5.05 Assignment for Thursday

This assignment has multiple components in addition to the assigned readings.
The purpose of this assignment is to compare the bonding in ethane to that in isoelectronic ammonia-borane from the point of view of the electron density and its topology.

C2H6 versus H3NBH3

  1. Use Gaussian to optimize the structure of ethane in D3d symmetry and also the structure of ammonia-borane in C3v symmetry.
  2. Use the optimized structures to generate corresponding *.wfn files.
  3. Locate C-C and C-H as well as B-N, B-H, and N-H critical points using the ext94b routine.
  4. Print out a sketch of the optimized structures and on these write the values of ρb and ∇2ρ near each respective critical point.
  5. Now, use the gridv and contorpg routines to generate electron density plots for the two molecules; the plots should contain H-C-C and H-N-B planes.
    Also, use 5 contours from 0.04 minimum to 0.2 maximum.
    Use of these routines is described in the Spinney tutorial as well as on Bader's AIM page.
  6. Print out the plots so you can hand them in.
    On the printouts, mark the location of the bond critical points with solid circles.
    Write on the printout what you notice is different about the electron density distribution for ethane as comparted with ammonia-borane.
    Which of the bond critical points are not located near a bond midpoint?
  7. Now, use the gridv and contorpg routines to produce plots of ∇2ρ for both molecules.
    Print these out and write down your observations on how they contrast with one another.
    Use the guidelines of Popelier to classify the C-C and B-N bond types, and write on the printouts the summary of your classification to be handed in.
  8. Finally, calculate the atomic charges on C, B, and N by integrating the electron density over the entire basin volume for the atom in question.
    This is done using the proaimv routine executed by typing "proaimv filename filename" where the two required files are a 4-line *.inp file and the *.wfn file.
    As an example of a minimal *.inp file you can download one by clicking here.
    You should also read more about proaimv on Bader's AIM pages as linked into our class home page.
    Write out the calculated charges for the C, B, and N atoms on a sketch of the structures.
    Write out a discussion of how these charges compare to the formal charges one would write down for these systems, as well as to what your intuition is for these molecules.
    Print out the three *.int files generated by the proaimv runs and hand these in as well.

What to hand in when you come to class:

All the printouts you were instructed to generate as well as the comments, discussions, and markups you were asked to make on the printouts.