9.10.3: Part I - FTIR spectrum of a mixture of HCl and DCl gas

Note

The ELN for this entire experiment is here: ELN03_FTIR2022.docx

In preparation for this experiment, read your textbook (McQuarrie, D.A. and Simon, J.D. "Physical Chemistry: A Molecular Approach", University Science Books, CA, 1997, Sections 13.2-13.4, 18.4-18.5); Atkins, P., Physical Chemistry, 5th ed., sections 16.8 – 16.11; and, Silbey, R.J., Alberty, R. A. and Bawendi, M.G., Physical Chemistry, 4th ed., Sections 13.4, 13.6 and 13.7, for a discussion of rotation-vibration spectroscopy of diatomic molecules. Each pair of students may share some of the data analysis as described in Section 4.3.3.

Preparation of Gas Sample Cell

• A mixture of HCl and DCl will be introduced into an IR gas cell. Note that because natural chlorine is a mixture of two isotopes \( \ce{^{35}Cl} \) and \( \ce{^{37}Cl} \) (in a ratio of 3 to 1), this sample is a mixture of four different gases, each with a distinct spectrum.
• Hydrogen chloride and deuterium chloride are colorless, pungent, and corrosive gases that fume in air. Contact will cause burns to the skin, severe burns to the eyes and burns to the respiratory system if inhaled. A vacuum line will be used to transfer the gases from their high-pressure containers to an evacuated IR cell.
• When not in use the IR gas cell must be stored in the desiccator. The cell is constructed of a glass body with KBr windows at each end. Ensure that the salt windows are kept free of moisture at all times. Please do not touch these windows.

Vacuum-Line Start-Up

Most of the following will be completed by the TA before the lab period.

1. Check that all external valves on the working and main manifold are closed to the outside atmosphere.
2. Attach an empty N₂ trap to the main manifold. Hold this trap in place until a vacuum has formed.
3. Turn on the floor vacuum pump to start generating a vacuum.
4. Turn on all valves in the manifold that are connected to gages (marked with purple tape).
5. Turn on the diffusion pump.
   • Ensure that the floor pump is on BEFORE the diffusion pump. Otherwise, the diffusion pump will generate heat and the oil within the pump will oxidize, causing long-term damage to the diffusion pump.
6. Turn on Digivac electronic monitor. Use this to actively monitor the pressure at the end of the main manifold.
7. Check the analog pressure gauge on the diffusion pump to ensure the pressure is stable.
   • Throughout the experiment, the pressure should remain below 30 mtorr if the system is closed and stable.
8. Fill a 4L dewar with liquid N₂ from the shared Chemistry department supply.
9. Place a second dewar underneath the N₂ trap and fill with liquid N₂ from the 4L dewar such that the N₂ trap is submerged in liquid N₂
10. Check the level of N₂ as the experiment progresses. Refill if necessary to ensure the trap remains cold.
11. Check to ensure the pressure at the end of the main manifold is below 30 mtorr before proceeding.

Transferring HCl and DCl to gas cell

1. Remove gas cell from the desiccator and attach to the middlemost section of the working manifold shown below in Figure \(\PageIndex{1}\).

   

   Figure \(\PageIndex{1}\): Vacuum line set-up: working manifold and main manifold

2. Evacuate the back of the gas cell and introduce a vacuum to the interior of the cell.
3. Remove cell from manifold, bring it to the FTIR instrument, and take a background spectrum (see "Obtaining an FTIR Spectrum using the Nicolet iS50" below).
4. Replace the gas cell onto the working manifold and open it to vacuum.
5. Connect the pressurized HCl cannister to the working manifold via a vacuum-rated line. This line should contain its own needle valve that may be closed to separate the cannister from the rest of the working manifold. Introduce a vacuum to the line and to the pocket between this needle valve and the main valve on the cannister. The HCl cannister should remain closed at this time. Close the needle valve once this pocket has been evacuated.
6. Close the working manifold off from the main manifold.
7. Introduce a small amount of HCl gas to the pocket between the cannister and the needle valve by turning the cannister's main valve 180 degrees open briefly, and returning 180 degrees to close the cannister once more.
8. After ensuring the HCl cannister is closed, open the needle valve to release HCl into the working manifold. Use the mercury pressure gauge on the working manifold to monitor this process.
   • If the pressure rises such that it threatens to expel mercury from the pressure gauge, open the working manifold to the main manifold to release pressure.
9. Slowly release HCl from the working manifold using the valve to the main manifold until the pressure in the gas cell reaches 10 mmHg.
10. Close the gas cell off from the working manifold and open the working manifold to the main line to release any excess HCl from the rest of the system. (Remember to include the pocket between the needle valve and the HCl cannister!)
11. Repeat steps 5-10 using DCl gas to introduce an additional 10 mmHg DCl into the gas cell such that the final contents of the cell are 10 mmHg HCl and 10 mmHg DCl. (What pressure DCl needs to be introduced to the working manifold for this to happen? Hint: it's not 10 mmHg.)
12. Record HCl/DCl spectrum using "Obtaining an FTIR Spectrum using the Nicolet iS50" below.
13. Once all data has been collected, return the gas cell to the working manifold and safely evacuate all remaining HCl/DCl.
14. Close and remove the evacuated gas cell and return it to the desiccator.

Vacuum Line Shutdown

1. Turn off the diffusion pump. Wait 5 mins for the pump to cool.
   • The pump should be cool before any oxygen is introduced to the system to prevent oil in the pump from oxidizing.
2. Close all valves with purple tape.
3. Turn off Digivac monitor.
4. Turn off the floor pump.
5. Pick an external valve and open it to release the vacuum and open both the working manifold and the main manifold to the air.
   • Do this quickly after step 4. The system should not be left under vacuum without an active vacuum pump.
6. Remove N₂ trap and transfer it to the fume hood to safely evaporate excess HCl/DCl gas.
   1. Do this quickly after step 5. Leaving the N₂ trap cold after the vacuum has been released may cause liquid O₂ to condense, which is highly volatile.

Obtaining an FTIR Spectrum using the Nicolet iS50

You will use a Nicolet iS50 FT-IR Fourier Transform Infrared (FTIR) Spectrometer to collect FTIR spectra. Below are instructions for its use.

1. Check the sample holder in the spectrometer to make sure there is nothing blocking the path of the infrared beam. Close the cover of the instrument.
2. The Omnic software may already be loaded. If not, load it by clicking the icon on the Windows desktop. As the software loads you will a blue light appear on the right side of the instrument and the main window with the menu items and button bar.

   

   Figure \(\PageIndex{1}\): The Omnic Button Bar. (CC-NC-BY-DUKE CHEM)

   Note on Collecting Data

   This is a single beam instrument. You must first record a background interferogram of the evacuated gas cell. A fast Fourier transform (FFT) will then be performed to give background transmittance spectrum. You will then introduce your gas sample into the SAME gas cell and record a new interferogram. FFT analysis of this interferogram will give a single beam transmittance spectrum of your sample. This spectrum is then ratioed against the single beam background spectrum to give the transmittance spectrum of your sample. Finally the absorbance spectrum of your sample is calculated and displayed.

3. Open the sample compartment of the FTIR spectrometer and mount the evacuated gas cell on the gas cell holder. Close the cover and allow the sample compartment to be purged with dry, CO₂-free air for at 5 minutes, noting the time so that the purge time is reasonably accurate.
4. Check the Experiment Setup from the Collect menu. It should be set to run 8 scans at 0.125 Cm^-1 resolution. You should be working in Absorbance format and a background scan should be “good” for 200 minutes.

   

   Figure \(\PageIndex{2}\): The Collect Pane of the Experiment Setup Window. (CC-NC-BY-DUKE CHEM)

   Click on the Bench tab and wait for the interferogram to appear. The Max signal should be around 4.5. Click OK.

   

   Figure \(\PageIndex{3}\): The Bench Pane of the Experiment Setup Window. (CC-NC-BY-DUKE CHEM)

5. Select Collect Background from the Collect menu (or with the Col Bkg button). Click OK when you see the Confirmation box. It will take about 4 minutes for the background scan to complete. Part way through, the background scan will be displayed on the main software screen. Once complete, add the spectrum to the window when prompted. Note the presence of water vapor and CO₂ in the spectrum. This arises from the atmosphere surrounding the cell (Remember: you gas cell is evacuated at this point). Keep a copy of the background both as a file (File \(\rightarrow \) Save as), and take a screen shot for your ELN.

   

   Figure \(\PageIndex{4}\): A Typical Background Spectrum. (CC-NC-BY-DUKE CHEM)

6. Remove the evacuated gas cell from the sample compartment. Under the guidance of your TA, introduce first 1 cm Hg pressure of DCl gas followed by about 0.8 cm Hg pressure of HCl gas into the cell. You must be careful to avoid the introduction of air into the cell. Replace the gas cell on the sample holder, and close the door of the sample compartment. Allow the sample compartment to purge for the same time as in the background spectrum.
7.  Select Collect Sample from the Collect menu (or use the Col Smp button). You can enter a title or leave the default date/time when prompted. Click OK, and OK again in the Confirmation box to start the run.
8. When the scan is complete, add the scan to the window and you will see both sample and background displayed. Save the data file (File \(\rightarrow \) Save as).

   

   Figure \(\PageIndex{5}\): Sample and Background Together. (CC-NC-BY-DUKE CHEM)

   Click on the background scan and select Hide Spectra from the View menu to remove the background spectrum from view.

   

   Figure \(\PageIndex{6}\): The Sample Spectrum. (CC-NC-BY-DUKE CHEM)

9. You will see two (or three) sets of peaks in the region 3200-1800 cm–1. Which of these is the spectrum of HCl and which the spectrum of DCl? Explain your reasoning. In your report, explain the origin of the third set of peaks (if observed). Click and drag a box around the H³⁵Cl/H³⁷Cl absorption and double-click in the box to expand the scale so that the spectrum of fills the screen.
10. Select Find Peaks from the Analyze menu or click the button. Move the horizontal threshold bar down so that it captures as many of the peaks as possible without getting into the noise at the baseline (see Figure 4.3.7 below). You can also adjust the sensitivity slider at the left of the screen to fine tune the peak selection. When you have a good selection of peaks, click the Clipboard button to put the peaks table into the Windows Clipboard. Open Excel and paste the clipboard into a spreadsheet. Save the spreadsheet along with a screenshot of the peak positions overlaid onto the speactum as seen in Figure 4.3.7.

    

    Figure \(\PageIndex{7}\): the Find Peaks Function. (CC-NC-BY-DUKE CHEM)

11. Click Replace at the top right of the screen to bring you back to the active window. Click and drag the white selection window over to the DCl spectrum.

    

    Figure \(\PageIndex{8}\): The Selection Window. (CC-NC-BY-DUKE CHEM)

    Re-scale the DCl spectrum in the window and repeat the peak-finding steps being sure to add the resultant peaks table to your Excel spreadsheet.

12.  Save screen shots of the spectra and the report for your ELN.
13.  If you wish to record the spectrum of a new sample, say of SO₂ in Part 2 of this experiment, repeat steps (4.3.2.6)-(4.3.2.12).
14. When you are done with the instrument and all of your work is saved, close the software. You will notice the blue light on the instrument turn off as the instrument goes into standby mode.
15. Remove your sample from the sample compartment and close the cover.
16. Re-evacuate the gas cell to remove your sample and store the evacuated cell (with stopcocks closed) in the desiccator.

Data Analysis for HCl/DCl FTIR spectra 

Note

This is probably the most intense data analyses you will conduct this semester. Let's minimize the number of repetitive tasks to save you some time. The following shortcuts are recommended:

• Collaborate with a partner on steps 1-3 (allowed in this specific case only):You may collaborate with your lab partners in steps 1-3 below. One student should work up the data (steps 1-3 ONLY) for H³⁵Cl and H³⁷Cl, and the second student should do the same analysis for D³⁵Cl and D³⁷Cl. You should then share your graphs of \( \Delta \widetilde{\nu}_{(m)} \) versus \(m\) and the values determined for the slopes and y-intercepts of these plots. Each student should do one complete analysis (either HCl or DCl) alone, and the other analysis can be the work of a partner. However, you are responsible for all work that is in your ELN, so be sure your partner's work is correct. *Steps 4 and onward should be completed individually, as usual.
• Calculate errors for one isotope only: The propagation of errors that you calculate for one isotope should similar (perhaps not identical, but close enough) as for another isotope. Save time by propagating errors carefully for (\(B_e\), \(\alpha_e\), \(\widetilde{\nu}_{o}\), Ie, re, and k for one isotope, then use those errors as an approximation for the errors for analogous values for other isotopes.
• Use what you know! Do you remember that MatLab Introduction module way back in the beginning of this course? It was a long time ago, but it was a template for data analysis in this module! Go back and use what you already know, and what you already created as a template, for this data analysis.

Collaborate with a partner 

1. Label each peak in your spectra as "P" or "R" and its J-value as: P(1), P(2),..., R(O), R(1),... etc., as shown in Figure 2 on page 399 of Shoemaker, Garland and Nibler. (Note: Spectra recorded using your iS50 FTIR spectrometer are plotted with the wavenumber (abscissa) decreasing from left to right.)
2. For each of the four isotopic combinations (H³⁵Cl, H³⁷Cl, D³⁵Cl and D³⁷Cl), make a table of the m values and the corresponding peak frequencies \( \widetilde{\nu}_{(m)} \). For each isotopic combination, calculate the separation between adjacent peaks.\[ \large \Delta \widetilde{\nu}_{(m)}=\widetilde{\nu}_{(m+1)}-\widetilde{\nu}_{(m)} \]

   Note

   \( \Delta \widetilde{\nu} \)_(m) cannot be calculated for m = –1 since there is no peak for m = 0. Therefore m = –1 must be deleted from the vector of m values.
3. Plot the differences between adjacent absorption frequencies (i.e., \( \Delta \widetilde{\nu}_{(m)} \) versus \(m\). Then perform a linear least squares fit of the data with the understanding that
   \[ \large \Delta \widetilde{\nu}_{(m)}=\widetilde{\nu}_{(m+1)}-\widetilde{\nu}_{(m)}=\left ( 2B_e - 3 \alpha_e\right )-2 \alpha_e m  \label{fit}\]
   Use the standard deviations of the slope and intercept in your error analysis (Q14).

Work on your own 

4. Compute \( B_e \) and \( \alpha_e \) for each isotope from the intercept \( \left ( 2B_e - 3 \alpha_e\right ) \) and the slope \( -2 \alpha_e \) of each best fit line (Equation \ref{fit}). 
5. Calculate the frequency of the forbidden transition \(\widetilde{\nu}_{o}\) using your calculated values of \( B_e \) and \( \alpha_e \). To do this, use equation 9 in expt 37 of Shoemaker, Garland and Nibler. The equation is \(  \widetilde{\nu}_{m} = \widetilde{\nu}_{o} + (2B_e -2 \alpha _e ) m - \alpha_e m^2\). Calculate \(\widetilde{\nu}_{o}\)  from several of the lines with low \(m\) values. Calculate the average value, and standard deviation, of \( \widetilde{\nu}_{o} \).

   Note

   You will get the best results by using only the lines closest to the center (e.g., m = ±1, ±2).
6. Create a table of \( \widetilde{\nu}_{o} \), \( B_e \), and \( \alpha_e \) for the four isotopic combinations The units should be cm^–1.
7. Calculate the moment of inertia (\(I_e\)), and the internuclear distance (a.k.a. equilibrium bond length, \(r_e\)) for all four isotopic combinations.
8. Calculate the harmonic oscillator force constant, \(k\), for each isotopic combination from your calculated values of \( \widetilde{\nu}_{o} \). In doing so you will need to assume that \( \widetilde{\nu}_{o} \) and the harmonic oscillator frequency \( \widetilde{\nu}_{e} \) are the same, thus ignoring the effects of anharmonicity (see the formulas and discussion on pages 400 and 401 of Shoemaker, Garland and Nibbler).
9. Compare your values of r_e and k with those reported in the literature. Why are these values independent of isotopic substitution?
10. Determine the isotope effects: Compute the ratios \( \large \frac{\widetilde{\nu}_{o}^*}{\widetilde{\nu}_{o}} \) and \( \large \frac{B_{e}^*}{B_e} \) for ³⁵Cl and ³⁷Cl in the case of HCl, and again in the case of DCl. Compare these with the predicted values given by Eqs. (11) and (13) on page 400 of Shoemaker, Garland and Nibbler.
11.  From the results in 10, above, explain why is the ³⁵Cl /³⁷Cl isotope effect so much larger in DCl than it is in HCl? That is, when looking at your FTIR spectrum, the ³⁵Cl/³⁷Cl line separation is significantly greater in the DCl case then when compared to the HCl case. A descriptive answer is a good start, but your experimental observations should also be supported with a calculation that justifies the experimental observation.
12. Apply the Boltzmann distribution to explain observed band intensities in terms of the populations of the ground-state levels (see section 4.2). Again, a descriptive answer is a good start, but it should be supported with calculated results. You might create a spreadsheet to do these calculations. Does a calculation based on the Boltzmann population of rotational states match what you see experimentally in terms of which rotational lines show the greatest intensity?
13. From the mean line intensities for at least 10 features (for example five or more lines from the HCl spectrum and five or more from the DCl spectrum for a total of at least 10), determine the relative abundance of the two Cl isotopes. Estimate the weighted average atomic mass of Cl (that you might see on a periodic table for example) assuming that ³⁵Cl and ³⁷Cl have masses of 34.968 and 36.956 amu, respectively. Compare your result with the reported atomic mass of Cl.
14. For one isotope only, calculate the estimated uncertainty in each of the parameters determined for this experiment (\(B_e\), \(\alpha_e\), \(\widetilde{\nu}_{o}\), I_e, r_e, and k). (Another table would be helpful)
15. Why is it important to purge the sample compartment of the FTIR spectrometer for both the background and sample runs? Why is it important to purge both background and sample for the same amount of time?

References and further Reading for Part I

1. Moore, W.J., Physical Chemistry, 4th ed., Prentice-Hall, Englewood Cliffs, New Jersey, 1972, pp. 767-770; or Pauling, L.; Wilson, E.B. Introduction to Quantum Mechanics, McGraw-Hill, New York, 1935.

2. Pitzer, K.S. Quantum Chemistry, Prentice-Hall, Englewood Cliffs, New Jersey, 1953.

3. Herzberg, G. Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules, 2nd ed., Van Nostrand, Princeton, New Jersey, 1950, Chapter III.

4. Hill, T.L. Introduction to Statistical Thermodynamics, Addison-Wesley, Reading, MA 1960, Chapter X.

5. Prais, M.G. J. Chem. Ed., 1986, 63, 747.

6. Shoemaker, D.P., Garland, C.W., Nibler, J.W. Experiments in Physical Chemistry, 6th ed., McGraw-Hill, New York, 1996, Chapter XIV, Experiments 35 and 37.

7. Richards, L.W. J. Chem. Ed., 1966, 43, 552. See also, McQuarrie, D.A. and Simon, J.D., Physical Chemistry: A Molecular Approach, University Science Books, CA, 1997, p. 169.

8. Sime, R.J., Physical Chemistry: Methods, Techniques and Experiments, Saunders, Philadelphia, PA, 1990, pp. 676-687.

9. Silbey, R.J., Alberty, R.A. and Bawendi, M.G., Physical Chemistry, 4th ed., Wiley, NY, 2005, Chapter 13, Section 13.4, 13.6, 13.7.

10. Atkins, P.W. Physical Chemistry, 5th ed., W. H. Freeman, NY, 1994, Sections 16.8 – 16.11., or 6th ed., W. H. Freeman, NY, 1997, Sections 16.9 – 16.12.