Molecular Modeling of acid-base and other
properties. This exercise will provide direct experience
in using molecular modeling software to calculate wavefunctions for selected
molecules and ions to obtain and to interpret information from quantum
mechanical predictions of specific atomic properties to include bond distances,
geometries, partial charges on individual atoms. Spartan Student
Software is available from any network PC on the Winthrop campus; it can
be accessed via the
Start, All Programs, Chemistry and Biology,
and selecting the "
Spartan ST" program to run.
Spartan should also automatically start once you download one of the spartan
files that are linked below.
To use Spartan to conduct a quantum mechanical calculation,
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Click on the link for the molecule or ion in the table being examined and
save the file being used to your local drive. If Spartan does not
automatically open, start Spartan and load the saved file.
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Select Setup, Calculations from the Menu
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Select Hartree-Fock / 3-21G methodology (to be used
in determining wave functions)
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Select the Print box marked Orbitals and Charges
to ensure your job output has the required information for this exercise.
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Run the wavefunction calculation for the selected molecule or ion by selecting
Submit
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Electrostatic charges can be added by...
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Distances can be measured by....
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Bond angles can be measured by....
Exercise Requirements
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From quantum mechanical calculations for the conjugate bases of acetic
acid, chloroacetic acid, dichloroacetic acid, and trichloroacetic acid;
determine the relative basicity of these ions by comparing the electrostatic
charges on individual atoms among these ions. Clearly support your
answer. Then rank order the acids in terms of increasing acidity.
Arrange the four structures on the screen showing all charges, copy and
paste them on a separate page in the word document listing your answers
to the questions in this assignment..
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Draw the correct Lewis structures for acetic acid and the acetate ion;
properly label all formal charges. Identify the C-O bond orders predicted
for each of these from their Lewis structures (note: there are two different
CO bond orders in acetic acid).
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Using your quantum mechanical calculation results, compare the two carbon-oxygen
bond distances in acetic acid with those found in the acetate ion.
Explain what you found, clearly explain why this was seen, and indicate
whether the observed distances are comparable with what would be expected..
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For acetic acid, compare the hybridizations of each of the two carbons
from the Lewis structure.
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Identify all bond angles found around each of the carbons in acetic acid
in the quantum mechanical calculation. Relate to your predictions.
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Compare the carbon-carbon bond distances among the four acids being examined.
Provide an explanation for your obervation.