PES: Exercise 1

C₄H₆ isomers

In this exercise we will learn how to locate different isomers of a molecule on a potential energy surface via geometry optimization from different starting structures. We will characterize the structures by vibrational frequency calculations and try to understand the chemical implications of the different minima located.

Build up the molecule (use the molecular editor Chemcraft and then copy/paste the coordinates (Cartesian coordinates in Angstrom units) to the ORCA inputfile) for the three different isomers of butadiene shown below (trans/cis-butadiene and cyclo-butene). These isomers share the same molecular formula, C₄H₆ and thus share a common potential energy surface.

Figure 1. Isomers of C₄H₆.

1. Execute a combined geometry optimization (Opt keyword) and frequency job (Freq keyword) with the HF-3c method (HF-3c is a Hartree-Fock method with a minimal basis set and 3 correction terms). Try different starting geometries if you don’t find the correct conformer immediately (distort the molecule in Chemcraft). Look at the trajectories (open the .out file with Chemcraft).

2. Determine whether the obtained stationary points are local minima by checking the vibrational frequency output (a local minimum should have all frequencies positive). Mention this in report.

3. Compare the relative energies of the isomers that are confirmed to be local minima. Which one is the most stable one? Why is it more stable?

4. Does the inclusion of zero-point energy change the relative energies significantly? I.e. compare the relative energies for the 3 isomers once you have included the zero-point energy for each molecule.

5. Calculate the fractional population of each isomeric form using Boltzmann statistics. Will you necessarily observe the different isomers in this proportion in actual experiments?

6. Why does it not matter whether the starting structures prepared in Chemcraft, show double bonds or not?

Optional question:

7. Calculate the silicon analogues of the C₄H₆ : Si₄H₆ . Compare the relative energies of Si₄H₆ to C₄H₆.

Is the energy landscape fundamentally different or similar?