Bond Rotation Energy Profile of 1,2-Dibromoethane
If a dihedral is marked/selected in Build Mode and the bond is not in a ring then the energy profile observed on rotating one end of the bond may be calculated using Tinker, Mopac or Gamess.
Load 1,2-dibromoethane from the library.
In Build Mode click MarkBond on the
menu and then use 
to select the atoms along the bonds in the order Br-C-C-Br and then click 
,
The C-C bond should now be blue if the default colour map is in use.
Exit Build
and go to the Mopac menu. Click RunMopac and OK to optimize the
structure. Click NewInput and
on dialog 10 check Bond Rot.Profile, Energy and click Run Mopac. The program will calculate the energy at each step as the rotation takes place when finished the results are in the GEO file. These may be plotted using Excel. To plot the results click the right mouse button to go back to display mode and click Edit.GEO.
Select the columns high lighted and then Edit/copy. Start Excel and in Excel click Edit/paste, then click Data, click Text to columns, click Fixed width then Finish. Select all three columns and click Data and Sort. The default of Column A ascending is fine. Select the first two columns click the plot button and select XY (Scatter).
Click Finish
There is one large peak which is observed at 0° where the Br atoms eclipse one another.
Bond Rotation Profile with Relaxation (partial
optimization)
Using GAMESS it is possible to allow the structure to
relax at each position along the profile. This is sometimes called partial
optimization. To do this select ZMAT and Bond Rot. Profile on the
GAMESS dialog. The optimization is performed in internal coordinates, more
details are given in the Moilin help. The optimization may fail in more complex
cases if you crash groups into one another and make the optimization gradient
too steep. To avoid this reduce the step size to 5 deg. The default step size can be changed on Setup.
A partial optimization run output is like the following;
While the overall shape of the profile is similar the height of the height of
the Br..Br interaction is much reduced by the relaxation.
