Chemistry 3730 Fall 2000 Assignment 6

Due: Friday, Dec. 1, 10:00a.m.

1.

In assignment 5, we calculated the dissociation energy of NaI in two different ways, first by fitting the vibrational energy levels, and then by a HyperChem calculation. The fitting procedure gave a value of $5.404\times\,10^{-19}\,\mathrm{J}$. This is only an approximate value since it uses equations from the Morse oscillator which only imperfectly describe the potential energy curves of real diatomic molecules. Nevertheless, it should give a reasonable approximation to the dissociation energy. On the other hand, the HyperChem SCF calculation gave a rather poor estimate of the dissociation energy.

Compute an accurate value for the dissociation energy of NaI by a HyperChem calculation. Explain your computational procedure in sufficient detail to allow replication of your results. Compare your value to that obtained by fitting the vibrational levels. [10 marks]

2.

(a)

Calculate the ground-state energy of ethane with and without the correlation energy using the 6-311G^* basis set. [5 marks]

HyperChem hint: The calculation goes much more quickly if you first optimize the geometry with smaller basis sets before moving on to the 6-311G^* level.

(b)

Find the torsional transition state. (This calculation takes a little while.) Do a single-point calculation at the transition state to get the energy with and without correlation. Calculate the torsional activation energy, again both with and without the correlation energy. Do correlation effects make any difference here? Why or why not? [10 marks]

3.

Calculate the electronic excitation spectrum of benzene using the 3-21G basis set. Explain your computational procedure in sufficient detail to allow its replication. Report the lowest-energy transition which has a nonzero intensity. Would you expect benzene to have an observable UV-visible spectrum using a standard spectrometer? [10 marks]

HyperChem hint: It is not feasible to include all possible excitations in this calculation. The best way to compute the spectrum in this case is to include only a band of orbitals near the HOMO. Look for gaps in the energies of the orbitals and set the CI excitation energy to a value which will allow excitations from occupied orbitals near the HOMO to unoccupied orbitals near the LUMO.