In this experiment, we will calculate the potential energy surface, as a function of internuclear distance, for the ground state of I2. At the internuclear distance associated with the minimum potential energy, we will determine the vibrational frequency.
We would also like to compute the potential energy surface of the relevant excited state of I2, and compare the results to your experimental results. Modeling of excited states is still an inexact science in computational chemistry, and one must proceed with caution. Electron correlation effects are often very important in shifting from the ground to the excited state, and limited detailed experimental information on the nature of the excited state limits our ability to best refine our models. For a molecule containing high atomic number elements such as iodine, relativistic effects (including spin-orbit coupling) become important in correctly modeling both the ground and excited state. For these reasons, we will look at literature results from high level computations of the ground and excited states of I2, which include greater degrees of electron correlation and relativistic effects than we can do with our tools and time constraints in lab.
As a 5th row element, iodine has quite a few electrons, making it more computationally time-consuming to calculate its electronic structure than the halides F, Cl, Br. It is also much safer to work with in the bench laboratory than the lower atomic number halides, which is why this is the halide used in the "wet" portion of this lab. We will use Hartree-Fock and the LANL2DZ basis set to model the ground state of the I2 molecule, a crude approximation at best. But amazingly, many of the results aren't that bad…
1. Using HF/LANL2DZ, do a geometry optimization and frequency calculation for I2. Call the file i2gnd.
2. Record the internuclear distance, the vibrational frequency (use a scale factor of 0.9), and the total energy for the resulting structure.
3. Perform a scan of the potential surface. Set the file prefix to i2gndscan1 and scan the variable r2 (the interatomic distance) from 2.0 Å to 3.5 Å in 10 steps. Reset the file prefix to i2gndscan2, and scan r2 from 3.7 Å to 6.0 Å in 10 steps.
4. Combine your scan data and plot the potential energy surface as function of interatomic distance. Label the plot.
5. Compare your results to both the wet lab data and the literature computational chemistry study.