 gives water-exchange energy barriers and reaction energies for all aqueous Al-F complexes (i e 4) and the leaving H2O in different position is considered as well. We reasonably refer to the lowest energy barrier as favorable energy barrier for the isomers. It is clear that the water-exchange reactions of isomers are dominated by trans H2O based on the labeled favorable energy barriers. The water molecule trans to fluoride is easier to leave due to trans effect (47), which is consistent with the above-mentioned the longer trans AlsOH2 bond length. The trans effect is interpreted as inducing polarization and disruption of the symmetry caused by substitution of anion ligand, as described in previous work (25). Combined with energy barrier (70.1 kJ mol-1) of Al(H2O)63+ calculated previously by our group (27), as expected, the favorable energy barriers decrease significantly with increasing number of F- ligands, reflecting the labilizing effect of F- ligand. Phillips ascribed the labilizing effect to the weaker metal-water bond induced by reduced formal charge in the complexes (25). Since F- and OH- are isoelectronic and isochoric, computational AlsOH distance (1.75-1.82 Å) (48) is close to AlsF distance; however, different labilizing effect exists clearly between them. The labilizing effect of hydroxyl is about 102 times higher than that of fluoride according to their experimental water-exchange rate constants and Table 3 compares the labilizing effects of fluoride and hydroxyl, theoretically. Table 3 suggests that the charge on central aluminum atom for monofluoro complex is more positive than that of Al-OH complex, and the corresponding AlsOH2 bond length is shorter. This is mainly induced by different electronegativity between fluoride and oxygen atom. Swaddle et al. (49) ascribed the rate difference to pentacoordinate AlOH2+ in the previous study (more details see SI Section 4). The Prediction of Log kex for AlF3(H2O)30 and AlF4(H2O)2-. The water-exchange rate constants can be predicted semiempirically by computational methods for the species whose kex is difficult to obtain experimentally. In the case of similar complexes structurally, three ways were employed for the prediction in previous work: (1) by a correlation between average AlsO bond lengths and log kex (51); (2) by assuming transmission coefficient to be the same (41, 52); (3) by a correlation between the activation free energies and log kex (41). For AlF3(H2O)30 and AlF4(H2O)2- the rate constants can not be obtained due to the limitation of experimental means. The activation parameters (∆H*, ∆S*, ∆G*), rate constants
TABLE 3. Comparison of Labilizing Effect Between OH- and F- Ligand complex
r(AlsOH/F)/Å
r(AlsOH2)/Å
q(Al)/e
kex(expl.) /s-1
∆E/kJ mol-1a
Al(H2O)63+ Al(H2O)5OH2+ Al(H2O)5F2+
1.803 1.787
1.916 1.951 (1.967b) 1.929 (1.943b)
1.942 1.945 1.970
1.3c (3.1 × 104)c (2.3 × 102)d
70.1e 36.7e 55.4 (this work)
a Calculated activated energy barriers. ref 24. e From ref 27.
b
The bond length of AlsOH2 trans to fluoride or hydroxyl.
c
d
From ref 50.
From
TABLE 4. Activation Parameters, Rate Coefficients and Structural Parameters for a Variety of Mono- And Polyaluminuma complexes 3+
Al(H2O)6 Al(OH)(H2O)52+ AlF(H2O)52+ AlF2(H2O)4+ AlF3(H2O)30 AlF4(H2O)2Al137+ GaAl127+ GeAl128+
∆H* (kJ mol-1) 70.1 (84.7) 36.7 (36.4) 58.8 (65) 46.7 (66) 36.2 10.1 29.1 (53) 31.3 (63) 37.0 (46)
∆S* (J mol-1 K-1)
∆G* (kJ mol-1)
32.8 (41.6) 31.8 (-36.4) 6.2 (19) 0.7 (59) 2.6 -9.7 -11.1 (-7) -10.1 (13) -11.5 (-46)
60.3 (72.3) 27.2 (47.3) 56.9 (59.3) 46.5 (48.4) 35.4 13.0 32.4 (55.1) 34.3 (59.1) 40.4 (59.7)
Al3018+ 58.0/79.6/43.2d 21e -24.2 Al6(OH)18(H2O)120 28.6 Al6(OH)12(H2O)186+ 27.2 -9.9
36.9e 35.8 30.1
log kTST (s-1)
log kex (s-1)
logK
2.2 8.0 2.8 4.7 6.6 10.5 7.1 6.8 5.7 4.3/4.8/4.7/ 1.9/2.0/7.5f 6.5 7.5
0.1 4.5 2.4 4.2 6.1* 10.0* 3.0 2.4 2.3
-2.1 -3.5 -0.4 -0.5 -0.5b -0.5b -4.1 -4.4 -3.4 -2.4/-1.6/-2.5/ -2.3/-2.4/-1.7f -4.1c -4.1c
2-4 2.4* 3.4*
AlsO bond length (Å)
ref
1.916 1.953 1.929 1.944 1.963 1.992 1.969 1.962 1.939
27, 50 27, 50 this work, 24 this work, 24 this work this work 26, 53 28, 54 52, 55
-
56, 57 41 41
a All the average AlsO bond lengths and charges are from SM-PCM model at the level of 6-311+G**. “-” denotes no data available. Experimental values are listed in parentheses. “*” denotes the predicted values (in bold type). b From the assumption of the same transmission coefficient as AlF(H2O)52+ and AlF2(H2O)4+. c From the assumption of the same transmission coefficient as Al137+. d Correspond to S1, S4, and S6 sites on Al3018+, respectively. e Correspond to S6 sites on Al3018+. f Correspond to S1-S6 sites on Al3018+, respectively.
(kex, kTST), transmission coefficient (κ) and AlsO bond lengths for a variety of mono- and polyaluminum complexes are compiled in Table 4. First, SI Figure S8A shows a strong correlation (R2 ) 0.953) between the average AlsO bond lengths and log kex for monomer aluminum complexes with the linear equation of log kex (s-1) ) 121r(AlsO) (Å) - 231. According to the correlation, the log kex was deduced to be 6.0 and 10.0 for AlF3(H2O)30 and AlF4(H2O)2-, respectively. Similarly a correlation (R2 ) 0.848) was found for polyaluminum complexes with the linear equation of log kex (s-1) ) 48.7r(AlsO) (Å) - 92.8. Second, as expected, the transmission coefficient is similar for AlF(H2O)52+ and AlF2(H2O)4+. We consider the value of -0.5 as the transmission coefficient of AlF3(H2O)30 and AlF4(H2O)2- in light of the above assumption. Thus 6.1 and 10.0 are estimated for log kex of AlF3(H2O)30 and AlF4(H2O)2-, respectively. Third, no strong correlation was found between the activation free energies and log kex for monomer aluminum complexes (see SI Figure S8B) although a strong correlation (R2 ) 0.962) exists for polyaluminum complexes (41). This indicates that the semiempirical method might not be appropriate to predict log kex for monomer aluminum complexes. Comparing the log kex data from the above two methods, an error of (0.1 was presumed. 3.3. Environmental Significance. According to previous studies, DFT method using SM-PCM model is appropriate to simulate the water exchange on a wide range of Al complexes including mono- and polyaluminum due to the simultaneous consideration of long-range and short-range solvent effects (26-29). Since the Al-F complexes are dominating inorganic aluminum forms in natural water environment (58) the water exchange on the typical Al-F system is investigated using SM-PCM model. The intrinsic relationships of theoretical and experimental values are established by the structural parameters such as bond length and transmission coefficient κ. According to the relationships, we are able to not only obtain κ values to understand the related dynamics character for the system with high-quality rate data based on the equation of kex ) κ kTST, but also
estimate kex for the system inaccessible experimentally, such as mineral surface. The environmental significance of this work lies in the extension of the method toward the following three fields: (1) Polyaluminum system. Based on the measured kex of Keggin-MAl12 (M ) Al, Ga, Ge), κ and kex of mineral surface which is difficult to sample experimentally can be estimated using the correlation of experimental and theoretical parameters (41, 56). This is very important since the water exchange reaction on mineral surface is fundamental reaction of environmental surface process which controls the whole solution chemistry and Earth’s surface environment; (2) Monomer Al-organic complexes system. The toxicity of aluminum in aquatic systems is sensitive to the aluminum species and the species are influenced directly by the ubiquitous ligand in environment, such as F-, OH-, and organic ligand (9, 10). In the case of Al-OH system, the correlations will allow one to probe the influence of pH on Al species microscopically. Similarly, for bidentate Al-organic complexes it is possible to predict unknown κ and kex value for a large number of structurally similar species according to a series of known kex (see SI Tables S12 and S13). The data is helpful to understand the effect of the ligands on the dissolution and transfer of toxic Al species in Earth’s surface, which has long been concerned by many environmental researchers. SI Table S12 shows that κ values depend on the Al species and the κ values of monomer system are larger than those of polyaluminum system and Al-F system is larger than Al-OH system. Although the factors inducing these differences are not clear, the results are useful to predict κ values of different aluminum species, thereby revealing dynamics influence of various ligands on these environmentally relevant systems; (3) Other metal ions system with high charge-to-radius ratio (Z/r). The SM-PCM is suitable to treat the high Z/r system which is often environmentally relevant, such as Al3+, Fe3+, Cr3+, V3+, and Ti3+, etc. Thus a new method can be provided to study the hydrolysis-polymerization behavior VOL. 45, NO. 1, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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and shed light on the toxicity mechanism on a molecular scale for these metal ions (more details see SI Section 5). The dissolution and transfer of these metal ions in terrestrial and aquatic ecosystems are strongly influenced by the dissolved ligands (59, 60). It can be anticipated that the influences of a variety of ligands and polymerization on the reactivity might be estimated using these correlations in present study for a certain amount of metal ions.
Acknowledgments This project is supported by the NSFC (20777030) and the Scientific Research Foundation of Graduate School of Nanjing University (2009PL04). We are also grateful to the High Performance Computing Center of Nanjing University for the award of CPU hours to accomplish this work.
Supporting Information Available Computational models and the parameters, structures of Al-F complexes and F(H2O)6-, the models and parameters for water exchange of aqueous Al-F complexes, environmental significance, and the related references as well as Cartesian coordinates of all the Al-F complex structures. This material is available free of charge via the Internet at http://pubs.acs.org.
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