Charge Transfer Complexes
Aim to present final and key results of your study. Use concise language and ensure to communicate if your findings are consistent or inconsistent with the expected outcome.
INTRODUCTION
Provide a comprehensive introduction that contains a historic and theoretical perspective on the topic of charge transfer complexes and their roles in chemical reactions.
Describe the graphical method used by Benesi and Hildebrand and the fact that the concentration of free iodine should not be neglected (graphing according to EQ 10 in the lab manual).
Make sure that it is clear to the reader how the spectroscopically extracted data allows you to determine thermodynamic values of the charge complex.
EXPERIMENTAL
Outline the details of your experiment in the experimental section and note any observations that were made during this study in the results section. Note the apparatus used for any measurements.
RESULTS AND DISCUSSION
Analysis point 1 (show the overlay spectra at one temperature in the text, and the rest in the appendix). Table with all absorbance values of all the different solutions at different temperatures (as indicated in the data analysis section).
All the fits of data required to extract the values of K at different temperatures. Also, show your ANOVA output for the linear regression. Obtain ΔG values at different temperatures and graphically extract values of ΔH and ΔS. For all the graphs, indicate the results of the ANOVA output, and include error bars on graphs.
Use error propagation if needed to determine the uncertainty of the physical parameters that you have determined. Show all your calculations in the Appendix.
In the DISCUSSION section describe the validity of your collected and calculated data.
Compare them with the literature data. Comment on the magnitude of the enthalpy associated with the formation of the charge transfer complex compared to hydrogen bonding or chemical bond formation.
Discuss the sign of the entropy change – where your predictions right in the Pre–Lab question Q6?
PRELAB QUESTIONS – TO BE HANDED IN BEFORE THE LAB STARTS
1. [2] Find the density of dioxane and explain how you will prepare the stock solution of dioxane. Calculate both molar concentrations and mole fractions of dioxane and iodine in each of the solutions you will prepare for the charge transfer experiment.
2. [1] In this experiment, what species will absorb at the maximum absorption in the CT band? What is the expression describing total absorbance at that point?
3. [2] Which one, the donor or the acceptor is present in a much higher concentration in your experiment (consult Q1)? If one of them is present in large access, you can assume that their concentration won’t change too much. Starting from the equilibrium constant expression for the formation of the complex, and with the approximation mentioned, show that one can derive the Benesi–Hildebrand equation using absorbance data.
4. [1] You are going to use the Ketelaar equation for the fitting of your data. How does that relationship differ than the one derived by Benesi–Hildebrand? What is the first run in your experiment used for?
5. [2] The Figure below illustrates the appearance of CT (charge transfer) spectra of
tetracyanoethylene (TCE) and a variety of enol ether donors. The ionization potentials (IP) of the donors are given in the Figure. The maxima of the CT bands shift to longer wavelengths as the IP of the donor drops.
Give a brief written explanation or interpretation of this generalization. For example, draw energy level diagrams for the general CT absorption process and discuss the trend in maxima wavelength with IP of the donor.
6. [1] What signs do you expect for ΔH and ΔS in this experiment? Justify your answer.
7. [1] What assumption about the temperature dependence of the thermodynamics constants had to be made, in order to use equations 11–13 from the manual.
