Lab 9. Procedure.

You will collect data with a lab partner (who should be noted in your write-up). However, all the data analysis and write-up should be done without collaboration.

Before the experiment
You should have

• gas cell with KBr windows (pathlength=10 cm)
• concentrated H₂SO4
• D₂O
• CO₂ tank
• C₂H₂ lecture bottle
• C₂D₂ lecture bottle
• vacuum line filling apparatus
• FTIR spectrometer Spectrum One from Perkin-Elmer Inc. The local version of the manual for Spectrum One is available here (courtesy of K.Burke).

During the experiment

Part A. CO₂

In this experiment, several vibrational-rotational infrared bands of CO₂ will be recorded at medium-to-high resolution (ca. l cm^-1). These spectra will be analyzed to extract information about hot bands.

• Pump out the system for 10-15 minutes (have IR cell attached with its valve open).
• Close off the line to the cell.
• Take your background spectrum. Use the highest resolution (0.4 cm^-1 if possible), set the signal gain to 1 and the number of scans to 16 or greater.
• Return cell to apparatus. Add approximately 1atm of CO₂. Record IR absorption spectrum. Two spectral ranges are of interest: 550-750 cm^-1 and 2250-2450 cm^-1. At this high pressure you should observe that absorption extends quite far from the maxima due to rotational unresolved P and R branches.
• Repeat the same with 150 Torr of CO₂. In the region of bending mode absorption, besides seeing unresolved rotational P and R branches, you should see sharp lines at 618, 648, 720 cm^-1 and maybe more. These lines are due to hot bands.
• Repeat the same with a piece of dry ice. Did the hot bands disappear?

Part B. HCl and DCl

In this experiment rotational fine structure of the infrared vibrational spectrum of HCl and DCl will be recorded at medium-to-high resolution (ca. l cm^-1). These spectra will be analyzed to extract rotational constants and obtain the moment of inertia of the molecule and thus the internuclear separation. The pure vibrational frequencies will also be measured.

• HCl and DCl can be synthesized by addition of D₂O to sulfuric acid (1:1) and poring it dropwise over solid NaCl or KCl. As a result, gaseous mixture of DCl/HC will be obtained: D₂SO₄(l) + 2NaCl(s)  ->  2DCl(g) + Na₂SO₄(s). The trick is to collect water vapor before the IR cell so that it won't damage the windows. Use a trap cooled by dry ice for that.
• Pump out the system for 5-10 minutes (have IR cell attached with its valve open). There should also be a dewar of liquid nitrogen on the gas trap leading to the pump; this will condense out the HCl and DCl so these do not go through the pump and into the air.
• Close off the line to the cell.
• Take your background spectrum. Use the highest resolution (0.3 cm^-1 if possible), set the signal gain to 1 and the number of scans to 16 or greater.
• Return cell to the vacuum system. Open its valve connected to the line with HCl/DCl. You should fill to about 200 Torr (mm Hg) of the mixture.
• You may want the TA to assist here in filling the cell until you are comfortable doing it.
• Close off the IR cell valves, remove itl and take spectrum using the same parameters as for background spectrum.
• Repeat procedure for other gases.
• When completely finished, turn off the pump, remove the gas trap and put it in a fume hood, and return the IR cell to a desiccator.

For HCl, you are interested in the region around 2900 cm^-1. For DCl, the region of interest is around 2100 cm^-1. Use the peak report function to create a table of the wavenumber position and absorbance of each peak within these regions.

Part C (if time allows).

In this experiment, several vibrational-rotational infrared bands of C₂H₂ and C₂D₂ will be recorded at medium-to-high resolution (ca. l cm^-1). These spectra will be analyzed to extract rotational constants for use in the calculation of accurate values for the C-H and C-C bond lengths. The role of symmetry and nuclear spin in determining the activities and intensity patterns of the spectral transitions is also examined. From such considerations, the infrared bands can be assigned to specific modes of vibration and values can be deduced for the fundamental vibrational frequencies of C₂H₂ and C₂D₂ [1,2]

TABLE 2
Infreared regions to be scanned for acetylenes

• Pump out the system for 10-15 minutes (have IR cell attached with its valve open).
• Close off the line to the cell.
• Take your background spectrum. Use the highest resolution (0.3 cm^-1 if possible), set the signal gain to 1 and the number of scans to 16 or greater.
• Return cell to apparatus. Add approximately 300 Torr of C₂H₂ for a survey scan. Table 2 indicates the spectral regions of interest and the approximate pressures that give satisfactory intensities. These pressures may require some adjustment depending upon the resolution capabilities of the instrument since the peak absorbance of a narrow line increases as the spectral resolution improves. For the survey scan, a resolution of 4 cm^-1 is adequate to permit rapid recording at reasonable signal-to-noise ratio. The regions to be studied in detail should be scanned more slowly at an expanded scale to permit accurate frequency measurements. A resolution of at least 1.5 cm^-1 is needed to resolve the rotational structure of the acetylene bands, and a value of 0.5 cm^-1 or better is desirable. Detailed instructions for operating the spectrometer will be given in the laboratory.
• Reduced the cell pressure to about 25 Torr so that the strong n₃ and n₅ fundamentals and the n₄ + n₅ combination band have a more reasonable intensity. Expanded scans of the latter bands are recorded according to Table 2, preceded by a second survey scan.
• We plan to have it already done for you, but just in case - C₂D₂ can be synthesized by addition of D₂O to calcium carbide using the apparatus shown in SGN. About 2.5 g of calcium carbide is placed in flask F which is then evacuated to remove traces of H₂O; 0.5 mL D₂O is added with a syringe to flask F through the rubber septum and the entire system up to the vacuum stopcock V is allowed to "cure" at room temperature for about 5 to 10 min to allow deuterium exchange with H₂Oadsorbed on the walls of the system. The pressure should be monitored and kept below 1 atm during this period, reducing it if necessary by opening and closing V. The system is then evacuated, after which the cold traps are put in place and 0.5 mL D₂O is added to flask F. The pressure will rise and then drop as the C₂D₂ is condensed in the storage vessel cooled by liquid nitrogen. When the pressure drops to a few Torr, another increment of D₂O is added and the procedure is repeated until a total of 4 mL has been added. When gas evolution has slowed or stopped, stopcock A is closed and the reaction flask and water trap are placed in the hood, open to allow any further reaction to occur harmlessly.
• Fill the infrared cell with C₂D₂ by evacuated the system with the liquid nitrogen trap still in place. Stopcock V is then closed and the nitrogen Dewar is lowered to allow the storage vessel to warm slowly until the pressure is about 300 Torr. The stopcocks C and C' leading to the infrared cell are then closed and, as warming continues, the system pressure is monitored and adjusted with stopcock V if necessary to keep it below 1 atm. When room temperature is reached, stopcock B' can be closed to save some residual C₂D₂ as "insurance" until all cell pressure adjustments and spectral measurements are completed. Do not open stopcock B' to air with the storage vessel in liquid nitrogen since liquid oxygen will condense on top of the acetylene, forming a potentially explosive mixture. At the end of the experiment, the acetylene in the infrared cell and in the storage vessel can be disposed of by simply exhausting it through the roughing pump of the vacuum system.
• Repeat measurements the same way as with C₂H₂

{Commercial acetylene is widely used for welding purposes and is shipped dissolved in acetone, in which it is extremely soluble. The acetone is retained by a porous filler material within the cylinder so that the discharged acetylene is typically >99%. If desired, residual traces of acetone can be eliminated by passage through a Dry Ice/isopropanol trap. In its free state, acetylene may decompose violently; the stability decreases at higher pressures. At pressures below 1 atm, the sampling conditions of this experiment, the gas can be handled safely but one should, of course, wear safety glasses and exercise reasonable judgement. Unalloyed copper, silver, and mercury should never be used in direct contact with acetylene, particularly when wet, owing to the possible formation of explosive acetylides.}

{A higher D/H ratio can be achieved by adding the acetylene to a storage bulb containing 5 mL of 99 + % D₂O and about 1 g of basic alumina. The latter serves to promote the exchange of acidic protons on C₂H₂ with the D₂O and thereby improves the D content of the acetylene. For best results, the protons on the basic alumina should first be exchanged by adding a few mL of D₂O and evacuating the storage bulb prior to addition of more D₂O and acetylene. After exchange at room temperature for a few hours, the storage bulb is cooled in a Dry Ice/isopropanol bath and the enriched C₂D₂ is distilled into the infrared cell.}

After you finish

• turn off the pump and disconnect it from the vacuum line (let the air into the pump)
• clean up the mess after yourself
• show you notebook and the apparatus to the TA or the instructor before you leave

Tentatively, plan to run CO₂ during first day, HCl/DCl on a second day and acetylenes on a third day.

Last updated on 02/12/07