© 1999 Rainer Glaser. All rights reserved.
The University of Missouri at Columbia
Computational Organic Chemistry, FS99
Instructions Organizing and Topic Assignments
of the Specific Exercises Related to
Theoretical Level Dependency
"Theoretical Level Dependency" always is a central issue in any
theoretical discussion. One needs to know how well a given
model performs. The theoretical level dependency of different properties
can be quite different. Many methods give good geometries, for example,
while there are fewer methods that provide accurate dipole moments.
So, we will take a look at "Theoretical Level Dependency" for a
few cases that exemplify selected issues well. A few topics covered in
the past ar available for study are listed below. A topic can be
claimed either by an individual student or by a group of students.
If you have a preference for a certain topic, do let me know as soon as
Here is what you should do. Carry out the computations using Gaussian94
on Shiva. Tabulate your results so that the parameters show up on the
horizontal and the theoretical levels on the vertical. We will be adding
data obtained at many theoretical levels as we go along in the course.
Eventually, we will produce html files of your tables and post them on the
web. You can include as many parameters in the tables as you like. Start
with the key parameter I am asking for (e.g. the dipole moment of CO, the
rotational barrier in formamide, ...) and then add parameters that you
think are of interest as well. Feel free to talk to me if you need some
feedback. Of course, it would not hurt to run a CAS search on your topic
and learn what others have thought about the issue before.
I will update this file as we go along specifying additional levels at
which you should examine the problem. Of course, do not feel limited by
my requests! You can do as many theoretical levels as you like (just do
not crash the system).
Update 10/11/99: Carry out calculations at the levels RHF/STO-3G,
RHF/3-21G, RHF/6-31G, RHF/6-31G* and RHF/6-31G**. In all cases, optimize
the structures using the keyword opt=z-matrix. In addition, you should use
the keyword GFP (stands for Gaussian Function
Printout) in these calculations so that you get some idea about the
magnitudes of the exponents. From now on, also please keep track of the
time used for the calculation. Record the computation time given at the
bottom of the output file.
Possible Topics and Assignments
Acetonitrile and its isomer, MeCN and MeNC.
These highly polar molecules are very different bonding situations. It
will be of interest to see how well theory can reproduce the isomer
preference energy. Keep an eye also on the lengths of the multiple bond
and the dipole moments.
The rotational barrier in formamide.
A classical case. The barrier that governs the conformations of peptides.
You will need to compute the equilibrium structure (which may not be
planar at the amino-group) and two transition
state structures (the ones with a pyramidal NH2 group that have
the N-lone pair either syn or anti with the C=O bond)
at each level.
Comparison of 1,3-pentadiene and 1,4-pentadiene.
One is conjugated and one is not. Does the theoretical model account for
this difference well?
Activation barrier for nitrogen inversion in ammonia..
This assignment requires the computation of ammonia in C3v and
D3h symmetry at every level.
Activation barrier for H-abstraction reaction from methane by fluoride
This assignment requires the computation of methane in Td, of
fluoride ion, and of the transition state structure in
C3v symmetry at every level.