Chemistry 3730 Fall 2000 Quiz 5

Name:

1.

In this question, you will study the effect of the excitation energy in the computation of electronic spectra. Compute the electronic spectrum of fluorobenzene ( C₆H₅F) from ab initio calculations with the 3-21G basis set for several different rationally chosen excitation energies. Start with the smallest sensible excitation energy and work your way up from there. After each calculation, record the positions and intensities of all lines occuring at wavelengths of 140nm or more. Does the excitation energy affect the predicted positions or intensities much? Based on your experience with this set of calculations, suggest a strategy for reliably predicting UV-visible spectra. [20 marks]

2.

Calculate the dissociation energy of N₂. Describe your calculation in detail. You will be marked on the correctness of your results, on your use of appropriate and reasonably accurate computational methods, and on your ability to describe these procedures in sufficient detail to allow the replication of your calculations. Note that you don't have to give a click-by-click description of your HyperChem session: You may assume that the reader knows how to use HyperChem (or a similar program). [10 marks]

Hint: Think about the multiplicity carefully!

The ghost-atom procedure: Select the atom you want to turn into a ghost atom. Choose Name Selection from the Select menu. Name the selected atom ``ghost-atoms''. Unselect the atom. Enable ghost atoms in the advanced options dialog of the ab initio setup window. Verify that the charge and multiplicity are correct in the options dialog.