3 Answer any two questions in this section.

Each question is worth 10 marks.

1. Describe in detail the LCAO MO treatment of the bond in HCl. Develop the secular equation and explain how the expansion coefficients would be obtained.
2. Suppose that a fluorescent molecule with a singlet ground state G and a singlet excited state S is in a solution with a quencher Q. A quencher is a molecule to which excitation is efficiently transferred during collisions. The excited state is deexcited by the following processes:

   

   (The last step assumes that the quencher itself rapidly loses its excitation.) Let be the quantum yield for fluorescence in the absence of the quencher and be the quantum yield with the quencher. Derive the Stern-Volmer equation

   

   where is the fluorescence lifetime in the absence of quencher.

3. Discuss the major assumptions, strengths and weaknesses of Hückel molecular orbital theory.