I have one host molecule and one guest molecule. The host molecule contains Zn+2 interacting with four nitrogens that belong to the same host. Initially, negatively charged oxygens each from two counterions form coordination bonds, making the whole system take octahedral coordination geometry. Then the guest molecule with a negatively charged oxygen replaces one of the counterions. Using quantum calculations, is it possible to calculate the binding energy of the host and the guest in the given situation? If possible, what is the procedure to be followed and what would be the final equation for binding energy?
Thank you all in advance.
Mr. Kompella,
There is nothing unusual in your system. The computations are according to the well known equation computing energies of interacting complex, and both interacting species separately. The level of theory is governed by the metal ion.
Calculate the energies of the reactants and products and get the overall reaction energy. Be sure to include solvation. You can compare the energy against experimental values from Keq or Calorimetry experiments. If its in an aqueous solution, be sure to consider pH and protonation states of your reactants and products to more accurately calculate the overall reaction energy.
Thank you for the responses.
@Bojidarka B. Ivanova If I understand your answer correctly, the procedure is - calculate the total energies of the host+counterion complex ('Ecomplex' ) and the guest ('Eguest') separately first, then the total energy of the full system ('Efull'). The binding energy can be determined using
BE = EFull - (Ecomplex + Eguest)
Please note that the coordination number of Zn+2 in the full system is 6, whereas in the host+counterion complex it is 5. If my knowledge is to be believed, Zn+2 is stable with a coordination number of 4 or 6. I think the coordination number in all the calculations involving Zn+2 needs to be the same (6 in this problem). That's where I'm stuck.
In such a case, you could try to saturate your metal's coordination sphere with an additional explicitly modeled solvent molecule. Choose one that is appropriate for the reaction you are trying to model. Usually, coordinating solvents like THF, diethylether, dimethylether, or water are used.
Of course, you then have to incorporate the solvent on both sides of the reaction equation.
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