Computational Chemistry at GVSU

Offweek Exercise: Singlet vs. Triplet Energies for O₂

1. Figure 14.29 in your text book (p. 401, Reference 3) gives the molecular orbitals of O₂. Observe there is one unpaired electron in each of the two 2pi*. This is the experimentally observed electronic configuration of O₂. Could this be a singlet, a doublet, or a triplet? (Check all that apply.)

   Singlet _____         Doublet _____          Triplet _____

The other choice is to pair the two electrons in one 2pi* orbital. Is this a singlet, doublet, or triplet state?

Singlet _____         Doublet _____          Triplet _____

2. Using Gaussian B3LYP/6-31G(d), determine whether the singlet or the triplet state of O₂ is lower in energy (do the singlet energy first). To do this, set the spin multiplicity to the desired value, run the calculation, and find the total energy. Record the energy (w/ units) below.
   

Singlet Energy ________________________             Triplet Energy _________________________

Which is more stable? Do your computational results agree with experiment, which shows O₂ is a triplet?

3. The electrons in O₂ could be thought of as being confined in space to the area near the two nuclei. To a first approximation, this is sort of like a particle in a box. Using this analogy, examine each valence sigma orbital of the triplet O₂ as they increase in energy. Do they follow a pattern similar to the wavefunction (i.e., one electron orbital) found for the particle in a box? Describe.
   
   
   
   
   
   
   
   
4. Why aren't the O₂ molecular orbitals exactly like a 3-d particle in a box? Is the potential energy surface the same?