CHEMISTRY 163, SPRING, 2002

GRADED EXERCISE 1

Your written work is due in class on Thursday, 18 April. You may consult your notes and the text but may not seek assistance from anyone but the instructor.

1) Using the C 2pz and O 2pz atomic orbitals (i.e. those valence orbitals perpendicular to the plane established by the heavy-atom framework) and the method of Hückel, construct the p molecular orbitals of the species whose structure is given below. Also perform the following calculations:

a) Obtain the energy eigenvalues in terms of a and b , the bond orders, and the charge distribution of your species.

b) Using the Cambridge Crystallographic Database, locate the crystal structure of your compound or the crystal structure as closely related as possible to your compound. Extract the bond lengths and bond angles. Discuss the results of your calculations in light of the crystal data.

c) Draw the energy diagram and label each energy level.

d) Add the number of p electrons for a neutral species to the diagram and determine the term symbol for the ground electronic state.

d) Determine the delocalization (resonance) energy of your species. Locate an experimental value of the standard enthalpy of hydrogenation for your compound. Estimate using model compounds this quantity in the absence of delocalization. By comparing these two values, generate an experimental value for the delocalization energy. Compare the experimental result with the result from your calculation.

e) Determine if the HOMO® LUMO electronic transition is allowed.

2) Following the example presented in class, perform a symmetry analysis of the s and p molecular orbitals of the ABn species provided below. Use only valence shell electrons, e.g. 3s, 3p, and 3d for Cl.

Using SpartanPro, perform a complete ab initio calculation on your species. A demonstration on the use of Spartan will be provided. You will want to use a feature of the program that minimizes the energy by varying the structure. Use 6-31G* basis functions. Compare the quantitative results generated by the program with the results of the symmetry analysis. Viewing of the molecular orbitals will probably be instructive.

Also, compare the structure calculation by Spartan with that derived experimentally. Structures derived from microwave spectroscopy are generally the best.

ClF5 (C4v)

exer1_2002.htm

WES, 25 Feb. 2002