Ultraviolet-Visible (UV-vis) Spectroscopy
Ultraviolet-Visible (UV-vis) Spectroscopy Practical Report Worksheet University of the Witwatersrand, Johannesburg JoVE Experiment Video Link: Ultraviolet-Visible (UV-vis) Spectroscopy | Protocol (jove.com) Student Number: Date: University of the Witwatersrand, Johannesburg School of CHEMISTRY SENATE PLAGIARISM POLICY Declaration by Students I ___________________________ (Student number: _________________) am a student registered for ______________________ in the year __________. I hereby declare the following: · I am aware that plagiarism (the use of someone else’s work without their permission and/or without acknowledging the original source) is wrong. · I confirm that ALL the work submitted for assessment for the above course is my own unaided work except where I have explicitly indicated otherwise. · I have followed the required conventions in referencing the thoughts and ideas of others. · I understand that the University of the Witwatersrand may take disciplinary action against me if there is a belief that this in not my own unaided work or that I have failed to acknowledge the source of the ideas or words in my writing. Signature: _________________________ Date: ________________________ QUESTIONS 2 Question 1 [5 Marks] Answer the following multiple-choice questions by circling or highlighting the correct answer. There is only one correct choice. You are advised to watch both the video and read through the notes for the practical. Question 1.1 What is required for a molecule to absorb a photon? (a) The photon’s energy must exactly match the molecule’s frontier MO gap. (b) The photon’s energy must be greater than the frontier MO gap. (c) The photon’s energy must be less than the frontier MO gap. (d) The photon and the molecule must be the same colour. (e) The photon’s energy must exactly match the energy difference between the excited state and the ground state for the transition of interest. Question 1.2 What is a monochromator? (a) A device that spatially focuses all wavelengths to a single point through an exit slit. (b) A lens that converts all wavelengths to a single colour. (c) A device that renders everything in greyscale. (d) A device that spatially separates wavelengths of light and directs a selected wavelength through to the sample being analysed. (e) A device that modulates the intensity of the light passing through the sample. Question 1.3 How can UV-vis spectroscopy be used to qualitatively identify a compound? (a) Measure the decrease in absorbance over time. (b) Make a calibration curve. (c) Measure the absorbance of a solvent blank. (d) Match the absorbance spectrum of the sample to published data. (e) Compare the UV-vis spectrum of a compound with its NMR spectrum. Question 1.4 How can you calculate the concentration of a solution with UV-vis spectroscopy if you do NOT know the molar absorption coefficient? (a) Add blue dye to the sample and then subtract the blank sample’s absorbance. (b) Use Beer’s Law by assigning the molar absorption coefficient a value of 1 ( = 1). (c) Create a calibration curve which is a plot of absorbance vs concentration. (d) Calculate the rate constant. (e) Divide the measured absorbance by the product of l . Question 1.5 Why is it important to wipe the cuvette before every measurement? (a) To make sure that the cuvette will reflect light. (b) To remove fingerprints or contaminants. (c) To cover the transparent sides in fingerprints and dust. (d) To keep the cuvette warm. (e) To charge the cuvette surface electrostatically for the measurement. Question 2 [23 Marks] In this section, questions relating to the theory discussed in the practical manual will be considered. It is advisable to read through the background theory several times until you have grasped the material. It is also important to study the figures very carefully as they contain much of the relevant information. Moreover, you may need to consult your course notes on molecular orbital theory as one goal of this exercise is to show you the link between MO theory and UV-vis spectroscopy. Question 2.1 Consider the energy of the * transition for molecular hydrogen (H2) shown in Scheme 2 (Page 3, Practical Manual). (a) Explain briefly why this energy is not equivalent to the FMO energy gap (ELUMO–HOMO). Your answer should reflect on the physical shape of the HOMO and LUMO for H2 to earn full credit. (5) (b) The far UV-absorption spectrum of H2 has a peak calculated to have an energy of 11.75 eV. Showing all working, first report the wavelength of this peak in nm and then calculate the frequency of the transition in Hz. (4) Question 2.2 DFT simulations indicate that ELUMO-HOMO for helium cation (He2+) is 7.70 eV. However, the DFT-calculated UV-visible spectrum shows an absorption peak at 10.276 eV for the first excited state. (a) Guided by Scheme 2 (Page 3, Practical Manual), use the provided template to sketch the relevant MO energy level diagrams and state energy level diagram for He2+ (helium cation). Include all electrons, energy quanta, and labels. Prove that the ground state for He2+ has a spin multiplicity = 2. (5) (b) Use the diagrams in Part (a) above and information in the Practical Manual to explain why the first excited state lies higher in energy than the energy gap predicted by ELUMO-HOMO for He2+. (3) (c) Use the information provided for He2+ to calculate the wavelength of the absorption peak in the UV-vis spectrum of this species. (2) (d) The DFT-calculated bond distance measures 0.7462 Å for H2 and 1.1377 Å for He2+. If the atomic radii for atomic H and He are 53 pm and 31 pm, respectively, explain why the bond length for He2+ is much longer than that for H2. Note: you should refer to the MO energy level diagrams for the two molecules to answer this question. (4) Question 3 [34 Marks] Question 3.1 Consider the UV absorption spectra of 1,4-diphenylbutadiene and trans-stilbene shown below. The spectra are plotted on the same vertical scale with the absorption intensity normalized (i.e., scaled in each case to extend from 0 to 1). (a) Assign the spectra to the compounds whose structures are indicated in the figure giving reasons for your assignments. (4) (b) Assign the peak in each spectrum that corresponds to the HOMOLUMO electronic transition. (2) (c) From the wavelengths of the lowest-energy peaks in the spectra for 1,4-diphenylbutadiene and trans-stilbene, quantify the spectral red shift in wavenumbers (cm–1) induced by the addition of one conjugated C=C bond to the delocalized structure of trans-stilbene. Is this consistent with what was said more generally in the JoVE video? Explain. (6) (d) Draw qualitative molecular orbital energy level diagrams for 1,4-diphenylbutadiene in which you depict two filled MOs and two unfilled MOs for both the ground state and the first excited state of the molecule. Note: none of the MOs are degenerate (i.e., have equivalent energy) and all are either or * in character. Make sure you label all MOs correctly (e.g., HOMO, HOMO-1, etc.) in the diagram. Use an appropriate arrow to indicate the lowest-energy electronic transition for the molecule. Hint: look at Figure 9 in the Practical Manual to see how this is done. (5) (e) Consider the figure shown above. The experimental UV spectrum of 1,4-diphenylbutadiene (red) is plotted on the same axes as the DFT-calculated spectrum (blue). The DFT calculations[footnoteRef:2] predict that the first excited state of 1,4-diphenylbutadiene occurs at 3.59 eV (345 nm). Construct a state energy level diagram for 1,4-diphenylbutadiene in which you include the information above and briefly explain what the diagram shows and why it is different from an MO energy level diagram. Note: The first excited state for the molecule is described by the wavefunction 11; the ground state will have the wavefunction 10. You should consult Figure 10 (Practical Manual) and, in a similar fashion, include at least five vibrational levels (v0, v1, v2, v3, and v4) for the excited state in your energy level diagram below. [2: Gas phase, HSEH1BPE/6-311G(2d,p) level of theory; peak width = 1500 cm–1 (fwhm).] (5) (f) From your state energy level scheme for 1,4-diphenylbutadiene in Part (e), comment on what the DFT calculations did not take into account. (4) (g) Calculate the quantum of energy (in units of cm–1) between the vibrational levels v0 and v1 for the first excited electronic state of 1,4-diphenylbutadiene using the experimental spectrum of the compound and the state energy level scheme you have constructed above in Part (e). Show all working. (3) 1. Assign the peaks in the spectrum of 1,4-diphenylbutadiene which occur at 330, 316, 303, and 290 nm. Use notation such as: 10 (v0) 11 (v0), 347 nm.[footnoteRef:3] Regarding vibrational levels of an excited state, these are typically more closely spaced in energy as the vibrational level number increases. Does the quantum of energy between the vibrational levels of the first excited electronic state of 1,4-diphenylbutadiene decrease with increasing level number? Explain why we may not get the expected energy sequence. [3: Note: The term symbols or symmetry labels of the ground and first excited states are 1Ag and 1Bu, respectively, for the molecule with D2h molecular symmetry. So, the transition can be written thus: 1Ag 1Bu. Revisit this when you do Chemistry Honours and check that you can derive these symmetry labels yourself.] (5) EXERCISES In this section of the assignment, which comprises several exercises, you will gain experience at plotting UV-vis spectral data and assigning peaks to transitions in the spectrum. Exercise E1 [15 Marks] Download the Excel spreadsheet called Tris(2,2'-bipyridyl)ruthenium(II) spectra.xlsx from Canvas/Ulwazi. Make use of Excel, or Google Sheets, or the free graphing App called SciDavis (best choice) to plot the electronic absorption and emission spectrum for [Ru(bipy)3]Cl2 on the same axes so that the Stokes shift of the emission spectrum is easy to see and quantify. You will need to produce a spectrum that looks something like the following. [Delete the image below and insert your own. Note: you should add wavelength labels for the key peaks (a–h) in the absorption and emission spectra in your plot. Yes–the “bumps” at b, c, d, and e are due to electronic transitions and must be included!] Figure E1. Write a suitable figure caption here. [Include the following text in your caption: “The absorption and emission spectra were recorded at 298 K in water.”] Delete the figure shown above, replace it with your own, and write your figure caption. Then delete these instructions. Exercise E2 [10 Marks] The transitions labelled “b” and “c” in Figure E.1 are called vibronic transitions, i.e., they are transitions to excited vibrational levels (v1, v2, v3, etc.) of the first excited electronic state. Construct a state energy level scheme by hand or with PowerPoint/Google Slides to explain the experimental absorption spectrum in this region (380–500 nm) for [Ru(bipy)3]Cl2. Assign the transitions “b” and “c” in the spectrum