Midterm Exam
Question 1. The rate of decay of the triplet state of benzophenone (kobs) was measured as a function of naphthalene concentration in benzene solvent. You are provided with a series of observed rates at various concentrations of naphthalene in a file. You can access the file below. Provide a plot of kobs versus naphthalene concentration, give the bimolecular rate constant for the reaction and give the lifetime of the benzophenone triplet under these conditions (in microseconds) in the absence of naphthalene. Compare the measured rate constant with that expected for a diffusion controlled reaction.
Rate Constant Data File.
Question 2. For the reaction scheme shown below, give the fluoresence quantum yield and the quantum yield for product formation for the values of the rate constants shown and a quencher concentration of .0002 M. What would the maximum quantum yield for product formation be at infinitely high quencher concentration
Question 3. Which of the reactions shown below would you expect to occur at the diffusion controlled rate? (hint, you will have to look up some excited singlet and triplet energies and oxidation and reduction potentials in a reference book. I suggest The Handbook of Photochemistry. There is a copy of this book in my lab! For the electron transfer reactions, ignore the coulomb terms when calculating the energies of the electron transfer products. In essence, this is saying, assume that you are in acetonitrile solvent).
Question 4. Given below are three files containing charge-transfer emission spectra for the electron donor/acceptor system hexamethylbenzene/tetracyanobenzene, measured in three solvents. These are carbon tetrachloride (ccl4.txt), chloroform (chcl3.txt) and trichloroethylene (tce.txt). Use the program SS5 to obtain the best fits to these spectra (absorption spectra are not provided in this case). SS5 also contains an option to calculate a rate constant for the (thermal) electron transfer reaction that corresponds to the emission process, in these cases a return electron transfer reaction with the contact radical-ion pair that is formed in the excitation process. This is shown on the "Display Spectra" page at the top right (the graph page that you see after hitting "D"). You will need to make sure that you select the "Charge Transfer Spectra" option when you start the program. Make sure that the matrix element parameter is set to 800 cm-1. Provide your best estimates of the excited state energy and solvent reorganization energy. When simulating the spectra, keep the lambda v and nuv fixed at 0.31 eV and 1400 cm-1, respectively, vary only the excitation energy and lambda s. The first column in each file in the energy in kK, and teh second is the intensity, normalized to unity.
CCl4 Spectrum Data.
CHCl3 Spectrum Data.
TCE Spectrum Data.
Question 5. The decay of benzophenone triplet was monitored in a transient absorption experiment in oxygen saturated acetonitrile solution. We have not yet discussed this in class, but oxygen reacts with triplet states. A file containing the time-resolved absorption as a function time data is given below. Provide a plot of absorbance versus time, show an exponential fit to the decay part of the data, and provide the value of the observed first-order rate constant, kobs, that best fits the data. The corresponding rate constant for decay of the triplet in normal aerated solution is 5.5 x 106 s-1, and in argon purged solution, it is 1.1 x 106 s-1. Provide a plot of kobs versus oxygen concentration and the bimolecular rate constant for reaction between benzophenone triplet state and oxygen. Compare this rate constant with the expected diffusion controlled value. (hint, you will need to know the concentration of oxygen in oxygen saturated acetionitrile. This is also given in the Handbook of Photochemistry, among other places.)
Bzp Triplet Decay Data.
Question 6. Give a reference to a paper that describes an example of the Marcus inverted region for an electron transfer reaction. (hint, you should probably use Sci-Finder for this. If you don't know how to use Sci-Finder, come and see me and I will help you.)
Question 7. Give the units of the matrix element that appears in the Golden Rule expression. Give the units of the FCWD in the Golden Rule expression. If the rate constant of an intersystem crossing process is 1.7 x 107 s-1, and the spin-orbit coupling matrix element is 500 cm-1, what is the value of the FCWD for the process?
None of these questions should take a long time to do. if you spend more than ca. 20 minutes on any question, please stop, and come and see me and I will help you.