Computational Chemistry at GVSU

F₂ Dissociation Energy

In our I₂ calculations, we found the bond distance and vibrational frequency for both the ground state and the excited state I₂ were predicted fairly well, but our dissociation energies were incorrect. As mentioned, I₂ has many electrons, and we were unable to use higher order methods, such as B3LYP, that include electron correlation. Let's treat the dissociation as a reaction. There are two possible reactions:

Reaction 1: F₂ –> F + F or Reaction 2: F₂ –> F⁺ + F-.

1. Using HF/6-31G(d),

1. calculate the bond distance, vibrational frequency, and total energy for F₂ (remember to do an AM1 calculation first to get to a good starting point). Record these below.

 

F₂ bond distance _______________________

 

F₂ vibrational frequency ________________________

 

F₂ total energy _______________________________

 

 

2. For F alone, calculate total energy. (Why don't we have to do an AM1 calculation first?)

F total energy _______________________________

 

3. For F⁺, calculate total energy

F⁺ total energy _______________________________

 

4. For F^-, calculate total energy.

3. Repeat the above using B3LYP/6-31G(d)

5. Compare your results to those given in Table 16.2 in your text, and comment on the usefulness of better methods in determining dissociation constants.