Molecules that bind at multiple points in a coordination complex are said to be chelated. The non-protonated form results with the removal of the hydrogen atoms on each of the four carboxylic acid groups. In a non-protonated molecule, the EDTA has extra, unpaired electrons on the four oxygen atoms that have single bonds with the carbons and on the two nitrogen atoms. Molecules that chelate tend to be more thermodynamically stable, and chelating compounds tend to displace monodentates in coordination complexes in solution.
This is due, in part, to the fact that entropy-favoring reactions are spontaneous, and "want" to happen more. For instance, see the following sample equation in which EDTA displaces the water ligands:. Here, an aqua-complex of iron is changed into an iron molecule with a single EDTA molecule surrounding it. This is because on the left side of the equation, there are two molecules - the iron complex and the EDTA - while on the right side of the equation there are seven molecules - the new iron complex and the six water molecules displaced by the EDTA ligand.
Entropy-favored reactions are spontaneously occurring, and an increase in the number of molecules is an increase in entropy. Thus, the above reaction is spontaneous, with a K value exceeding 10 Notice that the atoms with the unpaired electrons the two nitrogens and four oxygens make these coordinate covalent bonds.
For pressures lower than mb, an Alpert ion gage was employed, whereas a three-range McLeod gauge served for pressures between mb and 5 mb. Heme is a porphyrin that is coordinated with Fe II and is shown in Figure 4. The ion collector electrode C is supported by the high-leakage-resistance feed-through insulator D and is connected to the electrometer tube of a d-c amplifier outside the vacuum chamber. It is interesting to note that from Table 1, the EA of m- and o-xylene were calculated and, as our chemical intuition suggested, revealed negative values. Institute of Technology, Harbin, China, in and  U. Use of this web site signifies your agreement to the terms and conditions. It is assumed that the electric field intensity is not strong enough to cause any ionization by collision; the field is, however, sufficient to render any contribution due to diffusion current negligible.
Credits go to Wikipedia for picture. As it is so good at displacing molecules in coordination complexes, EDTA can be used to prevent undesired metals in trace amounts from reacting and having detrimental effects on products. This is known as sequestering. For instance, in regards to cosmetics, EDTA serves to increase the cosmetic product's resistance towards molecules in the air.
Simmilarly, in personal care and skin care products, EDTA binds to free metal ions and serves as a purifying agent and perservative. It basically reduces the "hardness" or presence of metal cations in tap water so that other ingredients in shampoos and soaps can work to cleanse more efficiently.
Along the same lines, EDTA is used in laundry detergents to soften water that comes into contact with it so the other active ingredients can cleanse better. In textiles, EDTA prevents the discoloring of dyed fabrics by removing harmful free metal ions and it also gets rid of residue left on industrial equipment that must be used at high temperatures i.
In general, EDTA reduces the reactivity of a metal, preventing any unwanted effects that may result from its presence. In addition to its usefulness in industries, EDTA can also be utilized in medicine. Doctors can prescribe EDTA treatments for patients suffering from lead poisoning.
Such a treatment is known as chelation therapy , in which EDTA renders the toxic ions present in the body harmless. The EDTA is administered intravenously and makes its way through the blood stream. Given its hexadentate nature, EDTA has a molecular structure much like a claw. Because of this very structure, the EDTA pulls toxic heavy metals detected in the bloodstream towards itself and attaches itself to these metal ions. This attachment forms a compound that can be excreted from the body through urine, not allowing them to bind to enzymes and cytochromes.
A chelation therapy may take many sittings and may last anywhere from one to three hours per sitting. Not only can chelation therapy aid in excreting harmful lead ions from the body, but it can also aid in safely getting rid of mercury, chromium, cobalt, nickel, zinc, arsenic and thallium ions from the bloodstream. In cases of excess consumption of digoxin, a medication used to treat atrial fibrillation, atrial flutter and even heart failure, EDTA has been used to clear the bloodstream of the unused ions.
At first the EA of the three isomers will be determined using equation 1 in the absence of an electric field. After which the same will be done when a gradually changing electric field will be applied to the molecules under investigation. In Figure 4, p-xylene is used as an example to show possible conjugation of the anion radical after addition of an electron. Stevenson et al. Figure 4: Possible molecular conjugation of p-xylene 1 after addition of an extra electron to the anion radical.
Structure 3 is one of the valence bond representations given by Stevenson et al. The total energy of the cation will also be determined in the absence and under the effect of an electric field. The main reason for this calculation method is for comparison purposes, so as to see whether the adiabatic EA of xylene changes in the presence of an electric field and also to see the stabilities of the three different states relative to one another.
In order to also be able to comment on the method's accuracy, I will also calculate the EAs of nitrogen, oxygen, and SF 6. Piechota et al. There are three methods that are particularly useful in determining a molecule's EA and are shown in Figure 5. The proposed calculation method stated above is known as the adiabatic EA, while the other two are known as the vertical detachment energy VDE and vertical attachment energy VAE.
VDE is the energy required for the removal of an electron from the anion, whereas the VAE is the energy released from the addition of an electron to a neutral molecule. During each of these processes there is no time for geometry optimization and as a result, the two species are either at the equilibrium geometry of the anion VDE or the neutral molecule VAE.
E tot X is the total energy of the neutral molecule in its equilibrium geometry E tot X - is the total energy of the anion in its equilibrium geometry E tot A is the total energy of the neutral molecule at the equilibrium anion geometry E tot A - is the total energy of the anion at the equilibrium neutral geometry. Figure 5: A diagram of potential energy surfaces of an anion R - and a neutral molecule R representing all diatomic and polyatomic molecules, although not all anions are necessarily energetically more stable. As can be seen in Figure 5, in addition to electronic effects, molecules also contain vibrational and rotational effects which can contribute to the overall molecular EA.
Equations 1, 3, and 4 can be corrected by adding the respective zero-point vibrational energies to the total energies of the neutral and anionic species. However, in most cases the vibrational energies are neglected, as they are usually very similar. The calculation of VAE as a function of electric field could also be an acceptable method of determining the EA of xylene.
However, this process would incorporate the addition of an electron into the equilibrium neutral geometry which, under the effect of an electric field may be unsuitable. Depending on the direction of the applied electric field the addition of an electron from the vacuum may be difficult. If added into the system from the anode side, then electron addition would be easy, whereas the opposite would be difficult due to like charges.
Therefore, the adiabatic EA calculation of the equilibrium structures in the absence of an electric field could possibly allow for a more accurate determination of the EA in the presence of an electric field, as the electron has already been added and the factor of the direction of the applied electric field would be eliminated. De Proft et al. They investigated EAs of neutral, closed-shell molecules according to three different calculation methods based on HOMO and LUMO orbital energies and compared their results to experimental results of electron transmission spectroscopy ETS.
One of their methods which they state as "eqn 6" in Table 1 is based on equation 3 above. The other two methods stated as "eqn 9" and "eqn 11" are given in equations 5 and 6. Table 1 summarizes their results.
All experimental values were obtained from ETS measurements. All three of their methods were based upon the assumption that during anion formation, the extra electron enters the LUMO, which allows their results to be directly compared to ETS measurements. They came to the conclusion that the latter approach in combination with equation 6 yielded the best results.
Also, even though results were not given, they state that their methods would be satisfactory for molecules with positive EA values, yet less appropriate for negative EA values due to the fact that the anions are unstable and are strongly resistent towards the uptake of charge. It is interesting to note that from Table 1, the EA of m- and o-xylene were calculated and, as our chemical intuition suggested, revealed negative values.
However, I am interested to see whether the EA of xylene will change from a negative to a positive value under the effect of a strong electric field.
The computation of EA of atoms and molecules using ab initio methods is an active area of research, especially with respect to the choice of basis set and exchange-correlation functional used. Calculating EA presents challenges because " 1 the correlation energy of anions is usually large and requires sophisticated methods to reproduce accurately, and 2 diffuse functions must usually be included in the basis set to represent the charge density distribution of the anion correctly.
An ab initio method always uses a basis set, which usually consists of atom-centered Gaussian functions. A proper description of the electronic structure of a molecule under investigation requires basis sets with diffuse functions. Boesch et al. Both methods gave extremely accurate geometeries for the different benzoquinone derivatives and there was excellent agreement between calculated EAs and experimental values.
Table 2: Calculated and experimental adiabatic EAs for an array of different methylated and halogenated p-benzoquinones using the B3LYP functional along with the G 3d,p basis set. Takahata et al. A are the recommended approaches. A summary of their results can be seen in Table 3. It can also be seen that the calculated VDE values agree better to the experimental adiabatic EA results than the calculated adiabatic EAs, yet the group fails to determine why this could be.