Subscriber access provided by NEW YORK UNIV

Article 2-

2-

A Comparative Study of [CaEDTA] and [MgEDTA] : Structural and Dynamical Insights from Quantum Mechanical Charge Field Molecular Dynamics Andreas O. Tirler, and Thomas S. Hofer J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b03938 • Publication Date (Web): 19 Jun 2015 Downloaded from http://pubs.acs.org on June 25, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

A Comparative Study of [CaEDTA]2− and [MgEDTA]2−: Structural and Dynamical Insights from Quantum Mechanical Charge Field Molecular Dynamics Andreas O. Tirler, Thomas S. Hofer∗ Theoretical Chemistry Division Institute of General, Inorganic and Theoretical Chemistry University of Innsbruck, Innrain 80-82, A-6020 Innsbruck, Austria E-Mail: [email protected] Tel.: +43-512-507-57102 Fax: +43-512-507-57199

May 28, 2015



Corresponding author

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Structure and dynamics of [MgEDTA]2− and [CaEDTA]2− complexes in aqueous solution have been investigated via quantum mechanical/molecular mechanical (QM/MM) simulations. While for the first a six-fold octahedral complex has been observed, the presence of an additional coordinating water ligand has been observed in the latter case. Due to rapidly exchanging water molecules this seven-fold coordination complex was found to form pentagonal bi-pyramidal as well as capped trigonal prismatic configurations along the simulation interchanging on the picosecond timescale. Also in case of [MgEDTA]2− a trigonal prismatic configuration has been observed for a very short time period of approx. 1 ps. This work reports for the first time the presence of trigonal prismatic structures observed in the coordination sphere of [MgEDTA]2− and [CaEDTA]2− complexes in aqueous solution. In addition to the detailed characterisation of structure and dynamics of the systems the prediction of the associated infrared spectra indicates that the ion-water vibrational mode found at approx. 250 cm−1 provides a distinctive measure to experimentally detect the presence of the coordinating water molecule via low-frequency IR setups.

1

ACS Paragon Plus Environment

Page 2 of 24

Page 3 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

1

Introduction

Ethylenediaminetetraacetate (EDTA) belongs to the group of aminopolycarboxylic acids, which are popular for their polydentate coordination properties. 1–4 Since EDTA is a hexadentate chelating ligand, it is especially suitable to complex a large number of different metal ions including alkaline, earth alkaline, transition metals as well as lanthanoids. 5–25 Because of these excellent chelating properties, EDTA is used as a complex agent in many processes in chemistry and industry, as well as in medicine and pharmacy. This on the one hand includes inhibition of metals catalysing undesired reactions in the paper and food industry, 1,26 on the other hand EDTA is used to reduce water hardness in laundry applications 1 or to improve the iron solubility for agricultural purposes. 27 EDTA is further used for the removal of harmful ions from ground- and waste water. 16,17 Regarding medical purposes, EDTA is used to treat heavy metal poisoning generally caused by mercury or lead, 28 while in cosmetics it is used to stabilise personal care products. 29 EDTA is also used as a masking agent or in complexometric titrations in analytical chemistry. 2,30 Because of the manifold ways EDTA is used on a daily basis, it is important to investigate the structural and dynamical properties of EDTA complexes in the aqueous medium. In fact, many studies have been carried out so far to characterise metal chelating EDTA in the solid 5,10–15,18–20,23,31 and liquid phase 6–9,16,17,20–22,24,25,32 using a number of different experimental methods including NMR, IR/RAMAN, X-RAY and EXAFS/XANES spectroscopy. It was concluded that the coordination behaviour of EDTA strongly depends on the nature of the complexed metal as well as on the pH value of the medium. 7,8,24 Basically a six-fold hexadentate coordination of EDTA to the metal is preferred at environmental conditions, 7,8,15,24 but also a pentadentate coordination of EDTA to the metal with a water molecule substituting a carboxyl group in the six-fold coordination sphere was observed. 10,23 Even a seven-fold coordination has been reported in the case of Ca(II), Mg(II), Na(I), Co(III) and Fe(III), where EDTA adapts a six-fold coordination and an additional water molecule is present in the coordination sphere of the metal. 12–14,18,19,24 Regarding the EDTA complexes of Ca and Mg generally a hexadentate six-fold coordination of EDTA with a distorted octahedral conformation has been reported. 7,15 Nevertheless, seven-fold coordination spheres adopting a pentagonal bi-pyramidal conformation have been mentioned in the literature. 12,13 Also several theoretical investigations of metal complexed EDTA and similar ligands using different approaches have been reported so far. 33–38 Theoretical studies, e.g. molecular simulations of such complex ligands have been shown to be a very suitable approach to gain information about the structural and dynamical properties of such compounds in an aqueous medium on a time scale, which is currently beyond the experimental feasibility. In particular molecular dynamics (MD) simulations 39 are especially suitable to investigate ultra-fast phenomena such as ligand exchange, ligand re-orientation or other associated structural and dynamical phenomena occurring in an aqueous environment. 40–43 For instance, no data on hydrogen bonding of EDTA complexes or configurational conversions of the EDTA ligand is available in the literature as of yet. Therefore, this work gives insight into new structural and dynamical properties of [CaEDTA]2− and [MgEDTA]2− in aqueous solution using molecular dynamics on a hybrid 2

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 24

quantum mechanical / molecular mechanical (QM/MM) level of theory applying pairwise radial distributions, angular distributions, root-mean-square deviations, mean residence time values and time correlation functions as means of analysis.

2

Computational Methods

A critical choice when performing MD simulations is the level of theory employed for the evaluation of energies and associated forces of the nuclei to obtain a proper description of the corresponding motion of molecular species present in the system. Whether the resulting energetic hyper surface of such systems is modeled by empirically derived potential functions in the case of molecular mechanics (MM), or on a quantum mechanical (QM) level of theory has been a widely discussed subject in the past. However, the choice strongly depends on the chemical problem to be addressed in the MD simulation study. Levitt M. and Warshel A. therefore suggested a hybrid simulation technique combining the advantages of both QM and MM and are referred to as QM/MM approaches. 44 This on the one hand allows an acceptable large number of particles in the QM region to obtain a sufficient sophisticated description of chemical problems, on the other hand the computational cost is kept on a manageable level. In this work an extended QM/MM technique, known as QMCF, 40–43 is used which has proven to yield excellent results for the investigation of coordination complexes. 45–49 For more information on the methodology the reader is referred to the respective original literature. 40–43 A crucial step for QM calculations is the assignment of suitable basis sets for the chemical species in the QM region, i.e. the QM core and QM layer, respectively. It has been evidenced in the past that double-ζ + polarisation basis sets provide an adequate compromise between accuracy and computational demand, yielding a sufficiently accurate description of aqueous systems. 40–43 Nevertheless, test computations have been performed including additional diffuse functions to evaluate the influence of such functions on bond distances and vibrational spectra. Bond distances were found to be nearly identical (difference smaller than 0.005 ˚ A for ion-ligand interaction). The only difference in vibrational spectra can be observed for the vibrational modes of the carboxyl groups (approximately 3 %), whereas all normal modes corresponding to ion-ligand interactions are largely unaffected (data not shown). From these results it was concluded that a basis without diffuse functions provides an excellent balance between effort and accuracy. Therefore, the Stuttgart RSC (Relativistic Small Core) ECP basis set 50 was assigned to Ca, whereas 6-31G(d,p) basis sets were chosen for Mg, C, O, N and H. 51,52 The applied level of theory for the QM region is also an important task to be faced in an MD simulation. Density Functional Theory (DFT) or Hartree-Fock (HF) are valid options, because post-HF methods such as second order Møller-Plesset (MP2) Perturbation Theory are not feasible at present for the chosen size of the QM region. Although DFT methods have become popular because of the associated relatively low computational effort, recent literature on pure water show up methodologically inherent shortcomings preventing DFT approaches from an accurate description of hydrogen bonded aqueous systems. Xantheas et al. 53 have shown that a description of 3

ACS Paragon Plus Environment

Page 5 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

water by using DFT methods yield in a melting point of approximately 410 K which led Mundy et al. 54 to the assumption that water treated at room temperature by DFT rather shows characteristics of a supercooled liquid. HF on the other hand lacks of a proper description of electron correlation, but when double-ζ basis sets + polarisation functions are applied excellent results are obtained. 45–49,55 Thus, HF was chosen as compromise between accuracy of results and computational cost. For both MD simulations of [CaEDTA]2− and [MgEDTA]2− in aqueous solution a cubic periodic water box containing 2500 explicitly treated water molecules was used. The MD trajectory was collected in the NVT ensemble with a solvent density of 0.997 g/cm3 and a temperature of 298.15 K using the Berendsen algorithm. 56 The configurations were propagated in time applying the Velocity-Verlet algorithm 57 using a time step of 0.2 fs. Coulombic interactions were taken into account within a radius of 15.0 ˚ A. Additionally 58,59 the Reaction Field method was used to consider the long-range nature of electrostatic contributions. The QM core included the metal ion, whereas the QM layer contained the EDTA ligand together with an average of 30 water molecules giving a total QM region with a diameter of 14.0 ˚ A. The software package TURBOMOLE 60 was used for the evaluation of QM energies and associated QM forces. For the QM/MM coupling of the EDTA ligand in the QM region and solvent molecules in the MM region, standard generalised AMBER force field (GAFF) parameters were used. 61 The solvent-solvent interactions were modeled using the flexible SPC-mTR water model. 62 Since it is important to fully equilibrate all solvent degrees of freedom before invoking the QM/MM treatment, both systems have been pre-equilibrated for 150 ps corresponding to 750000 MD steps employing a AMBER/OPLS type force field description. The typical hydrogen bond lifetime for water is estimated to be between 0.5 and 1.4 ps according to the literature. 63–66 Therefore, after the extensive MM pre-equilibration and application of the QM/MM treatment, both systems have been equilibrated for 5 ps corresponding to 3-10 times of the period of a hydrogen lifetime to ensure a proper relaxation of hydrogen bonds on a QM/MM level. Subsequently a trajectory of 35 ps was collected and analysed.

3 3.1

Results and Discussion Structure of [CaEDTA]2− and [MgEDTA]2−

Pairwise radial distribution functions (RDFs) are useful to determine bond distances and to distinguish different chemical species which take part in a coordination complex. Fig.1 shows RDFs of [CaEDTA]2− (left column) and [MgEDTA]2− (right column). Fig.1a and 1b show an overall M-O RDF for all oxygens within a radius of 12 ˚ A of the metal ion. Fig.1c and 1d show M-O RDFs of the carboxyl oxygens and the metal. It can be seen that in both systems two distinct peaks arise from the carboxyl oxygens, denoting the coordinatively bound carboxyl oxygens and those pointing into the solution. From the integration of the peaks it can be concluded that four carboxyl oxygens coordinate to the metal ion corresponding to a monodentate coordination scheme of each of the four carboxyl groups. 4

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 24

Figure 1: RDFs of selected M-O and M-N species for [CaEDTA]2− (left column) and [MgEDTA]2− (right column): a)+b) overall M-O RDFs, c)+d) M-O RDFs of the EDTA ligand e)+f) M-O RDFs of water g)+h) M-N RDFs of the nitrogens of the ethylenediamine backbone. The dotted lines denote the overall M-O RDFs of a)+b) and the dashed lines represent the respective integration over the RDF.

5

ACS Paragon Plus Environment

Page 7 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

The M-O bond distance in the case of calcium(II) is 2.32 ˚ A, which is in excellent agreement with theoretical findings by Kov´acs et al. (2.33–2.35 ˚ A). 33 and in good agreement with experimental data for the solid (2.37–2.47 ˚ A). 12 For magnesium(II) the M-O bond distance is 2.05 ˚ A, which is also in excellent agreement with theoretical data (2.05–2.12) 33 as well as with crystallographic data (2.07–2.27 ˚ A). 13 A considerable difference of nearly 0.3 ˚ A 2− 2− between [CaEDTA] and [MgEDTA] is observed. This difference in the M-O bond distance is also observed for the respective hydrated ions with ion-oxygen distances of 2.48 ˚ A 67–69 and 2.08 ˚ A 70 for aqueous calcium(II) and magnesium(II), respectively. Therefore, it can be safely assumed that the significantly smaller ion radius of magnesium(II) is responsible for the bond shortening with respect to the calcium(II) analogon in the EDTA complexes. A further argument for the longer M-O bond distance in the [CaEDTA]2− case can be deduced from the RDFs in fig.1e and 1f, where M-O RDFs of water oxygens and the metal ion are depicted. The peak at 2.45 ˚ A for calcium(II) in fig.1e is absent in the case of Mg(II). This peak can be attributed to a metal coordinating water molecule and the respective integration yields one. From studies on crystalline systems the Ca-O water bond distance was indicated to be 2.47–2.52 ˚ A, 13 which is in excellent agreement with the findings from the MD simulation. Therefore, it can be deduced that in the case of calcium(II) a solvent molecule is part of the coordination sphere, whereas according to the RDF, no coordinating water molecules can be observed in the case of magnesium(II). The increase in the coordination number thus leads to an elongation of the coordinative bonds. Fig.1g and 1h show the M-N RDF for [CaEDTA]2− and [MgEDTA]2− . Average ion-N distances of 2.52 ˚ A and 2.20 ˚ A have been observed in case of calcium(II) and magnesium(II), respectively. This is in excellent agreement with theoretical work reporting ion-N distances in the range of 2.52–2.54 ˚ A 33 for calcium(II) and 2.32–2.33 ˚ A 33 for magnesium(II). Furthermore, this data compares well to data obtained from crystal structures as 2.62–2.71 ˚ A 12 for calcium(II) and 2.37 ˚ A 13 for magnesium(II), respectively. Also in the case of M-N bond distances a pronounced difference of approximately 0.3 ˚ A is noted. Collecting the findings from the RDFs, it can be said that calcium(II) is coordinated hexadentately by EDTA with an additional solvent molecule coordinating to the metal. Thus, a coordination number of seven is observed, which has already been evidenced experimentally in the past. 12 Nevertheless, in the case of magnesium(II), EDTA is coordinating in a hexadentate way without further coordination of the solvent molecules. Therefore, a six-fold coordination is observed, but also in this case it can not be excluded that seven-fold coordination might occur as it has been observed by Stezowski et al. 13 It is noted that the hexadentate coordination of the EDTA ligand is persistent for the entire duration of the MD simulation and no carboxyl group is replaced by a water molecule in the coordination sphere of the metal ion as has been mentioned by Wheeler et al. using NMR. 8 A further important conclusion from the RDFs is that EDTA encloses the smaller magnesium(II) ion entirely with its carboxyl groups forming a cage, and therefore, no additional ligands can access the metal ion. Calcium(II) on the other hand has a bigger ion radius and EDTA does not enclose it completely. Therefore, an additional coordination site for the solvent is available. Aime et al. came to the same conclusion according to their NMR studies on solid structures. 11 This can be further confirmed by the work of Solans et al., who state that an increase of 6

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

the bite angle of the carboxyl groups leads to an increase of the coordination number. 15 Fig.2a shows angular distribution functions (ADFs) of the angle between the equatorial coordinating carboxyl groups and the metal ion, which is schematically depicted in fig.2b. Fig.2a evidences that in the case of calcium(II) (black) an angle of 162◦ is obtained (Kov´acs et al. 147◦ ), 33 whereas for magnesium(II) (red) the angle is ∼40◦ lower, i.e. 124◦ (Kov´acs et al. 127◦ ). 33 The considerable difference between Kov´acs et al. and our simulation data can be attributed to the different geometries taken into account in both cases. While the presented simulation data derives from an ensemble of different conformations at finite temperatures and aqueous solution including a coordinating water molecule, Kov´acs et al. considered a single minimum configuration in vacuum assuming a six-fold coordination pattern without the additional first shell water ligand. Fig.2c shows the partial charge distribution of the metal ions in the MD simulation. The partial charge is derived quantum mechanically using a Mulliken population analysis 71 and is 1.29 on calcium(II) (black) and 1.20 on magnesium(II) (red). Apparently a more pronounced charge transfer takes place in the case of magnesium(II) which consequently leads to a bond shortening between the metal ion and the ligand. Since, in the case of calcium(II) an additional ligand coordinates to the metal, the bond distance is further elongated and amounts in an average increase of ∼0.3 ˚ A for the metal–ligand bond as has already been discussed. According to the literature, a hexadentate coordination of the EDTA ligand with an overall coordination number of six for the metal ion results in a distorted octahedral coordination polyhedron. 7,10,15 Therefore, such a configuration was also used as the starting configuration for the MD simulations. In the case of [CaEDTA]2− , a water molecule enters the coordination sphere of the metal within the 5 ps of equilibration time forming a sevenfold coordinated pentagonal bipyramide, which has already been reported by Barnett et al. and Stezowski et al. 12,13 Nevertheless, the distorted octahedral coordination is temporarily maintained by the EDTA ligand. Fig. 3a evidences this observation according to the two-dimensional root-mean-square deviation (rsmd) plot of the sampled trajectory of 35 ps. It is seen that within the first 10 ps of sampling the configuration does not deviate significantly from the starting configuration corresponding to the pentagonal bipyramide which is depicted in fig. 4.1. However, at 3 ps of sampling time a ligand exchange occurs, which can be evidenced looking at the Ca-O distance plot of the metal ion with water oxygens in fig. 3c. Apparently, a water molecule is substituted (green graph) by another water molecule (red graph), i.e. a dissociative ligand exchange is observed. From fig. 3a it is further seen that at a simulation time of approximately 10 ps a considerable configurational rearrangement takes place. [CaEDTA]2− is adopting a monocapped trigonal prismatic coordination polyhedron, which is shown in fig. 4.2. After nearly 19 ps and 22 ps further dissociative ligand exchanges occur according to fig. 3a and fig. 3c. In the latter case the ligand exchange provokes a temporarily adoption of a pentagonal bi-pyramidal configuration which again converts to the monocapped trigonal prism after 4 ps. It can thus be concluded that the structure of [CaEDTA]2− is dynamically changing between a pentagonal bi-pyramidal and a monocapped trigonal prismatic configuration in aqueous solution. This is particularly interesting, since a monocapped trigonal prismatic configuration for [CaEDTA]2− has not been mentioned in the literature so far. However, Mizuta 7

ACS Paragon Plus Environment

Page 9 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Figure 2: a) ADFs for the angle between the metal ion and equatorial coordinating carboxyl groups. In the case of calcium(II) (black) a considerable higher angle is obtained than for magnesium(II) (red). b) Schematic representation of the EDTA ligand highlighting the depicted ADFs in a). The discussed angle is depicted using dashed lines and a spherical representation of the considered atomic species. The higher angle in the case of [CaEDTA]2− permits a further coordination of a solvent molecule, which is unfavourable in the case of [MgEDTA]2− . c) The partial charge of the metal (calcium(II) in black and magnesium(II) in red) is shown. A higher partial charge is noted for calcium(II), indicating a weaker charge transfer from the ligand to the metal ion.

8

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

et al. 72 and Font-Bardia et al. 14 observed such a configuration for iron(II) and sodium(I), respectively. Since the monocapped trigonal prism is a very short living species with rapid interconversion to the octahedral configuration, it can be assumed that it is difficult to detect experimentally. Further it should be noted that during the dissociative water exchange the coordination number formally drops to six. Fig. 3b shows the two-dimensional rmsd plot for the trajectory of [MgEDTA]2− in aqueous solution. It is clearly seen that in this case the EDTA ligand is not significantly deviating from its starting structure corresponding to a distorted octahedron which is depicted in fig. 4.3. However, at approximately 28 ps a structural rearrangement for one ps takes place. This configurational change is seen in fig. 3b and the adopted configuration is shown in fig. 4.4. Thus, also for [MgEDTA]2− a trigonal prismatic configuration with a very short time of existence of ∼ 1ps can be observed. This is beyond the resolution of most experimental methods and therefore it is not very surprising that such short living structural changes have not been reported in literature so far. Due to the short time of existence of the trigonal prism, the capping by a solvent molecule cannot be observed but might occur if such a configuration is adopted for a sufficient long time. Fig. 3d shows a two-dimensional cross-rsmd plot of the trajectories of [CaEDTA]2− against [MgEDTA]2− . From the depiction it is seen that at the beginning of the MD simulation both systems are structurally similar, but in the case of calcium(II) configurations are adopted which are not observed for magnesium(II), except when magnesium(II) also adopts the monocapped trigonal prismatic configuration for a marginally short time as it has already been discussed.

3.2

Dynamics of [CaEDTA]2− and [MgEDTA]2−

Time correlation functions C(t) may be used to obtain dynamical data on the structural relaxation of hydrogen bonds and the associated lifetime: C(t) =

< h(0)h(t) >

(1)

with h(t) being a defined hydrogen bond variable. 73 Since the so-called intermittent hydrogen bond correlation function C(t) typically has a double-exponential form, it may be fitted according to eq. 2. From the fitted time correlation functions (fig. 5a and b), the long and short contribution τl and τs are obtained. τl is a direct measure for the structural relaxation of a formed hydrogen bond. Fig. 5a and b show the intermittent time correlation functions for carboxyl oxygens of [CaEDTA]2− and [MgEDTA]2− , respectively. Since there are coordinating and non-coordinating carboxyl oxygens of the EDTA ligand, they are distinguished by different colours in the graphs (blue are coordinating and orange non-coordinating oxygens), which is also shown schematically in fig. 5d. In fig. 5c the effective number of hydrogen bonds for the carboxyl oxygens of [CaEDTA]2− (left graphs) and [MgEDTA]2− (right graphs) are shown.

9

ACS Paragon Plus Environment

Page 11 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Figure 3: a) 2d rmsd plot of the trajectory of [CaEDTA]2− . Two distinct configurations can be evidenced from the colour-coded depiction. b) The rmsd plot of the trajectory of [MgEDTA]2− . It is seen that the starting configuration is kept throughout the whole duration of the MD simulation, except one single event at approximately 28 ps. c) Ca-O water distance plot for the calcium(II) EDTA simulation. The time scale corresponds to that of a) evidencing several dissociative ligand exchanges which are assigned accordingly. d) A cross rmsd plot of the trajectories of both MD simulations. While in the case of magnesium(II) (y-axis) few structural rearrangement is observed, it is recognised that in the case of calcium(II) (x-axis) several re-orientations take place.

10

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

Figure 4: Coordination polyhedra of [CaEDTA]2− and [MgEDTA]2− : 1 The pentagonal bi-pyramidal configuration of Ca[EDTA]2− ·H2 O. 2 The trigonal prismatic configuration of the EDTA ligand with a solvent molecule capping it. 3 The dominant distorted octahedral configuration of [MgEDTA]2− . 4 The trigonal prismatic configuration of [MgEDTA]2− . An intermittent configuration which can temporarily be observed.

11

ACS Paragon Plus Environment

Page 13 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

y = a· e

− τt

l

+ (1 − a)· e− τs t

(2)

From fig. 5c it can be seen that for both systems up to four hydrogen bonds are formed

Figure 5: Hydrogen bond analysis for coordinating and non-coordinating carboxyl oxygens of [CaEDTA]2− and [MgEDTA]2− : a) The intermittent H-bond correlation functions for [CaEDTA]2− b) The intermittent H-bond correlation functions for [MgEDTA]2− c) Representation of the number of H-bonds of the carboxyl functions for [CaEDTA]2− (left graphs) and [MgEDTA]2− (right graphs) d) Schematic representation of the carboxyl oxygens: coordinating oxygens (blue) and non-coordinating oxygens (orange). at carboxyl oxygens pointing into the solution, whereas for the metal ion coordinating oxygens one or two hydrogen bonds can be observed, but also no hydrogen bonds to bulk solvent molecules is frequently observed. Table 1 depicts the long and short contributions of the fitted intermittent hydrogen bond correlation functions for the carboxyl oxygens of the EDTA ligand. It is seen that in both cases coordinating oxygens show a shorter structural relaxation of hydrogen bonds than the non-coordinating oxygens pointing into the bulk. On the other hand it is seen that the structural relaxation takes longer in the case of [CaEDTA]2− , which can be attributed to the weaker charge transfer to the metal 12

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ocoord Onon−coord

[CaEDTA]2− τl τs 1.08 0.09 1.70 0.15

Page 14 of 24

[MgEDTA]2− τl τs 1.02 0.089 1.54 0.125

Table 1: The long-range and short-range contributions τl and τs in ps for the intermittent hydrogen bond correlation functions C(t) for [CaEDTA]2− and [MgEDTA]2− . ion by the carboxyl oxygens. Fig. 6 shows the vibrational frequency of Ca-O for the coordinatively bound water molecule. A wave number of 244 cm−1 is obtained corresponding to a force constant of 40 N/m. This is an important information, since such results are very difficult to obtain by experimental techniques and offers particular insight into the binding strength of water ligands to the metal ion. It can be said that a force constant of 40 N/m is quite strong which can be further confirmed by the relatively high mean residence time (MRT) of 2.75 ps for the water molecule in the coordination sphere of calcium(II), which was calculated using the direct method. 74 For the hydrated calcium(II) similar wave numbers of 260 cm−1 (HF) and 276 cm−1 (DFT) corresponding to force constants of 46 N/m and 51 N/m are obtained. 67 Furthermore, MRT values of 40-60 ps for water molecules in the first hydration shell of calcium(II) are reported 67 confirming the strength of a coordinative Ca-O bond in aqueous solution. This further shows the influence of heteroatoms on the mean residence time of water ligands in the coordination sphere. It has been previously shown that the water exchange at several metal ions including calcium(II) significantly increases upon complexation by heteroatoms such as nitrogen. 75–78 Also in our case EDTA is coordinating with two nitrogen atoms towards the calcium(II) ion and the mean residence time for water drops approx. an order magnitude from 40-60 ps to 2.75 ps, thus being in accordance with the literature.

3.3

Cluster Computations

In order to probe the IR activity of the above mentioned ion-water vibration (cf. fig. 6) IR spectra of both systems using the quantum chemical package GAUSSIAN09 79 have been performed. The IR spectra of the clusters have been calculated employing the advanced implicit solvation model developed by Truhlar et al. 80 applying different levels of theory, i.e. HF and B3LYP using the same basis set employed in the MD simulation. To overcome the inherent overestimation of vibrational frequencies using HF and B3LYP in combination with the 6-31G(d,p) basis set, the obtained spectra were scaled by a factor reported in the literature. 81,82 The final spectra of [MgEDTA]2− (red) and [CaEDTA]2− (black) are shown in fig. 7. For both levels of theory multiple peaks in the low-frequency region are seen in case of calcium(II), which are attributed to the ion-water vibration since they are absent in case of magnesium(II). The corresponding frequency is 239 cm−1 in case of HF, which is in agreement with the value obtained from the QMCF simulation. In case of B3LYP the

13

ACS Paragon Plus Environment

Page 15 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Figure 6: The vibrational frequency for the Ca-O vibration of the calcium coordinating water molecule.

14

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

frequency is 282 cm−1 . Since the Ca-water vibration is typically IR inactive for the solution structure of calcium, it has been clearly shown that upon complexation with EDTA the Ca-water vibration mode is IR active and thus, provides a valuable route to experimentally verify the seven-fold coordination of [CaEDTA]2− .

Figure 7: Calculated IR spectra for [MgEDTA]2− (red) and [CaEDTA]2− (black) at different levels of theory: a) HF b) B3LYP

4

Conclusions

The current work presenting a hybrid ab initio/molecular mechanical simulation study of [MgEDTA]2− and [CaEDTA]2− demonstrates the capabilities of a theoretical characterisation of solvated coordination complexes. Computer simulations not only provide results of experimental quality, but also give insight into elusive properties and provides information how to investigate such phenomena via experiment. In particular the presence of a coordinating water molecule in case of [CaEDTA]2− corresponding to a seventh coordination site can be directly observed in the simulation. It is shown that for both systems the EDTA ligand adopts either a distorted octahedral or a trigonal prismatic configuration when coordinating to calcium(II) or magnesium(II). For the [MgEDTA]2− an octahedral configuration of the EDTA ligand resulting in a coordination number of six with respect the metal ion is the dominant configuration, while the trigonal prismatic configuration is an intermittent transition state with a short time of existence of about 1 ps. In the case of 15

ACS Paragon Plus Environment

Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

[CaEDTA]2− an equilibrium between a pentagonal bipyramide and a monocapped trigonal prism is observed. Therefore, several configurational conversions are observed during the simulation. The increased coordination number of seven in case of calcium(II) correlates with the larger ionic radius and a weaker ion-ligand charge transfer. In addition to the detailed structural and dynamical characterisation of the complexes the comparison of the predicted spectra not only illustrates that the ion-water vibrational mode found at approx. 250 cm−1 is infrared active, but may serve as an unambiguous probe to distinguish the species in aqueous solution via extended spectroscopic setups. This prediction of a distinct vibrational mode highlights the main capabilities of atomistic simulations executed at a quantum chemical level of theory, providing extensive insight into a broad variety of properties of the investigated systems.

5

Acknowledgments

Financial support for this work by a PhD scholarship of the Leopold-Franzens-University of Innsbruck (Rector Prof. Dr.Dr.hc.mult. Tilmann M¨ark) for A.O. Tirler is gratefully acknowledged. This work was supported by the Austrian Ministry of Science BMWFW as part of the Konjunkturpaket II of the Focal Point Scientific Computing at the University of Innsbruck.

References [1] Holleman, A.; Wiberg, E. Inorganic Chemistry, 1st ed.; San Diego: Academic Press, 2001. [2] Harris, D. Quantitative Chemical Analysis, 8th ed.; Freeman, W.H., 2002. [3] Fischer, R.; Peters, D. Basic Theory and Practice of Quantitative Chemical Analysis; Chemical Society, London, 1968; p 690. [4] Schwarzenbach, G. Complexometric Titrations; Interscience, New York, 1957; p 101. [5] Shevchenko, L. Infrared Spectra of Salts and Complexes of Carboxylic Acids and Some of their Derivatives. Russ. Chem. Rev. 1963, 32, 201–207. [6] Sawyer, D.; Tackett, J. Properties and Infrared Spectra of Ethylenediaminetetraacetic Acid Complexes. V. Bonding and Structure of Several Metal Chelates in Solution. J. Am. Chem. Soc. 1963, 85, 2390–2394. [7] Han, S.; Mathias, E.; Ba, Y. Proton NMR Determination of Mg2+ and Ca2+ Concentrations Using Tetrasodium EDTA Complexes. Journal of Chemistry 2007, 1, 1–5. 16

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

[8] Wheeler, W.; Legg, J. Solution Structure of the Chromium(III) Complex with Edta by Deuteron NMR Spectroscopy. Inorg. Chem. 1984, 23, 3798–3802. [9] El Abdallaoui, H.; Champmartin, D.; Rubini, P. Complexes of EDTA in Aqueous Solutions. Structural Aspects from a 13 C NMR Relaxation Study. J. Chem. Soc., Dalton Trans. 2001, 2153–2156. [10] Alam, T.; Assink, R. Solid State 13 C CP/MAS NMR Investigations of EDTA-Metal Complexes. Magn. Reson. Chem. 1997, 35, 427–431. [11] Aime, S.; Gobetto, R.; Nano, R.; Santucci, E. 13 C Solid State CP/MAS NMR Studies of EDTA Complexes. Inorg. Chem. Acta 1987, 129, 23–25. [12] Barnett, B.; Uchtman, V. Structural Investigations of Calcium-Binding Molecules. 4. Calcium Binding to Aminocarboxylates. Crystal Structures of Ca(CaEDTA)·7H2 O and Na(CaNTA). Inorg. Chem. 1979, 18, 2674–2678. [13] Stezowski, J.; Countryman, R.; Hoard, J. Structure of the Ethylenediaminetetraacetatoaquomagnesate(II) Ion in a Crystalline Sodium Salt. Comparative Stereochemistry of the Seven-Coordinate Chelates of Magnesium(II), Manganese(II), and Iron(III). Inorg. Chem. 1973, 12, 1749–1754. [14] Font-Bardia, M.; Solans, X.; Font-Altaba, M. Sodium Ion Complexes with Ethylenediaminetetraacetic Acid. Acta Cryst. C 1993, 49, 1452–1456. [15] Solans, X.; Font-Altaba, M. Crystal Structures of Ethylenediaminetetraacetato Metal Complexes. I. A Comparison of Crystal Structures Containing Hexacoordinated Metal Ions, [(H2 O)4 X(C10 H12 N2 O8 )Y]n ·2nH2 O. Acta Cryst. C 1983, 39, 435–438. [16] Friedly, J.; Kent, D.; Davis, J. Simulation of the Mobility of Metal-EDTA Complexes in Groundwater: The Influence of Contaminant Metals. Environ. Sci. Technol. 2002, 36, 355–363. [17] Balaska, F.; Bencheikh-Lehocine, M.; Chikhi, M.; Meniai, A.-H.; Bouledjouidja, A. Experimental Study and Simulation of Complexation Reaction of Chromium by EDTA for its Recovery by Ultrafiltration. Energy Procedia 2012, 19, 249–258. [18] Zubkowski, J.; Perry, D.; Valente, E.; Lott, S. A Seven Coordinate Co-EDTA Complex. Crystal and Molecular Structure of Aquo(ethylenediaminetriacetatoacetic acid)cobalt(III) Dihydrate. Inorg. Chem. 1995, 34, 6409–6411. [19] Zabel, M.; Poznyak, A.; Pawlowski, V. Crystal Structure of Calcium Dihydroethylenediaminetetraacetate(2-)dihydrate Ca(H2 Edta)·2H2 O. J. Struct. Chem. 2006, 47, 581–584. [20] Sakane, H.; Watanabe, I.; Ikeda, S. EXAFS and XANES Spectra of Cobalt(III) EDTA Complexes in Solids and Solutions. Bull. Chem. Soc. Jpn. 1989, 62, 1513–1516. 17

ACS Paragon Plus Environment

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

[21] Sakane, H.; Watanabe, I.; Ono, K. Structures of Fe(III) Complexes with EDTA and EDDDA in Aqueous Solution by EXAFS and XANES. Inorg. Chim. Acta 1990, 178, 67–70. [22] Kanamori, K.; Dohniwa, H.; Ukita, N.; Kanesaka, I.; Kawai, K. The Raman Spectral Study on the Solution Structure of Iron(III)-EDTA Complexes. Bull. Chem. Soc. Jpn. 1990, 63, 1447–1454. [23] Mizuta, T.; Yamamoto, T.; Miyoshi, K.; Kushi, Y. The Ligand Field Stabilization Effect on the Metal-Ligand Bond Distances in Octahedral Metal Complexes with EDTAType Ligands. Redetermination of the Molecular Structure of (ethylenediaminetriacetatoacetic acid)-(aqua)iron(III), [Fe(Hedta)(H2 O)]. Inorg. Chim. Acta 1990, 175, 121–126. [24] Wagner, C.; Baran, E. Vibrational Spectra of Two Fe(III)/EDTA Complexes Useful for Iron Supplementation. Spectrochim. Acta Part A 2010, 75, 807–810. [25] Krishnan, K.; Plane, R. Raman Spectra of Ethylenediaminetetraacetic Acid and Its Metal Complexes. J. Am. Chem. Soc. 1968, 90, 3195–3200. [26] Furia, T. EDTA in Foods–A Technical Review. Food Techn. 1964, 18, 1874–1882. [27] Norvell, W.; Lindsay, W. Reactions of EDTA Complexes of Fe, Zn, Mn, and Cu with Soils. Soil Sci. Soc. Am. J. 1968, 33, 86–91. [28] Harrison, T.; Fauci, A.; Braunwald, E.; Isselbacher, K. Innere Medizin, 14th ed.; London: Mc Graw–Hill, 1999; pp 3017–3022. [29] Lanigan, R.; Yamarik, T. Final Report on the Safety Assessment of EDTA, Calcium Disodium EDTA, Diammonium EDTA, Dipotassium EDTA, Disodium EDTA, TEAEDTA, Tetrasodium EDTA, Tripotassium EDTA, Trisodium EDTA, HEDTA, and Trisodium HEDTA. Int. J. Toxicol. 2002, 21, 95–142. [30] Somashekar, B.; Ijare, O.; Nagana Gowda, G.; Ramesh, V.; Gupta, S.; Khetrapal, C. Simple Pulse-aquire NMR Methods for the Quantitative Analysis of Calcium, Magnesium and Sodium in Human Serum. Spectrochim. Acta, Part A 2006, 65, 254–260. [31] Faulques, E.; Perry, D.; Lott, S.; Zubkowski, J.; Valente, E. Study of Coordination and Ligand Structure in Cobalt-EDTA Complexes with Vibrational Microspectroscopy. Spectrochim. Acta Part A 1998, 54, 869–878. [32] Nowack, B.; Sigg, L. Adsorption of EDTA and Metal-EDTA Complexes onto Goethite. J. Colloid Interface Sci. 1996, 177, 106–121. [33] Kov´acs, A.; Nemcsok, D.; Kocsis, T. Bonding Interactions in EDTA Complexes. J. Mol. Struct.: THEOCHEM 2010, 950, 93–97. 18

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

[34] Gajewski, M.; Klobukowski, M. DFT Studies of Complexes Between Ethylenediamine Tetraacetate and Alkali and Alkaline Earth Cations. Can. J. Chem. 2009, 87, 1492– 1498. [35] J´oszai, R.; Purgel, M.; P´apai, I.; Wakita, H.; T´oth, I. Multinuclear NMR and DFT Studies of the Structure and Fluxionality for MIII-Ethylenediamine-Tetraacetate Complexes (M(EDTA)-, M=Al, Ga and In) in Solution. J. Mol. Liq. 2007, 131-132, 72–80. [36] Pesonen, H.; Aksela, R.; Laasonen, K. Density Functional Complexation Study of Metal Ions With Aminopolycarboxylic Acid Ligands: EDDHA and HBED in Comparison to EDTA, EDDS, ODS, and ISA. J. Mol. Struct.: THEOCHEM 2007, 804, 101–110. [37] Chen, L.; Liu, T.; Chen, J. A Density Functional Theory Study on the Electronic Structures and Properties of Methylenediaminetetraacetate Complexes (M=Co,Ni,Cu,Zn,Cd. Acta Chim. Sin. 2008, 66, 1187–1195. [38] Coskuner, O.; Jarvis, E. Coordination Studies of Al-EDTA in Aqueous Solution. J. Phys. Chem. A 2008, 112, 2628–2633. [39] Leach, A. Molecular Modelling: Principles and Applications, 2nd ed.; Prentice Hall, 2001. [40] Rode, B.; Hofer, T.; Randolf, B.; Schwenk, C.; Xenides, D.; Vchirawongkwin, V. Ab Initio Quantum Mechanical Charge Field (QMCF) Molecular Dynamics- A QM/MMMD Procedure for Accurate Simulations of Ions and Complexes. Theor. Chem. Acc. 2006, 115, 77–85. [41] Rode, B.; Hofer, T. How to Access Structure and Dynamics of Solutions: The Capabilities of Computational Methods. Pure Appl. Chem. 2006, 78, 525–539. [42] Rode, B.; Hofer, T.; Pribil, A.; Randolf, B. Advances in Inorganic Chemistry; Elsevier, 2010; Vol. 62; Chapter 4, pp 143–175. [43] Rode, B.; Hofer, T.; Pribil, A.; Randolf, B. Advances in Quantum Chemistry; Elsevier, 2010; Vol. 59; Chapter 7, pp 213–246. [44] Warshel, A.; Levitt, M. Theoretical Studies of Enzymic Reactions: Dielectric, Electrostatic and Steric Stabilization of the Carbonium Ion in the Reaction of Lysozyme. J. Mol. Biol. 1976, 103, 227–249. [45] Tirler, A.; Weiss, A.; Hofer, T. A Comparative Quantum Mechanical Charge Field Study of Uranyl Mono- and Dicarbonate Species in Aqueous Solution. J. Phys. Chem. B 2013, 117, 16174–16187.

19

ACS Paragon Plus Environment

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

[46] Tirler, A.; Hofer, T. Structure and Dynamics of the Uranyl Tricarbonate Complex in Aqueous Solution: Insights from Quantum Mechanical Charge Field Molecular Dynamics. J. Phys. Chem. B 2014, 117, 16174–16187. [47] Lutz, O.; Messner, C.; Hofer, T.; Glaetzle, M.; Huck, C.; Bonn, G.; Rode, B. Combined Ab Initio Computational and Infrared Spectroscopic Study of the Cis- and TransBis(glycinato)copper(II) Complexes in Aqueous Environment. J. Phys. Chem. Lett. 2013, 4, 1502–1506. [48] Messner, C.; Lutz, O.; Rainer, M.; Huck, C.; Hofer, T.; Rode, B.; Bonn, G. Structure and Dynamics of Chromatographically Relevant Fe(III)-Chelates. J. Phys. Chem. B 2014, 118, 12232–12238. [49] Tarique, M.; Hofer, T. Hydration of Porphyrin and Mg-Porphyrin: Ab Initio Quantum Mechanical Charge Field Molecular Dynamics Simulations. Mol. BioSyst. 2014, 10, 117–127. [50] Kaupp, M.; Schleyer, P.; Stoll, H.; Preuss, H. Pseudopotential Approaches to Ca, Sr, and Ba Hydrides. Why are some Alkaline Earth MX2 Compounds Bent? J. Chem. Phys. 1991, 94, 1360–1366. [51] Hariharan, P.; Pople, J. The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies. Theoret. Chim. Acta 1973, 28, 213–222. [52] Francl, M.; Pietro, W.; Hehre, W.; Binkley, J.; Gordon, M.; DeFrees, D.; Pople, J. Self-Consistent Molecular Orbital Methods. XXIII. A Polarization Type Basis Set for Second Row Elements. J. Chem. Phys. 1982, 77, 3654–3665. [53] Yoo, S.; Zeng, X.; Xantheas, S. On the Phase Diagram of Water with Density Functional Theory Potentials: The Melting Temperature of Ice Ih with the Perdew-BurkeErnzerhof and Becke-Lee-Yang-Parr Functionals. J. Chem. Phys. 2009, 130, 221102– 221105. [54] Schmidt, J.; VandeVondele, J.; Kuo, I.; Sebastiani, D.; Siepmann, J.; Hutter, J.; Mundy, C. Isobaric-Isothermal Molecular Dynamics Simulations Utilizing Density Functional Theory: An Assessment of the Structure and Density of Water at NearAmbient Conditions. J. Phys. Chem. B 2009, 113, 11959–11964. [55] Hofer, T.; Weiss, A.; Randolf, B.; Rode, B. Hydration of Highly Charged Ions. Chem. Phys. Lett. 2011, 512, 139–145. [56] Berendsen, H.; Postma, J.; van Gunsteren, W.; DiNola, A.; Haak, J. Molecular Dynamics with Coupling to an External Bath. J. Chem. Phys. 1984, 81, 3684–3690. [57] Andersen, H. Molecular Dynamics Simulations at Constant Pressure and/or Temperature. J. Chem. Phys. 1980, 72, 2384–2393. 20

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

[58] Barker, J.; Watts, R. Monte Carlo Studies of the Dielectric Properties of Water-Like Models. Mol. Phys. 1973, 26, 789–792. [59] Watts, R. Monte Carlo Studies of Liquid Water. Mol. Phys. 1974, 28, 1069–1083. [60] TURBOMOLE V6.3 2011, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989-2007, TURBOMOLE GmbH, since 2007; available from http://www.turbomole.com. [61] Wang, J.; Wolf, R.; Caldwell, J.; Kolman, P.; Case, D. Development and Testing of a General Amber Force Field. J. Comput. Chem. 2004, 25, 1157–1174. [62] Liew, C.; Inomata, H.; Arai, K. Flexible Molecular Models for Molecular Dynamics Study of Near and Supercritical Water. Fluid Phase Equil. 1998, 144, 287–298. [63] Texeira, J.; Luzar, A.; Longeville, S. Dynamics of Hydrogen Bonds: How to Probe Their Role in the Unusual Properties of Liquid Water. J.Phys.: Condens. Matter 2006, 18, S2353–S2362. [64] Luzar, A.; Chandler, D. Hydrogen-Bond Kinetics in Liquid Water. Nature 1996, 379, 55–57. [65] Lock, A.; Woutersen, S.; Bakker, H. Femtochemistry and Femtobiology; World Scientific Publishing Co. Pte. Ltd., 2001; Chapter 2, pp 234–239. [66] Lawrence, C.; Skinner, J. Ultrafast Infrared Spectroscopy Probes Hydrogen-Bonding in Liquid Water. Chem. Phys. Lett. 2003, 369, 472–477. [67] Schwenk, C.; Rode, B. Ab Initio QM/MM MD Simulations of the Hydrated Ca2+ Ion. Pure Appl. Chem. 2004, 76, 37–47. [68] Schwenk, C.; Loeffler, H.; Rode, B. Molecular Dynamics Simulations of Ca2+ in Water–Comparison of a Classical Simulation Including 3-Body Corrections and Born-Oppenheimer Ab Initio and Density-Functional Theory Quantum Mechanical/Molecular Mechanics Simulations. J. Chem. Phys. 2001, 115, 10808–10813. [69] Schwenk, C.; Loeffler, H.; Rode, B. Dynamics of the Solvation Process of Ca2+ in Water. Chem. Phys. Lett. 2001, 349, 99–103. [70] Bhattacharjee, A.; Pribil, A.; Randolf, B.; Hofer, T.; Rode, B. Hydration of Mg2+ and Its Influence on the Water Hydrogen Bonding Network via Ab Initio QMCF MD. Chem. Phys. Lett. 2012, 536, 39–44. [71] Mulliken, R. Electronic Population Analysis on LCAO-MO Molecular Wave Functions. I. J.Chem.Phys. 1955, 23, 1833–1840.

21

ACS Paragon Plus Environment

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

[72] Mizuta, T.; Wang, J.; Miyoshi, K. A Seven-Coordinate Structure of Iron(II)Ethylenediamine-N,N,N’,N’-tetraacetato Complex as Determined by X-Ray Crystal Analysis. Bull. Chem. Soc. Jpn. 1993, 66, 2547–2551. [73] Chanda, J.; Chakraborty, S.; Bandyopadhyay, S. Sensitivity of Hydrogen Bond Lifetime Dynamics to the Presence of Ethanol at the Interface of a Phospholipid Bilayer. J. Phys. Chem. B 2006, 110, 3791–3797. [74] Hofer, T.; Tran, H.; Schwenk, C.; Rode, B. Characterisation of Dynamics and Reactivities of Solvated Ions by Ab Initio Simulations. J. Comput. Chem. 2004, 473, 211–214. [75] Fatmi, Q.; Hofer, T.; Rode, B. The Stability of [Zn(NH3 )4 ]2+ in Water: A Quantum Mechanical/Molecular Mechanical Molecular Dynamics Study. Phys. Chem. Chem. Phys. 2010, 12, 9713–9718. [76] Schwenk, C.; Hofer, T.; Randolf, B.; Rode, B. The Influence of Heteroligands on the Reactivity of Ni2+ in Solution. Phys. Chem. Chem. Phys. 2005, 7, 1669–1673. [77] Schwenk, C.; Rode, B. CuII in Liquid Ammonia: An Approach by Hybrid QuantumMechanical/Molecular-Mechanical Molecular Dynamics Simulation. Chem. Phys. Chem. 2004, 5, 342–348. [78] Schwenk, C.; Loferer, M.; Rode, B. Ultrafast Ligand Exchange Rates Determined by Ab Initio QM/MM Molecular Dynamics. Chem. Phys. Lett. 2003, 382, 460–465. [79] Frisch, M.; Trucks, G.; Schlegel, H.; Scuseria, G.; Robb, M.; Cheeseman, J.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.; et al., Gaussian 09 Revision A.1. Gaussian Inc. Wallingford CT 2009. [80] Marenich, A.; Cramer, C.; Truhlar, D. Universal Solvation Model Based on Solute Electron Density and a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378–6396. [81] Scott, A.; Radom, L. Harmonic Vibrational Frequencies: An Evaluation of HartreeFock, Møller-Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors. J. Phys. Chem. 1996, 100, 16502–16513. [82] Merrick, J.; Moren, D.; Radom, L. An Evaluation of Harmonic Vibrational Frequency Scaling Factors. J. Phys. Chem. A 2007, 111, 11683–11700.

22

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For Table of Contents Only

23

ACS Paragon Plus Environment

Page 24 of 24

A Comparative Study of [CaEDTA](2-) and [MgEDTA](2-): Structural and Dynamical Insights from Quantum Mechanical Charge Field Molecular Dynamics.

Structure and dynamics of [MgEDTA](2-) and [CaEDTA](2-) complexes in aqueous solution have been investigated via quantum mechanical/molecular mechanic...
10MB Sizes 0 Downloads 9 Views