Vibrational Structure on an I2 Electronic Transition
Author: J. M. McCormick
Last Update: July 15, 2009
This exercise is identical to the absorption experiment found in the provided reference,1 but you will not be measuring the emission spectrum. You are to attempt some sort of theoretical calculation (examples are described in reference 1) as part of your data analysis, but that is left up to you. Note that, because of the number of electrons involved, any Gaussian calculation with I2 will take some time and that you will probably have the best results if you use DFT (e. g., B3LYP with the accurate basis set). A number of references are provided2-7 for background information, additional details on how to work up the data, and other calculations that you can perform.
You will be using an Ocean Optics HR4000 high resolution spectrometer (resolution is approximately ±0.05 nm between 449 and 657 nm) and a 10-cm pathlength cell to obtain the absorbance spectrum of I2 between 449 and 657 nm. The cell will have already been charged with I2 solid and should contain sufficient I2 vapor for our purposes. The amount of I2 vapor present can be increased by gently warming the cell to about 40 °C.
Only one laptop is configured to operate the HR4000, and it is marked as such. The USB port (on the back of this computer) to use is indicated. The HR4000 is configured in the same way as the three-piece USB2000 spectrometers, except that actual components are different. Beside the HR4000 the system includes a CUV-VAR variable pathlength cell holder and a Mikropack DH2000 high-intensity deuterium-halogen light source. Do not turn on the DH2000 until all the fiber optic connections have been made (note that the fiber optic port on the DH2000 is behind a metal door on the front of the instrument). Once all of the connections have been made and the DH2000 has been plugged in, turn on the DH2000 (on the back) and then push the blue (deuterium) and red (halogen) lamp switches. The LED below the deuterium lamp switch will flash green and then change to a steady green. The LED below the halogen lamp switch should be a steady green. If either LED turns red, or continuously flashes, then the bulb has burned out, or there is some other problem with the system. In this event, notify the instructor. Allow the source to warm up for 10 to 15 minutes after the lamps have been switched on. To open the shutter (and allow light out of the source), flip the toggle switch on the front of the DH2000 to “Open”.
Start Ocean Optics SpectaSuite software. The instrument will start actively acquiring data in scope mode, and the screen shown in Fig. 1, or something similar will appear (note that the intensity may be off-scale initially). Set the integration time so that maximum intensity is about 15000 counts (~27 msec), as shown in Fig. 1. If you don’t see a signal like this, verify that light is passing from the DH2000 to the HR4000 by blocking the beam with a piece of paper (the intensity should drop to near 0 and you should see the beam on the paper).
Figure 1. Starting screen for the Ocean Optics SpectraSuite software when interfaced to the HR4000 spectrometer.
Set the dark current by blocking the beam at the end of sample holder nearest fiber optic cable to the detector. Do not touch the lens with your finger or anything wet or rough! Click on the black light bulb while the beam is blocked. Set the light current by removing the beam stop and clicking on the yellow light bulb light when the light path is clear. Once you have set the light and dark current do not change the integration time!
Enter absorbance mode (click on the letter A) and set the scale to fill window with the data region of interest. Enable stray light and electrical dark correction, if desired. Set the number of scans to average so as to minimize the noise and click on the button to start a single acquisition. Once the scan in completed, save the data (using the icon) as a tab-delimited text file for importing into Excel.
It is recommended that you immediately import the spectrum into Excel and print it out exactly as it is shown in reference 1. This will make assigning the peaks must easier. Once the peaks are assigned, you can then use the cursor function in SpectraSuite to determine peak positions as described in reference 1. Note that your spectrum may be noisier than the one shown in reference 1, so you will need to interpolate through the noise when determining peak positions.
For the computational component you use either WebMO (for quantum mechanical calculations) or Mathematica (e. g., for calculation of Franck-Condon factors). Remember to describe the quantum mechanical calculations in the Experimental section of your report, while other calculations should be described in the Results section. If you use Mathematica, a copy of the calculations must be included in the Supplemental Materialsection.
Results and Analysis
Prepare the various graphs and tables and perform the calculations described in reference 1. Calculated values must be presented in a table along with with their uncertainties at 95% confidence. Discuss whether your results agree with those quoted in references 2 – 7.
1. Garland, C. W.; Nibler, J. W. and Shoemaker, D. P. Experiments in Physical Chemistry, 8thEd.; McGraw-Hill; New York: 2003, p. 436-446.
2. Stafford, F. E. J. Chem. Educ. 1962, 39, 626-629. Click here to view as a PDF file (Truman addresses and J. Chem. Educ. subscribers only).
3. Pursell, C. J. and Doezema, L. J. Chem. Educ. 1999, 76, 839-841. Click here to view as a PDF file (Truman addresses and J. Chem. Educ. subscribers only).
4. Long, G.; Sauder, D.; Shaloub, G. M.; Stout, R.; Hamby-Towns, M. and Zielinski, T. J. J. Chem. Educ. 1999, 76, 841-847. (Click here to view as a PDF file (Truman addresses and J. Chem. Educ. subscribers only).
5. Snadden, R. B. J. Chem. Educ. 1987, 64, 919-921. Click here to view as a PDF file Truman addresses and J. Chem. Educ. subscribers only.
6. McNaught, I. J. J. Chem. Educ. 1980, 57, 101-105. Click here to view as a PDF file (Truman addresses and J. Chem. Educ. subscribers only).
7. Steinfeld; J. I.; Zare; R. N.; Jones; L.; Lesk, M. and Klemperer, W. J. Chem. Phys. 1965, 42, 25-33. Click here to view as a PDF file (Truman addresses and J. Phys. Chem. subscribers only).