HyperChemistry
strength
Q1:
The Jahn-Teller effect
Octahedral transition metal complexes of the form MX6 are highly symmetric follow the Oh point group. The Jahn-Teller distortion occurs when two ligands that are trans to each other move either closer or farther away from the metal than the other four ligands. In both cases the symmetry is lowered to D4h so the energy of the d orbitals change. Assuming the change take place at the z-axis, elongation of the M-L bond lowers the energy of the dxy , dyz and the dz2 orbitals. Comparison of the M-L bond would lower the energy of the dxy and the dx2-y2 orbitals.
For the two octahedral complexes MnF63- ( charge -3 and multiplicity = 3) and NiF63- ( charge = -3 and multiplicity = 2) examine the Jahn-Teller effect upon distortion along the z-axis. In each case plot a graph between the axial M-L bond length and the binding energy and predict the optimum bond length. Compare your results with a full geometry optimization at the ZINDO/1 level of theory.
Examine orbital occupancy upon elongation and compression of the M-L bond.
You need to write a report explaining the effect a different situations.
Q2:
Steric Energies of Cubane
Recent synthetic advances in cubane chemistry have spurred research into applications of the box-shaped eight-carbon compound. Cubane itself is a strained compound. Nitrocubanes have been studied recently as potential fuels. You will compute and compare the standard enthalpies of formation and combustion of the compounds in the following table. From this data you will evaluate their potential as fuels.
n-Octane
cubane trinitrotoluene
Build and optimize using MM+. Compute standard heats of formations at the PM3 ( single point).
compound |
MM+ energy |
Hf ( Kcal/mol) |
Hcomb(Kcal/mol) |
n-octane |
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Cubane |
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Tetranitrocubane |
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Octanitrocubane |
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trinitrotoluene |
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Carbon dioxide |
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Water |
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Q3:
7-aminopyrazolopyrimidine (N7H) can exist in tautomeric equilibrium with an N8H form. In this exercise, a comparison of the tautomeric preference of the protonated structure relative to the neutral state will be carried out.
Build the N7H and N8H tautomers , optimize using MM and use AM1 single point record the heat of of formation of each. Save the structures.
Next determine the relative stability of the protonated tautomers by adding a proton to N1 or N3. Repeat the calculation .Tabulate your results as follows (all values are in kcal/mol)