Unrelaxed scan of the dihedral angles of cis-1,2-dihaloethylenes.
Scanning a dihedral angle is a case where it is easier to switch to internal coordinates instead of Cartesian coordinates in the input. (Such internal coordinates are actually always used by ORCA during geometry optimizations and ORCA prints coordinates in both Cartesian coordinates and internal coordinates in a table, this is useful if you ever need internal coordinates and it's too difficult to set up manually).
The inputfile file for an unrelaxed scan for ethylene is shown below:
! HF-3c opt NOSOSCF
%paras
Alpha=0,180,18
end
* int 0 1
C 0 0 0 0 0 0
C 1 0 0 1.3385 0 0
H 1 2 0 1.07 120 0
H 1 2 3 1.07 120 180
H 2 1 3 1.07 120 {Alpha}
H 2 1 3 1.07 120 {Alpha+180}
*
It shows the molecule geometry defined in internal coordinates (bond lengths, angles and dihedral angles), also called a Z-matrix. See ORCA Manual, Chapter 4 for details on internal coordinates. The inputfile also makes use of the %paras block that defines a parameter Alpha that will be scanned from value 0 to 180 in 18 steps. The parameter Alpha and Alpha+180 is used to define the dihedral angle of 2 of the hydrogen atoms in the inputfile. Running this inputfile would twist the dihedral angle and swap position of 2 of the hydrogens.
1. Run the inputfile without changes. Open the outputfile in Chemcraft and confirm that the dihedral is varied as expected. Also note the table that you get at the bottom of the outputfile named "The Calculated Surface using the 'Actual Energy'" that contains the scanned parameters and energies that you can use to make your own plots (Excel, Matlab, R, etc.)
2. Create a new inputfile for cis-1,2-difluoroethylene, cis-1,2-dichloroethylene, cis-1,2-dibromoethylene and cis-1,2-diiodoethylene, using the inputfile above as template (all you need to do is swap the H for F/Cl/Br in the right places). Change the bond lengths of the carbon-halogen bonds to more sensible values (either look them up in a database or get them from separate geometry optimizations).
3. Run unrelaxed scans for cis -> trans transformations of cis-1,2-difluoroethylene, cis-1,2-dichloroethylene and cis-1,2-dibromoethylene.
4. Create a combined plot that shows how the relative energy changes in going from cis to trans for fluoro, chloro and bromo. Where is the highest barrier ? Why do you think that is?
5. Compare the unrelaxed scan for 1,2-difluoroethylene with both HF-3c method and the semiempirical PM3 method. The methods will give dramatically different total energies (due to the semiempirical method energy being defined differently) but is the relative energy landscape the same? Why do you think that is?
6. In this exercise you ran unrelaxed scans where you vary the relevant coordinates without the rest of the molecular coordinates changing. An alternative is to run a relaxed surface scan where the other coordinates are minimized during the scan. This will not be done in this exercise but what would you expect to happen? Would the barrier for cis->trans reaction get lower or higher in an relaxed scan?