Exercise 4:
Calculate energy curves for substituted alkanes

In this final exercise of the first module, you can edit molecules and perform calculations on them. You can go directly to the molecule-edit-and-submit page from here, to add substituents to ethane and compute the energy curve upon rotation.

For more information on the bakground of these calculations see "What background is required to run these tutorials?" for some information on MOPAC input files, or check the text below.

The MOPAC input file for the rotation around the C-C bond in ethane is given below. Alternatively, you can download it for local editing by clicking here with the right mouse button. An explanation of MOPAC input files can be found here. Some specific information for this one is given below.

AM1 T=600 STEP=10.0 POINT=37
   rotation around c-c bond
   in ethane
  C    0.00000000  0    0.0000000  0    0.0000000  0    0    0    0 
  C    1.51738930  1    0.0000000  0    0.0000000  0    1    0    0
  H    1.11721396  1  107.8021607  1    0.0000000  0    2    1    0
  H    1.11721396  1  111.37226    1 -122.3562469  1    1    2    3
  H    1.11783779  1  107.4581078  1  115.7104335  1    1    2    3
  H    1.11692070  1  111.0770073  1    0.0000000 -1    1    2    3
  H    1.11712199  1  111.0150133  1    0.8782819  1    2    1    4 
  H    1.11712199  1  110.9935404  1  120.0975124  1    2    1    4

Obviously, the two rows starting with C denote the two carbon atoms in ethane. The first line after that denotes a hydrogen attached to the second carbon atom, as do the last two lines starting with a H. The fourth, fifth and sixth atom line are the hydrogens attached to the first carbon atom. These are also the ones that are rotated.

You are encouraged to submit some substituted alkanes to see what happens to the energy profile upon substitution. If something goes wrong, check this. If you don't know where to start, here's a question:

1. Submit a job that shows the rotation of 1,2-dichloroethane. Look at occurrences of energy maxima and minima.
2. Add an extra chlorine and look at the energies of 1,1,2-trichloroethane.
3. Next, change one of the chlorines that are on the same carbon into a bromine and look at the rotational barriers in 1-bromo-1,2-dichloroethane.
4. See also the following suggestions of substituents to add.