MOMEC force field

It also emerges that parameter schemes based on thermodynamic (heats of formation, isomer distributions), spectroscopic (vibrational force constants) or structural experimental data are different and primarily useful for modeling the corresponding properties. The MOMEC force field is largely based on structural data. 

Open the menu item Edit/View, there Force Field and there Bond Stretch Parameters. Move the cursor to the row C03-NT, click on this line, click on the button Delete, confirm and leave the Force field editor with OK. Now, your MOMEC force field does not have any parameters for Com-amine bonds you are ready to develop your own parameterization Actually, see what happens if you try to refine a molecule with a missing parameter Open the [Co(NH3)6]3+ - hin file from Section 17.1 (Setup/Files) or from the CSD and refine it (set the maximum number of cycles (Setup/Optimization Controls) to 2 Execute/Geometry Optimization). At the bottom of the Summary window the Force Field Messages will tell you that there is no bond stretch function for atom types C03 NT. 

Use the structure of ofe3-[Co(en)3]3+ from Section 17.4 as a starting structure to build oh3-[Co(tmen)3]3+, and refine it with the original MOMEC force field (Setup Files/Force Field/c momec97 parm). 

Measure the trigonal twist angle of the four conformers of [Co(tmen)3]3+ (see Section 17.7 for the definition of and the method used to measure it with MOMEC). Check the accuracy of the MOMEC force field and those that you have developed in the last lessons by comparing the computed trigonal twist angle (j> with that obtained experimentally for o63-[Co(tmen)3]3+ (43.9°)[56J. 

Note that the isomer ratio computed using the MOMEC force field is different from that reported in Section 8.1. This difference is due to the fact that here we only consider the most stable conformer of each of the three isomers. The neglected conformers also contribute to the isomer abundance and, due to some relatively low energy conformers in the case of the u-fac isomer, this leads to the observed differences (see discussion above). Thus, for a more accurate computation of isomer distributions, all conformations need to be considered. 

Note that a combination of various types of potentials can be activated. For example, the coordination geometry for octahedral transition metal compounds can be modeled by 1,3-non-bonded interactions in combination with a multiple harmonic function. This is the approach used in the Momec force field for a number of metal... 

It also emerges that parameter schemes based on thermodynamic (heats of formation, isomer distributions), spectroscopic (vibrational force constants), or structural experimental data are different and primarily usefiil for modeling the corresponding properties. The Momec force field is largely based on structural data. However, some of the parameters have also been refined against thermodynamic and/or spectroscopic data. Thus, the hexaaminecobalt(III) force field leads to accurate structural and thermodynamic predictions. Since all parameters are highly correlated (i.e., the parameterization ofthe ligands and that of the chromophore), the prediction of thermodynamic properties of complexes for other metal centers is also expected to be reasonably accurate. However, this may have to be tested separately for each set of compounds. 

In terms of ofo3-[Co(tmen)3], there is some improvement However, the resulting Co—N distances are still too short, and the overall agreement for all five structures considered is worse. It emerges that the original Momec force field leads to the best overall agreement, but it has some deficiencies for compounds with very long Co—N... 

Construct and refine the other three conformers of [Co(tmen)3] + and refine them with the original Momec force field. Check the results, in particular the Co—N distances and the strain energies. Compute the conformer distribution and compare it with that of [Co(en)3] (see Section 17.5). 

Refine the structures (except that of the D3lel3 conformation) with the Momec force field. ChecJc your results against those given in Tables 17.13 and 17.14 with respect to the torsion angles and the strain energies. 

You have built and refined a number of hexaaminecobalt(lll) compounds in earlier lessons. These include [Co(NH3)6] (Section 17.3), the four conformers of [Co(en)3] (Section 17.5), the three conformers of [Co(traws-diammac)] (Section 17.12), the two isomers of [Co(trap)2] (Section 17.12), the four conformers of [Co(tmen)3] (Section 17.13), and five conformers of [Co(sar)] (Section 17.17). Here, we will calculate the reduction potentials of these six compounds we will consider all possible isomers, but only the most favorable conformer of each. Make sure that all have been fully refined with the original Momec force field. If you are not sure about that, refine the seven structures (19, if you include all conformers) again. Record all of the strain energies in a table. Check and record also the average Co—N bond distances for each compound. Try to list the compounds in the order of increasing reduction potential (remember that ligand sets that enforce short bond distances stabilize the oxidized form). 

Note that there are two conformers, one with the two Hg atoms on the same side of the coordination plane (syn isomer) and the other with one Hg atom on each side of the coordination plane (anti isomer). Refine and save both using the Momec force field. Section 17.15 describes how to enforce planarity in a coordination compound. Two-dimensional NMR methods can be used to determine which isomer dominates - as long as interconversion of the isomers is not rapid on the NMR timescale. The data used here are hypothetical and we have assumed that one isomer dominates to the exclusion of the other and that there is no interconversion that is, the observed NMR spectrum is that of an isomerically pure compound. 

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