Sixth Exercise

Ab Initio Calculations

Do either Problem #1 or Problem #2: your choice. The NIST WebBook is one possible source for the experimental geometries and enthalpies of formation; a Scifinder search for the individual compounds is another. Note that there is no easy way to obtain enthalpy of formation from an ab initio calculation. The total energy in the output is NOT an enthalpy of formation. (You would have to calculate total energies of the elements composing the molecule, correct everything to 25o, and then calculate enthalpy change for forming the molecule from the relevant numbers of atoms. This is more work than the information is worth.)

  1. Select one of the small molecules listed below. Carry out full geometry optimizations using the the 3-21G(*), 6-31G*, and 6-31+G* ab initio basis sets. Be sure to use an appropriate symmetry for each calculation.

    Construct a Table comparing such things as: major structural parameters, total energies, and HOMO and LUMO energies as computed with each basis set. Include experimental numbers, when available (where did you get them?), and comment on which model provides the best approximation to reality. Try to relate the changes to the changes in model provided by each basis set.

    CH2=CH2 CH3NH2
    CH3OH CH3SH
    CH3PH2

  2. Use 3-21G(*), 6-31G*,, and 6-31+G* ab initio calculations to compare the relative stabilities and the major geometrical parameters within one of the following isomeric series. Tabulate your results, including experimental numbers where available. Try to relate the changes to the changes in model provided by each basis set.

    Series I: 1,1-dichloroethylene; cis-1,2-dichloroethylene; trans-1,2-dichloroethylene

    OR

    Series II: propyne (methylacetylene), allene (1,2-propadiene); cyclopropene

E-mail your tables from the exercise you chose to me. I will post summaries to our Web page. What we are trying to do here is to get a sense of how accurate the various methodologies are, and the relative times required to do each type of calculation. (The time used is printed in each output file.)

This page last modified 10:32 AM on Thursday October 21st, 2010.
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