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 plot will appear and the results
are also in the GEO file.

There is one large peak which is observed at -120° where the Br atoms eclipse one another and two smaller peaks when Br eclipses H.

Bond Rotation Profile with Relaxation (partial optimization)
Using Firefly 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 Firefly 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.

While the overall shape of the profile is similar to the Mopac run above the height of the height of the Br..Br interaction is much reduced by the relaxation.

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