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Effects of hydration on the molecular structure of magnesium-fulvic acid complexes: a MOPAC (PM3) study
Journal of Molecular Structure (Theochem) 468 (1998) 51–58
Effects of hydration on the molecular structure of magnesiumfulvic acid complexes: a MOPAC (PM3) study Evangelos A. Nantsis, W. Robert Carper* Department of Chemistry, Wichita State University, Wichita, KS 67260-0051, USA Received 18 August 1998; received in revised form 24 September 1998; accepted 24 September 1998
Abstract A semi-empirical model of Suwannee River fulvic acid, based on spectroscopic and electrochemical evidence, has been used as a basis for determining the structural characteristics and computed enthalpies and entropies of both anhydrous and hydrated mixed metal ion-fulvic acid complexes containing magnesium and either zinc, lead or cadmium. Replacement of Mg 21 with Cd 21, Pb 21, or Zn 21 at alternate binding sites results in changes in binding of Mg 21 from cis to trans at the phthalate binding site in fulvic acid. Hydration of Mg 21, Cd 21, Pb 21 and Zn 21 changes the relative enthalpies of formation for the di-substituted metal-fulvic acid complexes while leaving the type of binding at the phthalate binding site unaffected. The entropies of complex formation for both the anhydrous and hydrated metal ion-fulvic acid complexes are similar. Consequently, standard enthalpies of formation are an indicator of the relative strength of metal ion binding for both metal ion-phthalic acid and metal ion-fulvic acid complexes. Hydrogen bonding contributes to the overall structure and to the co-ordination of waters of hydration in the metal-fulvic acid complexes q 1999 Elsevier Science B.V. All rights reserved. Keywords: Anhydrous metal ion-fulvic acid complexes; Hydrated metal ion-fulvic acid complexes; Thermodynamic properties; Hydrogen Bonding; Fulvic acid; Cd, Pb, Mg and Zn-fulvic acid complexes
1. Introduction Quantum mechanical studies on the hydration of water to large molecules at the ab initio level are either impossible or extremely difficult at best. This is in part due to the high level of ab initio theory necessary for accurate modelling of hydrogen bonding and because of basis set superposition error (BSSE) [1,2]. Consequently, the expense and facility requirements of the purely quantum mechanical method limits such studies to the inclusion of a few solvent molecules within the super molecule approach. At the same time, the semi-empirical PM3 * Corresponding author. E-mail address: [email protected] (W.R. Carper)
method [3] has proven successful in modelling hydrogen bonding between neutral molecules [4], nucleotide base pairs [5] and the solvated version of cyclic 3’5’-adenosine monophosphate [6]. Consequently, we have used the PM3 method for hydration studies of a series of large metal ion-fulvic acid complexes described in this and a previous report [7]. Fulvic acids are a class of large molecules that are part of humic substances that are a major contributor of dissolved organic carbon in water [8]. Humic substances also affect water quality by transporting metal ions and organic matter from a variety of sources. A detailed knowledge of the metal ion binding properties of humic substances (humic and fulvic acids) provides useful information in the areas of metal ion transport, waste storage and remediation.
0166-1280/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0166-128 0(98)00495-3
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Fig. 1. Phthalic Acid Model of Suwannee River Fulvic Acid.
In previous reports [7,9], a semi-empirical model of Suwannee River fulvic acid based on experimental evidence was used as a model for studying the binding of Cd 21, Pb 21, Mg 21 and Zn 21 to the lowest energy form of fulvic acid. The optimized structures of anhydrous [9] and hydrated [7] 1:1 and 2:1 metal ion-fulvic acid complexes were determined for Cd 21, Pb 21, Mg 21, and Zn 21 bound to pairs of carboxylate groups in fulvic acid. The basic model of Suwannee River fulvic acid used in this and other theoretical studies [7,9] is the phthalate model [10] shown in Fig. 1. The similarity in entropy changes of both the anhydrous and hydrated 1:1 and 2:1 metal ion-fulvic
acid complexes suggests the use of standard enthalpies of formation as predictors of complex formation [7,9]. Carboxylate moities are the main metal ion co-ordination sites in fulvic acid with sulfur, nitrogen and phosphorus containing functional groups also serving as possible albeit less prevalent ligands [8, 10-15]. The flexibility of the fulvic acid carboxylic acid groups [10] allows for binding of ions of differing ionic radii with minor accompanying variations in molecular structure. These properties were observed in the initial report [9] that contained anhydrous molecular structures of the divalent 1:1 and 2:1 metal ionfulvic acid complexes containing Cd 21, Pb 21, Mg 21, and Zn 21. In a later study, hydrated metal ion-fulvic acid complexes were compared with anhydrous forms of the same complexes and with hydrated forms of metal ion-phthalate complexes [7]. This theoretical study is focused on, (a) the effects of replacement of Mg 21 in various forms of Suwannee River Mg2-fulvic acid with either Cd 21, Pb 21 or Zn 21, and (b) the effects of metal ion hydration on the Suwannee River phthalate model (Fig. 1) of mixed metal ion-fulvic acid complexes containing Mg 21 and either Cd 21, Pb 21 or Zn 21. Enthalpies of formation of the anhydrous and hydrated mixed metal ionfulvic acid complexes are used to characterize the binding between Suwannee River fulvic acid and Mg 21, Cd 21, Pb 21 or Zn 21. The cis and trans model hydrated metal ion-phthalate complexes [7] are compared with the hydrated mixed metal ion-fulvic acid complexes and the structural changes that occur when hydration is introduced in the mixed metal ionfulvic acid complexes are discussed. The significance of hydrogen bonding in the various structures of anhydrous and hydrated metal-fulvic acid complexes is also evaluated.
2. Theoretical section
Fig. 2. MOPAC(PM3)-optimized structure of Fulvic Acid (Configuration III).
The semi-empirical calculations (PM3) [3] reported herein were accomplished with a Digital 3000/700 AXP and a Silicon Graphics Indy workstation using Spartan 4.0 and MOPAC [16]. A complete vibrational analysis was performed on all structures to insure the absence of negative vibrational frequencies and verify the existence of a true minimum.
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41, 55 and 82) provide a maximum of four possible binding sites containing two carboxyl groups. The binding sites include carbons 5 and 21 for site 1, carbons 21 and 41 for site 2, carbons 41 and 82 for site 3 and carbons 55 and 82 for site 4. The location of carboxyl groups containing carbons 5, 21 and 41 limits this region to only one dicarboxylate binding site. In this study, configuration III is evaluated for the effects of hydration on mixed metal ion-fulvic acid complexes containing Mg 21 and either Cd 21, Pb 21 or Zn 21. 3.2. Binding at Phthalate Site 1 Fig. 3. MOPAC(PM3)-optimized structure of hydrated Mg(site 1)Pb(site 3)-fulvic Acid.
3. Results and discussion 3.1. Fulvic Acid Model Fig. 1 represents the model structure of Suwannee River fulvic acid. This structure includes a phthalatetype metal ion binding site (site 1 – carbons 5 and 21) in addition to three other carboxylate groups that serve as possible binding sites. The fulvic acid functional groups shown in Fig. 1 have been identified by NMR ( 1H and 13C), infrared and ultraviolet spectrometry; and titrimetric studies [10–14,17]. In an initial report [9], three low energy configurations of fulvic acid whose enthalpies of formation were within 2 kcal of each other were identified. The lowest energy configuration of the fulvic acid phthalate model with the maximum number of binding sites is configuration III shown in Fig. 2 [9]. Configuration III is obtained by two modifications of the structure shown in Fig. 1. The first modification consists of rotating ring 2 by 1808 relative to ring 1 about the carbonyl carbon that connects the two rings (see arrow in Fig. 1). The second modification is accomplished by rotating ring 3 by 1808 relative to ring 1 about the oxygen that connects them (see arrow in Fig. 1). This last modification of ring 3 results in the formation of an intra-molecular hydrogen bond system between the carboxylic acid functions containing carbons 55 and 82 (site 4). As a result of these changes to Fig. 1, the five carboxyl groups in configuration III (carbons 5, 21,
The phthalate binding site (carbons 5 and 21, site 1) has been modeled previously for anhydrous and hydrated phthalate complexes of Mg 21, Zn 21, Pb 21 and Cd 21. In each case, molecular structures and thermodynamic properties of the cis and trans isomers were determined [7]. The metal phthalate cis isomers contain both metal binding oxygens on one side of a plane passing through the phenyl ring and its attached carbons. The trans isomer has the bound oxygens on opposite sides of this plane [7]. Enthalpies of formation of the anhydrous metal ionphthalate complexes are lowest for the cis configurations. The heats of formation for the hydrated metal ion-phthalate complexes are lowest for the cis Mgand Zn-phthalate complexes, whereas cis and trans hydrated Pb- and Zn-phthalate complexes were within 1 kcal of each other [7]. The configuration of the metal ion-site 1 complex is cis for the Mg2-fulvic acid compexes at sites 1 & 3 and 1 & 4 in both the anhydrous and hydrated versions, as would be predicted on the basis of the metal ionphthalate calculations [7]. Similar results are obtained for both anhydrous and hydrated versions of Mg complexes in which Mg is located at site 3 and either Zn 21, Pb 21 or Cd 21 is at site 1. However, all of the remaining anhydrous and hydrated complexes in which site 1 is occupied have the trans configuration at site 1. An example of the trans geometry in which Pb 21 occupies site 3 and Mg 21 occupies site 1 is shown for the hydrated metal-fulvic acid complex in Fig. 3. 3.3. Metal-Oxygen (water) distances The hydration of Mg 21 in fulvic acid produces
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Table 1 MOPAC (PM3) thermodynamic properties of metal ion fulvic acid complexes Fulvic Acid With Site 1 config. Anhydrous Config. III Mg site 1 Pb site 3 - trans Mg site 3 Pb site 1 - cis Mg site 1 Pb site 4 - trans Mg site 4 Pb site 1 - trans Mg site 2 Pb site 4 Mg site 4 Pb site 2 Mg site 1 Zn site 3 - trans Mg site 3 Zn site 1 - cis Mg site 1 Zn site 4 - trans Mg site 4 Zn site 1 - trans Mg site 2 Zn site 4 Mg site 4 Zn site 2 Mg site 1 Cd site 3 - trans Mg site 3 Cd site 1 - cis Mg site 1 Cd site 4 - trans Mg site 4 Cd site 1 - trans Mg site 2 Cd site 4 Mg site 4 Cd site 2 Mg site 1 & 3 - cis Mg site 1 & 4 - cis Mg sites 2 & 4 Hydrated Fulvic Acid Complexes With Site 1 Config. Mg site 1 Pb site 3 - trans Mg site 3 Pb site 1 - cis Mg site 1 Pb site 4 - trans Mg site 4 Pb site 1 - trans Mg site 2 Pb site 4 Mg site 4 Pb site 2 Mg site 1 Zn site 3 - trans Mg site 3 Zn site 1 - cis Mg site 1 Zn site 4 - trans Mg site 4 Zn site 1 - trans Mg site 2 Zn site 4 Mg site 4 Zn site 2 Mg site 1 Cd site 3 - trans Mg site 3 Cd site 1 - cis Mg site 1 Cd site 4 - trans Mg site 4 Cd site 1 - trans Mg site 2 Cd site 4 Mg site 4 Cd site 2 Mg site 1 & 3 - cis Mg site 1 & 4 - cis Mg sites 2 & 4
DfH8 (kcal/mol)
S8 (cal/mol-K)
DfG8 (kcal)
DrH8 (kcal)
2843.696 2833.177 2857.301 2839.479 2864.161 2863.652 2867.818 2761.515 2783.201 2770.36 2781.525 2800.693 2798.947 2773.727 2792.906 2780.145 2795.325 2803.845 2808.738 2886.908 2892.925 2938.395 – DfH8 (kcal/mol)
320.402 309.679 310.335 312.576 311.596 309.805 315.159 308.802 309.265 313.211 312.771 307.948 312.190 315.900 311.499 315.964 313.468 313.465 315.548 307.705 307.168 298.334 – S8 (cal/mol-K)
2939.176 2925.461 2949.781 2932.627 2957.017 2955.974 2961.735 2853.538 2875.362 2863.697 2874.731 2892.462 2891.980 2867.865 2885.733 2874.302 2888.738 2897.258 2902.771 2978.604 2984.461 21027.299 – DfG8 (kcal)
209.234 185.110 202.932 178.250 178.759 174.593 122.129 100.443 113.284 102.119 82.951 84.697 228.219 209.04 221.801 206.621 198.101 193.208 261.536 255.519 210.049 – DrH8 (kcal)
21127.322 21148.936 21138.263 21147.602 21154.957 21134.787 21058.974 21080.249 21072.074 21073.301 21082.368 21052.917 21048.697 21070.078 21059.262 21064.244 21079.009 21045.180 21223.011 21219.083 21246.545
352.520 359.366 354.983 353.060 354.096 347.859 349.824 356.734 350.895 357.796 353.430 354.622 359.249 375.398 361.641 369.063 356.584 363.449 343.194 335.174 329.053
21232.373 21256.027 21244.048 21252.814 21260.478 21238.449 21163.222 21186.556 21176.641 21179.924 21187.690 21158.594 21155.753 21181.947 21167.031 21174.225 21185.271 21153.488 21325.283 21318.965 21344.603
128.797 107.183 117.856 108.517 101.162 121.332 38.378 17.103 25.278 24.051 14.984 44.435 48.655 27.274 38.090 33.108 18.343 52.172 139.141 143.069 115.607
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metal-oxygen (water) distances varying from 1.862 ˚ that are similar to those calculated to 1.874 A ˚ ) for the lowest energy form (1.869 and 1.875 A (cis) of the hydrated Mg 21-phthalate complex. The Zn 21-oxygen (water) distances vary from ˚ , with the exception of Zn 2.181 to 2.221 A (located in site 4), and Mg (at site 2), where the ˚ . These Zn-oxygen distances are 2.213 and 2.427 A distances may be compared with values of 2.244 ˚ obtained for the cis form of hydrated and 2.242 A Zn-phthalate. The slightly higher energy form (by 5.5 kcal) of trans hydrated Zn-phthalate has Zn˚ . The Pboxygen distances of 2.213 and 2.224 A oxygen (water) distances vary from 2.787 to ˚ , with a similarity to those calculated for 2.828 A ˚ ) and trans (2.786, 2.806 A ˚) the cis (2.799, 2.818 A versions of Pb-phthalate whose enthalpies of formation are less than 0.1 kcal apart [7]. Finally, the Cd-oxygen (water) distances vary from 2.347 ˚ , in a manner similar to that observed to 4.531 A ˚ ) and cis (4.483, for the trans (2.513, 2.515 A ˚ ) hydrated Cd-phthalate complexes whose 4.472 A enthalpies of formation are less than 1 kcal apart [7]. In the case of the Cd-phthalate complexes, the higher entropy form (cis) has considerably longer metal ion-oxygen distances than the trans form. In particular, the long metal-water distances suggest a weak interaction between the oxygen and the metal ion due to a shallow potential well and therefore a higher entropy. In a similar manner, the highest entropy form of the hydrated Cdcontaining fulvic acid complex (Mg at site 3, Cd at site 1) is the only complex in which both Cd˚ ). oxygen (water) distances are long (4.406, 4.382A
3.4. Thermodynamic Properties of Mixed Metal IonFulvic Acid Complexes Table 1 contains the calculated standard enthalpies and entropies of both anhydrous and hydrated metal ion-fulvic complex formation (see equation below) at 298 K. The gas phase enthalpies of formation for H 1, Cd 21, Pb 21, Mg 21, and Zn 21 used to calculate the standard heats of reaction for metal ion-fulvic acid complexation are 353.586, 544.444, 503.979, 555.096 and 662.746 kcal/mol, respectively [18]. The binding sites in fulvic acid are carbons 5 and 21
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for site 1, carbons 21 and 41 in site 2, carbons 41 and 82 in site 3 and carbons 55 and 82 in site 4. Fulvic Acid 1 2(metal ion) 1 4(water) ! Complex 1 4H 1 The standard enthalpies of reaction are calculated assuming that fulvic acid combines with two metal ions and four water molecules (hydrated version only) to produce a metal ion-fulvic acid complex and 4 H 1. The fact that changes in entropy are relatively constant over a wide range of metal ions and variations in binding site geometry indicates that standard enthalpies of reaction are a reasonable indication of trends in hydrated metal ion-fulvic acid complexation. In the calculations of metal ion binding to phthalic acid, the cis configuration was the lowest energy form [7]. Site 1 in fulvic acid is virtually identical to phthalic acid with the exception that additional functional groups may be available for hydrogen bonding in the same vicinity as site 1. The cis form of metal ion binding was observed as the low energy form of hydrated Mg2-fulvic acid binding at sites 1 & 3 and 1 & 4, as would be predicted on the basis of the metal ion-phthalic acid calculations. However, in the mixed metal ion-fulvic acid complexes, the trans configuration predominates with the exception of Mg 21 bound at site 3 and either Cd 21, Pb 21or Zn 21 bound at site 1, irrespective of hydration. This latter combination of Mg 21 bound at site 3 and either Cd 21, Pb 21or Zn 21 bound at site 1 represents the highest entropy form of these complexes with the exception of Mg 21 and Zn 21 where Mg at site 3 and Zn at site 1 is only 1 e.u. from the highest entropy form containing Mg at site 4 and Zn at site 1. In addition to changes in binding configurations at site 1, the effects of hydration on the enthalpies of formation and reaction are considerable. The two mixed metal-fulvic acid complexes with the lowest enthalpy of formation for Pb 21, Zn 21or Cd 21 are generally combinations of sites 2 & 4 for the anhydrous form of these mixed metal-fulvic acid complexes. Hydration of these complexes produces the lowest enthalpy of formation with Mg 21 at site 2 and either Pb 21, Zn 21 or Cd 21 at site 4. The reverse combination of hydrated metal-fulvic acid complexes (Mg 21 at site 4 and either Pb 21, Zn 21 or Cd 21 at site 2) now has the highest or next to highest enthalpy of formation within each group.
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E.A. Nantsis, W.R. Carper / Journal of Molecular Structure (Theochem) 468 (1998) 51–58
Fig. 4. MOPAC(PM3)-optimized structure of hydrated Mg2(sites 1 & 3)-fulvic Acid.
, Zn 21 to the same order as ionic radii, Zn 21 , Pb 21 , Cd 21, whereas the enthalpies of reaction (see earlier equation) are in the order Zn 21 , Cd 21 , Pb 21. Substitution of Mg 21 in anhydrous Mg2-fulvic acid at sites 2 or 4 with either Pb 21, Zn 21 or Cd 21 yields enthalpies of formation in the order Pb 21 , Cd 21 , Zn 21 and enthalpies of reaction in the same order as the ionic radii, Zn 21 , Pb 21 , Cd 21. In the hydrated complexes, replacement of Mg 21 in Mg2fulvic acid at sites 2 or 4 with either Pb 21, Zn 21 or Cd 21 yields enthalpies of formation in the order Pb 21 , Zn 21 ù Cd 21 and enthalpies of reaction in the order Zn 21 , Cd 21 , Pb 21. In many cases, the differences in enthalpy between metal ion substitution is too small to be considered relevant.
3.6. Intramolecular Hydrogen Bonds 3.5. Effects of Ionic Radii The replacement of Mg 21 in Mg2-fulvic acid at site 1 with either Pb 21, Zn 21 or Cd 21 in the anhydrous forms of the metal-fulvic acid complexes follows the enthalpy of formation order Pb 21 , Cd 21 , Zn 21. The enthalpies of reaction in the demonstrated equation follow the same order as the ionic radii, Zn 21 , Pb 21 , Cd 21. Hydration has the effect of changing the order of both types of enthalpies. The enthalpies of formation order changes from Pb 21 , Cd 21
Fig. 5. MOPAC(PM3)-optimized structure of hydrated Pb(site 1)Mg(site 3)-fulvic Acid.
Intra-molecular hydrogen bonding is present in both anhydrous and hydrated versions of the mixed metal ion-fulvic acid complexes contained in this and prior reports [7,9]. In all hydrated and most anhydrous structures reported herein, there is at least one O–-H ˚ or less and two additional hydrogen bond of 2.0 A ˚ or less. In the majority O–-H hydrogen bonds of 2.5 A of structures reported herein, one finds that hydration produces limited changes in the existing intra-molecular hydrogen bonds. However, when intra-molecular hydrogen bond distances change, these changes accompany variations in distances between rings 1, 2 and 3 (Fig. 1) of the metal-fulvic acid complexes. An example of this effect is the substitution of Pb 21 for Mg 21 at site 1 of the hydrated Mg2-fulvic acid complex (sites 1 & 3). The Mg2-fulvic acid complex (sites 1 & 3) and the PbMg-fulvic acid complex (sites 1 & 3) are shown in Figs. 4 and 5. Figs. 4 and 5 look quite similar, however the substitution of Pb 21 for Mg 21 at site 1 results in considerable separation between rings 2 and 3 (Fig. 1) as indicated by a change in the intra-molecular O– –H distance between H29 ˚ ). Other intra-molecular and O66 (3.595 to 3.902 A O– –H distance changes include H40-O30 (3.540 to ˚ ) and H40-O66 (4.089 to 4.099 A ˚ ). Hydrogen 3.518 A bond distance changes are minimal and include H29˚ ), H40-O20 (1.781 to 1.778 A ˚) O57 (2.483 to 2.490 A ˚ and H29-O35 (2.709 to 2.620 A). These observations
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more distant water molecule forms a hydrogen ˚ (H10-O28) with the opposite bond of 1.793 A carbonyl function.
4. Conclusions
Fig. 6. MOPAC(PM3)-optimized structure of hydrated Cd(site 2)Mg(site 4)-fulvic Acid.
are typical for all of the mixed metal-fulvic acid complexes discussed herein. 3.7. Hydrogen Bonding and Waters of Hydration Hydration of the metal complexes of fulvic acid involves not only coordination between metals and the oxygens of water, but also hydrogen bonding between the water hydrogens and carbonyl oxygens of the coordinating carboxylate functions. A typical example of this type of bonding is the molecular structure of the hydrated form of MgPbfulvic acid (sites 1 and 3) shown in Fig. 3. The O–-H hydrogen bond distances are 1.812 (H96˚ (H95-O44). O11), 1.830 (H91-O20) and 1.803 A Van der waals’ distances [19] of 2.712 (H89-O4) ˚ (H94-O85) are also observed for this and 2.665 A structure. Even in those few cadmium-containing complexes where the metal-oxygen co-ordination distance is longer than expected, one observes hydrogen bonds between the waters of hydration and surrounding functional groups containing oxygen. An example of this is the hydrated version of CdMg-fulvic acid (sites 2 and 4 — shown in Fig. 6) in which the two water oxygens ˚ from Cd co-ordinated to Cd are 2.375 and 4.047 A at site 2. The closer water molecule forms O– –H ˚ (H97,H96hydrogen bonds of 1.860 and 1.822 A O44) with the same carbonyl function and the
A theoretical model of Suwannee River fulvic acid based on experimental evidence has been used to determine the structures of 2:1 hydrated metal ion-fulvic acid complexes containing Mg 21 and either Cd 21, Zn 21 or Pb 21. The similar changes in entropy of the mixed metal-fulvic acid complexes suggest that one can use standard enthalpies of formation as a predictor of complex formation. The binding of Mg 21 at the phthalate binding site (site 1) in fulvic acid changes from cis to trans upon binding of Cd 21, Zn 21 or Pb 21 at sites 3 or 4. In addition to co-ordination with the metal ions, the waters of hydration typically form O–-H hydrogen bonds of approximately ˚ with adjacent carbonyl functions. Cd-water 1.8 A (oxygen) distances are occasionally long compared with van der Waals’ distances [19]. The structures of both hydrated and unhydrated 2:1 metal ion-fulvic acid complexes of Mg 21 and either Cd 21, Zn 21 or Pb 21 with fulvic acid are similar at the site of metal ion co-ordination. However, there is evidence that these small changes are somewhat amplified in terms of distances between ring system 1,2 and 3 (Fig. 1). In addition, the hydration of the metal ions has the effect of equalizing the energies within each metal ion type, thus improving the chances of random rather than preferential binding within fulvic acid. As observed previously [7], the flexibility of fulvic acid results in complex formation energies that do not always correlate with the ionic radii of the metal ions.
Acknowledgements The authors wish to thank Professors P.G. Wahlbeck and G.J. Mains for numerous helpful discussions. Financial support was provided by NSF Grant CHE-9524865 to W.R.C.
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References [1] M.J. Frisch, J.E. Del Bene, J.S. Binkley, H.F. Schaefer III, J. Chem. Phys. 84 (1986) 2279. [2] B.J. Smith, D.J. Swanton, J.A. Pople, H.F. Schaefer III, J. Chem. Phys. 92 (1990) 1240. [3] J.J.P. Stewart, J. Comput. Chem. 10 (1989) 209–221. [4] M.W. Jurema, G.C. Shields, J. Comput. Chem. 14 (1993) 89. [5] T.N. Lively, M.W. Jurema, G.C. Shields, Int. J. Quantum Chem.: Quantum Biol. Symp. 21 (1994) 95. [6] K.N. Kirschner, E.C. Sherer, G.C. Shields, J. Phys. Chem. 100 (1996) 3293. [7] E.A. Nantsis, W.R. Carper, J. Mol. Struct. (Theochem) 431 (1998) 267. [8] E.M. Perdue, in I.H. Suffet and P. MacCarthy (Eds.), Aquatic Humic Substances, American Chemical Society, Washington D.C., 1989, Vol. 219, pp. 281-295. [9] E.A. Nantsis, W.R. Carper, J. Mol. Struct. (Theochem) 423 (1998) 203. [10] J.A. Leenheer, D.M. McKnight, E.M. Thurman, P. MacCarthy, in R.C. Averett, J.A. Leenheer, D.M. McKnight and K.A. Thorn (Eds.), Humic Substances in the Suwannee River Georgia, Interactions Properties and Proposed Structures, U.S. Geological Survey Water Supply Paper 2373, 1994, pp. 195-211.
[11] E.C. Bowles, R.C. Antweiler, P. MacCarthy, in R.C. Averett, J.A. Leenheer, D.M. McKnight and K.A. Thorn (Eds.), Humic Substances in the Suwannee River Georgia, Interactions Properties and Proposed Structures, U.S. Geological Survey Water Supply Paper 2373, 1994, pp. 115-127. [12] T.I. Noyes, in R.C. Averett, J.A. Leenheer, D.M. McKnight and K.A. Thorn (Eds.), Humic Substances in the Suwannee River Georgia, Interactions Properties and Proposed Structures, U.S. Geological Survey Water Supply Paper 2373, 1994, pp. 129-139. [13] K.A. Thorn, in R.C. Averett, J.A. Leenheer, D.M. McKnight and K.A. Thorn (Eds.), Humic Substances in the Suwannee River Georgia, Interactions Properties and Proposed Structures, U.S. Geological Survey Water Supply Paper 2373, 1994, pp. 141-182. [14] J.A. Leenheer, R.L. Wershaw, M.M. Reddy, Environ. Sci. Technol. 29 (1995) 393. [15] J.A. Leenheer, R.L. Wershaw, M.M. Reddy, Environ. Sci. Technol. 29 (1995) 399. [16] J.J.P. Stewart, MOPAC, QCPE Bull. 3 (1983) 43. [17] C.K. Larive, A. Rogers, M. Morton, W.R. Carper, Environ. Sci. Technol. 30 (1996) 2828. [18] C.E. Moore, Natl. Bur. Stand. Circ. 467, Vol. I, III, 1949, 1958. [19] A. Bondi, J. Phys. Chem. 68 (1964) 441.
Effects of hydration on the molecular structure of magnesiumfulvic acid complexes: a MOPAC (PM3) study Evangelos A. Nantsis, W. Robert Carper* Department of Chemistry, Wichita State University, Wichita, KS 67260-0051, USA Received 18 August 1998; received in revised form 24 September 1998; accepted 24 September 1998
Abstract A semi-empirical model of Suwannee River fulvic acid, based on spectroscopic and electrochemical evidence, has been used as a basis for determining the structural characteristics and computed enthalpies and entropies of both anhydrous and hydrated mixed metal ion-fulvic acid complexes containing magnesium and either zinc, lead or cadmium. Replacement of Mg 21 with Cd 21, Pb 21, or Zn 21 at alternate binding sites results in changes in binding of Mg 21 from cis to trans at the phthalate binding site in fulvic acid. Hydration of Mg 21, Cd 21, Pb 21 and Zn 21 changes the relative enthalpies of formation for the di-substituted metal-fulvic acid complexes while leaving the type of binding at the phthalate binding site unaffected. The entropies of complex formation for both the anhydrous and hydrated metal ion-fulvic acid complexes are similar. Consequently, standard enthalpies of formation are an indicator of the relative strength of metal ion binding for both metal ion-phthalic acid and metal ion-fulvic acid complexes. Hydrogen bonding contributes to the overall structure and to the co-ordination of waters of hydration in the metal-fulvic acid complexes q 1999 Elsevier Science B.V. All rights reserved. Keywords: Anhydrous metal ion-fulvic acid complexes; Hydrated metal ion-fulvic acid complexes; Thermodynamic properties; Hydrogen Bonding; Fulvic acid; Cd, Pb, Mg and Zn-fulvic acid complexes
1. Introduction Quantum mechanical studies on the hydration of water to large molecules at the ab initio level are either impossible or extremely difficult at best. This is in part due to the high level of ab initio theory necessary for accurate modelling of hydrogen bonding and because of basis set superposition error (BSSE) [1,2]. Consequently, the expense and facility requirements of the purely quantum mechanical method limits such studies to the inclusion of a few solvent molecules within the super molecule approach. At the same time, the semi-empirical PM3 * Corresponding author. E-mail address: [email protected] (W.R. Carper)
method [3] has proven successful in modelling hydrogen bonding between neutral molecules [4], nucleotide base pairs [5] and the solvated version of cyclic 3’5’-adenosine monophosphate [6]. Consequently, we have used the PM3 method for hydration studies of a series of large metal ion-fulvic acid complexes described in this and a previous report [7]. Fulvic acids are a class of large molecules that are part of humic substances that are a major contributor of dissolved organic carbon in water [8]. Humic substances also affect water quality by transporting metal ions and organic matter from a variety of sources. A detailed knowledge of the metal ion binding properties of humic substances (humic and fulvic acids) provides useful information in the areas of metal ion transport, waste storage and remediation.
0166-1280/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0166-128 0(98)00495-3
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Fig. 1. Phthalic Acid Model of Suwannee River Fulvic Acid.
In previous reports [7,9], a semi-empirical model of Suwannee River fulvic acid based on experimental evidence was used as a model for studying the binding of Cd 21, Pb 21, Mg 21 and Zn 21 to the lowest energy form of fulvic acid. The optimized structures of anhydrous [9] and hydrated [7] 1:1 and 2:1 metal ion-fulvic acid complexes were determined for Cd 21, Pb 21, Mg 21, and Zn 21 bound to pairs of carboxylate groups in fulvic acid. The basic model of Suwannee River fulvic acid used in this and other theoretical studies [7,9] is the phthalate model [10] shown in Fig. 1. The similarity in entropy changes of both the anhydrous and hydrated 1:1 and 2:1 metal ion-fulvic
acid complexes suggests the use of standard enthalpies of formation as predictors of complex formation [7,9]. Carboxylate moities are the main metal ion co-ordination sites in fulvic acid with sulfur, nitrogen and phosphorus containing functional groups also serving as possible albeit less prevalent ligands [8, 10-15]. The flexibility of the fulvic acid carboxylic acid groups [10] allows for binding of ions of differing ionic radii with minor accompanying variations in molecular structure. These properties were observed in the initial report [9] that contained anhydrous molecular structures of the divalent 1:1 and 2:1 metal ionfulvic acid complexes containing Cd 21, Pb 21, Mg 21, and Zn 21. In a later study, hydrated metal ion-fulvic acid complexes were compared with anhydrous forms of the same complexes and with hydrated forms of metal ion-phthalate complexes [7]. This theoretical study is focused on, (a) the effects of replacement of Mg 21 in various forms of Suwannee River Mg2-fulvic acid with either Cd 21, Pb 21 or Zn 21, and (b) the effects of metal ion hydration on the Suwannee River phthalate model (Fig. 1) of mixed metal ion-fulvic acid complexes containing Mg 21 and either Cd 21, Pb 21 or Zn 21. Enthalpies of formation of the anhydrous and hydrated mixed metal ionfulvic acid complexes are used to characterize the binding between Suwannee River fulvic acid and Mg 21, Cd 21, Pb 21 or Zn 21. The cis and trans model hydrated metal ion-phthalate complexes [7] are compared with the hydrated mixed metal ion-fulvic acid complexes and the structural changes that occur when hydration is introduced in the mixed metal ionfulvic acid complexes are discussed. The significance of hydrogen bonding in the various structures of anhydrous and hydrated metal-fulvic acid complexes is also evaluated.
2. Theoretical section
Fig. 2. MOPAC(PM3)-optimized structure of Fulvic Acid (Configuration III).
The semi-empirical calculations (PM3) [3] reported herein were accomplished with a Digital 3000/700 AXP and a Silicon Graphics Indy workstation using Spartan 4.0 and MOPAC [16]. A complete vibrational analysis was performed on all structures to insure the absence of negative vibrational frequencies and verify the existence of a true minimum.
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41, 55 and 82) provide a maximum of four possible binding sites containing two carboxyl groups. The binding sites include carbons 5 and 21 for site 1, carbons 21 and 41 for site 2, carbons 41 and 82 for site 3 and carbons 55 and 82 for site 4. The location of carboxyl groups containing carbons 5, 21 and 41 limits this region to only one dicarboxylate binding site. In this study, configuration III is evaluated for the effects of hydration on mixed metal ion-fulvic acid complexes containing Mg 21 and either Cd 21, Pb 21 or Zn 21. 3.2. Binding at Phthalate Site 1 Fig. 3. MOPAC(PM3)-optimized structure of hydrated Mg(site 1)Pb(site 3)-fulvic Acid.
3. Results and discussion 3.1. Fulvic Acid Model Fig. 1 represents the model structure of Suwannee River fulvic acid. This structure includes a phthalatetype metal ion binding site (site 1 – carbons 5 and 21) in addition to three other carboxylate groups that serve as possible binding sites. The fulvic acid functional groups shown in Fig. 1 have been identified by NMR ( 1H and 13C), infrared and ultraviolet spectrometry; and titrimetric studies [10–14,17]. In an initial report [9], three low energy configurations of fulvic acid whose enthalpies of formation were within 2 kcal of each other were identified. The lowest energy configuration of the fulvic acid phthalate model with the maximum number of binding sites is configuration III shown in Fig. 2 [9]. Configuration III is obtained by two modifications of the structure shown in Fig. 1. The first modification consists of rotating ring 2 by 1808 relative to ring 1 about the carbonyl carbon that connects the two rings (see arrow in Fig. 1). The second modification is accomplished by rotating ring 3 by 1808 relative to ring 1 about the oxygen that connects them (see arrow in Fig. 1). This last modification of ring 3 results in the formation of an intra-molecular hydrogen bond system between the carboxylic acid functions containing carbons 55 and 82 (site 4). As a result of these changes to Fig. 1, the five carboxyl groups in configuration III (carbons 5, 21,
The phthalate binding site (carbons 5 and 21, site 1) has been modeled previously for anhydrous and hydrated phthalate complexes of Mg 21, Zn 21, Pb 21 and Cd 21. In each case, molecular structures and thermodynamic properties of the cis and trans isomers were determined [7]. The metal phthalate cis isomers contain both metal binding oxygens on one side of a plane passing through the phenyl ring and its attached carbons. The trans isomer has the bound oxygens on opposite sides of this plane [7]. Enthalpies of formation of the anhydrous metal ionphthalate complexes are lowest for the cis configurations. The heats of formation for the hydrated metal ion-phthalate complexes are lowest for the cis Mgand Zn-phthalate complexes, whereas cis and trans hydrated Pb- and Zn-phthalate complexes were within 1 kcal of each other [7]. The configuration of the metal ion-site 1 complex is cis for the Mg2-fulvic acid compexes at sites 1 & 3 and 1 & 4 in both the anhydrous and hydrated versions, as would be predicted on the basis of the metal ionphthalate calculations [7]. Similar results are obtained for both anhydrous and hydrated versions of Mg complexes in which Mg is located at site 3 and either Zn 21, Pb 21 or Cd 21 is at site 1. However, all of the remaining anhydrous and hydrated complexes in which site 1 is occupied have the trans configuration at site 1. An example of the trans geometry in which Pb 21 occupies site 3 and Mg 21 occupies site 1 is shown for the hydrated metal-fulvic acid complex in Fig. 3. 3.3. Metal-Oxygen (water) distances The hydration of Mg 21 in fulvic acid produces
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Table 1 MOPAC (PM3) thermodynamic properties of metal ion fulvic acid complexes Fulvic Acid With Site 1 config. Anhydrous Config. III Mg site 1 Pb site 3 - trans Mg site 3 Pb site 1 - cis Mg site 1 Pb site 4 - trans Mg site 4 Pb site 1 - trans Mg site 2 Pb site 4 Mg site 4 Pb site 2 Mg site 1 Zn site 3 - trans Mg site 3 Zn site 1 - cis Mg site 1 Zn site 4 - trans Mg site 4 Zn site 1 - trans Mg site 2 Zn site 4 Mg site 4 Zn site 2 Mg site 1 Cd site 3 - trans Mg site 3 Cd site 1 - cis Mg site 1 Cd site 4 - trans Mg site 4 Cd site 1 - trans Mg site 2 Cd site 4 Mg site 4 Cd site 2 Mg site 1 & 3 - cis Mg site 1 & 4 - cis Mg sites 2 & 4 Hydrated Fulvic Acid Complexes With Site 1 Config. Mg site 1 Pb site 3 - trans Mg site 3 Pb site 1 - cis Mg site 1 Pb site 4 - trans Mg site 4 Pb site 1 - trans Mg site 2 Pb site 4 Mg site 4 Pb site 2 Mg site 1 Zn site 3 - trans Mg site 3 Zn site 1 - cis Mg site 1 Zn site 4 - trans Mg site 4 Zn site 1 - trans Mg site 2 Zn site 4 Mg site 4 Zn site 2 Mg site 1 Cd site 3 - trans Mg site 3 Cd site 1 - cis Mg site 1 Cd site 4 - trans Mg site 4 Cd site 1 - trans Mg site 2 Cd site 4 Mg site 4 Cd site 2 Mg site 1 & 3 - cis Mg site 1 & 4 - cis Mg sites 2 & 4
DfH8 (kcal/mol)
S8 (cal/mol-K)
DfG8 (kcal)
DrH8 (kcal)
2843.696 2833.177 2857.301 2839.479 2864.161 2863.652 2867.818 2761.515 2783.201 2770.36 2781.525 2800.693 2798.947 2773.727 2792.906 2780.145 2795.325 2803.845 2808.738 2886.908 2892.925 2938.395 – DfH8 (kcal/mol)
320.402 309.679 310.335 312.576 311.596 309.805 315.159 308.802 309.265 313.211 312.771 307.948 312.190 315.900 311.499 315.964 313.468 313.465 315.548 307.705 307.168 298.334 – S8 (cal/mol-K)
2939.176 2925.461 2949.781 2932.627 2957.017 2955.974 2961.735 2853.538 2875.362 2863.697 2874.731 2892.462 2891.980 2867.865 2885.733 2874.302 2888.738 2897.258 2902.771 2978.604 2984.461 21027.299 – DfG8 (kcal)
209.234 185.110 202.932 178.250 178.759 174.593 122.129 100.443 113.284 102.119 82.951 84.697 228.219 209.04 221.801 206.621 198.101 193.208 261.536 255.519 210.049 – DrH8 (kcal)
21127.322 21148.936 21138.263 21147.602 21154.957 21134.787 21058.974 21080.249 21072.074 21073.301 21082.368 21052.917 21048.697 21070.078 21059.262 21064.244 21079.009 21045.180 21223.011 21219.083 21246.545
352.520 359.366 354.983 353.060 354.096 347.859 349.824 356.734 350.895 357.796 353.430 354.622 359.249 375.398 361.641 369.063 356.584 363.449 343.194 335.174 329.053
21232.373 21256.027 21244.048 21252.814 21260.478 21238.449 21163.222 21186.556 21176.641 21179.924 21187.690 21158.594 21155.753 21181.947 21167.031 21174.225 21185.271 21153.488 21325.283 21318.965 21344.603
128.797 107.183 117.856 108.517 101.162 121.332 38.378 17.103 25.278 24.051 14.984 44.435 48.655 27.274 38.090 33.108 18.343 52.172 139.141 143.069 115.607
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metal-oxygen (water) distances varying from 1.862 ˚ that are similar to those calculated to 1.874 A ˚ ) for the lowest energy form (1.869 and 1.875 A (cis) of the hydrated Mg 21-phthalate complex. The Zn 21-oxygen (water) distances vary from ˚ , with the exception of Zn 2.181 to 2.221 A (located in site 4), and Mg (at site 2), where the ˚ . These Zn-oxygen distances are 2.213 and 2.427 A distances may be compared with values of 2.244 ˚ obtained for the cis form of hydrated and 2.242 A Zn-phthalate. The slightly higher energy form (by 5.5 kcal) of trans hydrated Zn-phthalate has Zn˚ . The Pboxygen distances of 2.213 and 2.224 A oxygen (water) distances vary from 2.787 to ˚ , with a similarity to those calculated for 2.828 A ˚ ) and trans (2.786, 2.806 A ˚) the cis (2.799, 2.818 A versions of Pb-phthalate whose enthalpies of formation are less than 0.1 kcal apart [7]. Finally, the Cd-oxygen (water) distances vary from 2.347 ˚ , in a manner similar to that observed to 4.531 A ˚ ) and cis (4.483, for the trans (2.513, 2.515 A ˚ ) hydrated Cd-phthalate complexes whose 4.472 A enthalpies of formation are less than 1 kcal apart [7]. In the case of the Cd-phthalate complexes, the higher entropy form (cis) has considerably longer metal ion-oxygen distances than the trans form. In particular, the long metal-water distances suggest a weak interaction between the oxygen and the metal ion due to a shallow potential well and therefore a higher entropy. In a similar manner, the highest entropy form of the hydrated Cdcontaining fulvic acid complex (Mg at site 3, Cd at site 1) is the only complex in which both Cd˚ ). oxygen (water) distances are long (4.406, 4.382A
3.4. Thermodynamic Properties of Mixed Metal IonFulvic Acid Complexes Table 1 contains the calculated standard enthalpies and entropies of both anhydrous and hydrated metal ion-fulvic complex formation (see equation below) at 298 K. The gas phase enthalpies of formation for H 1, Cd 21, Pb 21, Mg 21, and Zn 21 used to calculate the standard heats of reaction for metal ion-fulvic acid complexation are 353.586, 544.444, 503.979, 555.096 and 662.746 kcal/mol, respectively [18]. The binding sites in fulvic acid are carbons 5 and 21
55
for site 1, carbons 21 and 41 in site 2, carbons 41 and 82 in site 3 and carbons 55 and 82 in site 4. Fulvic Acid 1 2(metal ion) 1 4(water) ! Complex 1 4H 1 The standard enthalpies of reaction are calculated assuming that fulvic acid combines with two metal ions and four water molecules (hydrated version only) to produce a metal ion-fulvic acid complex and 4 H 1. The fact that changes in entropy are relatively constant over a wide range of metal ions and variations in binding site geometry indicates that standard enthalpies of reaction are a reasonable indication of trends in hydrated metal ion-fulvic acid complexation. In the calculations of metal ion binding to phthalic acid, the cis configuration was the lowest energy form [7]. Site 1 in fulvic acid is virtually identical to phthalic acid with the exception that additional functional groups may be available for hydrogen bonding in the same vicinity as site 1. The cis form of metal ion binding was observed as the low energy form of hydrated Mg2-fulvic acid binding at sites 1 & 3 and 1 & 4, as would be predicted on the basis of the metal ion-phthalic acid calculations. However, in the mixed metal ion-fulvic acid complexes, the trans configuration predominates with the exception of Mg 21 bound at site 3 and either Cd 21, Pb 21or Zn 21 bound at site 1, irrespective of hydration. This latter combination of Mg 21 bound at site 3 and either Cd 21, Pb 21or Zn 21 bound at site 1 represents the highest entropy form of these complexes with the exception of Mg 21 and Zn 21 where Mg at site 3 and Zn at site 1 is only 1 e.u. from the highest entropy form containing Mg at site 4 and Zn at site 1. In addition to changes in binding configurations at site 1, the effects of hydration on the enthalpies of formation and reaction are considerable. The two mixed metal-fulvic acid complexes with the lowest enthalpy of formation for Pb 21, Zn 21or Cd 21 are generally combinations of sites 2 & 4 for the anhydrous form of these mixed metal-fulvic acid complexes. Hydration of these complexes produces the lowest enthalpy of formation with Mg 21 at site 2 and either Pb 21, Zn 21 or Cd 21 at site 4. The reverse combination of hydrated metal-fulvic acid complexes (Mg 21 at site 4 and either Pb 21, Zn 21 or Cd 21 at site 2) now has the highest or next to highest enthalpy of formation within each group.
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Fig. 4. MOPAC(PM3)-optimized structure of hydrated Mg2(sites 1 & 3)-fulvic Acid.
, Zn 21 to the same order as ionic radii, Zn 21 , Pb 21 , Cd 21, whereas the enthalpies of reaction (see earlier equation) are in the order Zn 21 , Cd 21 , Pb 21. Substitution of Mg 21 in anhydrous Mg2-fulvic acid at sites 2 or 4 with either Pb 21, Zn 21 or Cd 21 yields enthalpies of formation in the order Pb 21 , Cd 21 , Zn 21 and enthalpies of reaction in the same order as the ionic radii, Zn 21 , Pb 21 , Cd 21. In the hydrated complexes, replacement of Mg 21 in Mg2fulvic acid at sites 2 or 4 with either Pb 21, Zn 21 or Cd 21 yields enthalpies of formation in the order Pb 21 , Zn 21 ù Cd 21 and enthalpies of reaction in the order Zn 21 , Cd 21 , Pb 21. In many cases, the differences in enthalpy between metal ion substitution is too small to be considered relevant.
3.6. Intramolecular Hydrogen Bonds 3.5. Effects of Ionic Radii The replacement of Mg 21 in Mg2-fulvic acid at site 1 with either Pb 21, Zn 21 or Cd 21 in the anhydrous forms of the metal-fulvic acid complexes follows the enthalpy of formation order Pb 21 , Cd 21 , Zn 21. The enthalpies of reaction in the demonstrated equation follow the same order as the ionic radii, Zn 21 , Pb 21 , Cd 21. Hydration has the effect of changing the order of both types of enthalpies. The enthalpies of formation order changes from Pb 21 , Cd 21
Fig. 5. MOPAC(PM3)-optimized structure of hydrated Pb(site 1)Mg(site 3)-fulvic Acid.
Intra-molecular hydrogen bonding is present in both anhydrous and hydrated versions of the mixed metal ion-fulvic acid complexes contained in this and prior reports [7,9]. In all hydrated and most anhydrous structures reported herein, there is at least one O–-H ˚ or less and two additional hydrogen bond of 2.0 A ˚ or less. In the majority O–-H hydrogen bonds of 2.5 A of structures reported herein, one finds that hydration produces limited changes in the existing intra-molecular hydrogen bonds. However, when intra-molecular hydrogen bond distances change, these changes accompany variations in distances between rings 1, 2 and 3 (Fig. 1) of the metal-fulvic acid complexes. An example of this effect is the substitution of Pb 21 for Mg 21 at site 1 of the hydrated Mg2-fulvic acid complex (sites 1 & 3). The Mg2-fulvic acid complex (sites 1 & 3) and the PbMg-fulvic acid complex (sites 1 & 3) are shown in Figs. 4 and 5. Figs. 4 and 5 look quite similar, however the substitution of Pb 21 for Mg 21 at site 1 results in considerable separation between rings 2 and 3 (Fig. 1) as indicated by a change in the intra-molecular O– –H distance between H29 ˚ ). Other intra-molecular and O66 (3.595 to 3.902 A O– –H distance changes include H40-O30 (3.540 to ˚ ) and H40-O66 (4.089 to 4.099 A ˚ ). Hydrogen 3.518 A bond distance changes are minimal and include H29˚ ), H40-O20 (1.781 to 1.778 A ˚) O57 (2.483 to 2.490 A ˚ and H29-O35 (2.709 to 2.620 A). These observations
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more distant water molecule forms a hydrogen ˚ (H10-O28) with the opposite bond of 1.793 A carbonyl function.
4. Conclusions
Fig. 6. MOPAC(PM3)-optimized structure of hydrated Cd(site 2)Mg(site 4)-fulvic Acid.
are typical for all of the mixed metal-fulvic acid complexes discussed herein. 3.7. Hydrogen Bonding and Waters of Hydration Hydration of the metal complexes of fulvic acid involves not only coordination between metals and the oxygens of water, but also hydrogen bonding between the water hydrogens and carbonyl oxygens of the coordinating carboxylate functions. A typical example of this type of bonding is the molecular structure of the hydrated form of MgPbfulvic acid (sites 1 and 3) shown in Fig. 3. The O–-H hydrogen bond distances are 1.812 (H96˚ (H95-O44). O11), 1.830 (H91-O20) and 1.803 A Van der waals’ distances [19] of 2.712 (H89-O4) ˚ (H94-O85) are also observed for this and 2.665 A structure. Even in those few cadmium-containing complexes where the metal-oxygen co-ordination distance is longer than expected, one observes hydrogen bonds between the waters of hydration and surrounding functional groups containing oxygen. An example of this is the hydrated version of CdMg-fulvic acid (sites 2 and 4 — shown in Fig. 6) in which the two water oxygens ˚ from Cd co-ordinated to Cd are 2.375 and 4.047 A at site 2. The closer water molecule forms O– –H ˚ (H97,H96hydrogen bonds of 1.860 and 1.822 A O44) with the same carbonyl function and the
A theoretical model of Suwannee River fulvic acid based on experimental evidence has been used to determine the structures of 2:1 hydrated metal ion-fulvic acid complexes containing Mg 21 and either Cd 21, Zn 21 or Pb 21. The similar changes in entropy of the mixed metal-fulvic acid complexes suggest that one can use standard enthalpies of formation as a predictor of complex formation. The binding of Mg 21 at the phthalate binding site (site 1) in fulvic acid changes from cis to trans upon binding of Cd 21, Zn 21 or Pb 21 at sites 3 or 4. In addition to co-ordination with the metal ions, the waters of hydration typically form O–-H hydrogen bonds of approximately ˚ with adjacent carbonyl functions. Cd-water 1.8 A (oxygen) distances are occasionally long compared with van der Waals’ distances [19]. The structures of both hydrated and unhydrated 2:1 metal ion-fulvic acid complexes of Mg 21 and either Cd 21, Zn 21 or Pb 21 with fulvic acid are similar at the site of metal ion co-ordination. However, there is evidence that these small changes are somewhat amplified in terms of distances between ring system 1,2 and 3 (Fig. 1). In addition, the hydration of the metal ions has the effect of equalizing the energies within each metal ion type, thus improving the chances of random rather than preferential binding within fulvic acid. As observed previously [7], the flexibility of fulvic acid results in complex formation energies that do not always correlate with the ionic radii of the metal ions.
Acknowledgements The authors wish to thank Professors P.G. Wahlbeck and G.J. Mains for numerous helpful discussions. Financial support was provided by NSF Grant CHE-9524865 to W.R.C.
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