In this exercise the semi-empirical method AM1 will be used to determine the energies of the molecular orbitals on borane, BH₃, and ammonia, NH₃, and to construct graphical representatins of the orbitals. Double click the Chemistry Software folder and then the Spartan ’10 icon..

Procedure:

1. Use Spartan '10 to build BH₃. Select the semi-empirical method AM1 and optimize the geometry. Print the molecular orbital coefficients, orbital energies, and electrostatic and atomic charges to the output file. Construct graphical representations of the molecular orbitals. Map the electostatic potential on an isodensity surface for the molecule. See the Tutorial for Molecular Orbitals on Borane for help.


Questions:


1. How do the molecular orbitals that you constructed with the aid of group theory (Problem Set I handout, problem 1) compare with the orbitals generated by the AM1 method? Are your e' orbitals and the e' orbitals generated by the AM1 method linear combinations of the same atomic orbitals? Are the orbitals labeled a₂" the same?
2. Use the orbital energies in the output file to construct an orbital energy level diagram. The most stable orbital is the one with the largest negative number for its energy. Is this diagram identical to the energy level diagram that you drew for problem 1 in the Problem Set I handout?
3. Which atoms in the BH₃ molecule are the most electon-rich?




2. Use Spartan '10 to build NH₃. Select the semi-empirical method AM1 and optimize the geometry. Print the molecular orbital coefficients, orbital energies, and electrostatic and atomic charges to the output file. Construct graphical representations of the molecular orbitals. Map the electostatic potential on an isodensity surface for the molecule.


Questions:


1. How do the molecular orbitals that you constructed with the aid of group theory (Problem Set I handout, problem 2) compare with the orbitals generated by the AM1 method? Are your e orbitals and the e orbitals generated by the AM1 method linear combinations of the same atomic orbitals? Are the various orbitals labeled a₁" the same?
2. Use the orbital energies in the output file to construct an orbital energy level diagram. The most stable orbital is the one with the largest negative number for its energy. Is this diagram identical to the energy level diagram that you drew for problem 2 in the Problem Set I handout?
3. Which atoms in the NH₃ molecule are the most electron-rich?