Effect of mass attached to the spring: 1. Replace the small stopper with the large stopper. Repeat steps 3-9 for each spring set.

Transcription

1 EXERCISE 1: Representing molecular vibrations with spring oscillations A spring is a common model for covalent chemical bonds. One of the interesting interpretations of quantum mechanics is that bonds are always oscillating; they never come to rest. In this activity we will use three different metal spring sets as models for covalent chemical bonds in order to study the oscillations (stretching vibrations) of bonds. This type of bond oscillation occurs when molecules absorb electromagnetic radiation in the infrared region, as in atmospheric warming. We will observe the factors which influence the frequency of bond vibrations. Procedure: 1. Weigh both the large and small stopper. Do not over- stretch the springs! Effect of spring strength: 1. Place the center of the single spring set on the clamp mounted on the ring stand. 2. Measure the distance between the table top and the bottom of the spring. 3. Hang the small stopper on the center of the spring set. Measure the distance between the table top and the bottom of the spring. 4. Calculate the displacement of the spring caused by hanging the mass on the spring. 5. Lift the stopper until the spring is almost back to its original position and release it. What happens? The up- and- down motion of the spring is called an oscillation. One complete oscillation is when the stopper starts at the top and then returns to its starting position. 6. One team member should act as the timekeeper; the other should act as the counter. The counter should lift the stopper and drop it. After the stopper oscillates once, the timer should start the stopwatch. The timer will measure how long it takes the counter to count 10 oscillations. 7. Repeat step 6 two more times. 8. The average time, in seconds, for one oscillation should be calculated, this is the period. Calculate the inverse of the period; this is the frequency in Hertz (Hz, 1/s). 9. Repeat steps 3-8 for the double and triple spring sets. Effect of mass attached to the spring: 1. Replace the small stopper with the large stopper. Repeat steps 3-9 for each spring set.

2 2. Record each spring set s displacement and frequency with the different masses. Follow- up Questions: 1. How is the spring displacement related to the strength of the spring set? 2. A spring is a useful model for a covalent chemical bond. Chemists have seen evidence that bonds are always oscillating. In fact they never come to rest. How would the frequency of oscillation correlate with the bond strength? 3. Consider the C- C, C=C and C C bonds. Predict the relative strengths of these three bonds, that is, which is strongest and which is weakest. Then predict the relative oscillation frequencies of these three bonds which has the highest frequency and which the lowest - - based on your observations in this activity. Explain your reasoning 4. What is the effect of different attached masses on frequency of vibration for a given spring? (Compare results small and large stoppers.) 5. Consider the three bonds, C - C, C - H, and C - O. Assume, for purposes of this discussion, that their bond strengths are approximately equal. Based on your observations of the springs with different attached masses, what do you predict to be the relative oscillation frequencies of these three bonds? Explain you reasoning. Spectroscopy Background You learned about visible spectroscopy in a previous topic. A UV- Vis spectrometer compares the intensity of a given wavelength of light before and after it interacts with a colored solution. You learned that different molecules absorb different wavelengths of light. The energy associated with UV- Vis light causes electrons in the molecules to be excited to higher energy levels. Now we are going to learn about two other spectroscopic techniques, IR spectroscopy and spectroscopy. IR Spectroscopy: When you expose molecules to infrared radiation (800nm < λ < 1.0mm), the bonds within the molecules will stretch and bend. Different covalent bonds absorb different wavelengths or frequencies of IR radiation. For example, a C- H bond will absorb IR radiation in the 2800 to 3100 cm - 1 range. We generally report the frequency of IR absorption using the units of cm - 1. We call this the wavenumber and it is simply calculated as λ 1, with the wavelength, λ, given in the units of cm. IR spectroscopy can be used to study and identify chemical compounds. If you have an unknown sample, you can take the IR spectra of the sample and determine what types of bonds are present in the compound.

3 Spectroscopy: spectroscopy often serves as a complement to IR Spectroscopy. In spectroscopy, the sample is exposed to a single frequency of light. This light is generally obtained from a laser source. The laser light is scattered by the sample. The difference between the frequency of the scattered light and the frequency of the incident light provides useful information about the types of bonds present in the compound, and can be correlated to specific bond vibrations. SPECTROSCOPY Molecules can absorb various types of electromagnetic radiation and attain excited states as indicated below. Region Wavelength range Type of excitation Ultraviolet nm electronic Visible nm electronic Infrared μm bond deformations Microwave 1mm 1m rotational Radio waves meters electron and nuclear spin transitions Specific regions in IR spectra of organic compounds are characteristic of the following types of bonds: Frequency bond indicated C- H bending ~ C- C stretching 1650 C=C stretching C C stretching

4 Figure 7.1: A summary of electromagnetic radiation.

5 ACTIVITY 2: GaussView: Using software to observe molecular vibrations In the "Springs and Bonds" activity, you observed macroscopic systems and saw that vibrational frequency depends on both bond (spring) strength and attached mass. This system was a model for covalent bond behavior. Unfortunately, we cannot view real molecular vibrations directly. In this activity you will observe computer animations of molecular vibrations using a program called GaussView. GaussView aids in the interpretation of molecular vibrations by using infrared and spectroscopy. It presents spectra and animations of the molecular vibrations that give rise to the absorption features on those spectra. These animations are not true to scale and are exaggerated to show the vibrations clearly. In Part A, you will be investigating the molecular motions in propane, propene, and propyne molecules. Tables V.1, V.2, and V.3 summarize the important frequencies for these molecules. In Part B, you will be investigating the molecular motions in isopropanol and acetone molecules. The purpose of this activity is to observe computer animations of molecular vibrations using GaussView. PART A: Propane, Propene, Propyne Spectra Procedure: 1. Log on to Blackboard and open the course folder titled gen chem. common site perm. 2. Click Assignments 3. In the Gaussian Files folder, right click on the propane.log link 4. Select Save Target As: 5. Choose to Save In: Desktop 6. Click Save 7. Once the download is complete, click Close 8. Repeat steps 3 7 with the propene.log and propyne.log files 9. Close the BlackBoard link. 10. Open GaussView 4.1 by clicking on the desktop icon 11. Close the GaussView Tips 12. Select File then enter 13. Select Open, then enter 14. In the Files of Type box, select Gaussian Output Files from the pulldown menu 15. Click on propane.log

6 16. Look at the structure of propane in the blue box. You can zoom in and out using the right mouse button. You can rotate the molecule using the left mouse button. Rotate the molecule so that you can see all 11 atoms. 17. In the GaussView window, select Results a. Select Vibrations b. Click the Start Animation button c. Click the Spectrum button. Expand the window so that you can see both the IR spectrum and the Activity spectrum. 18. By clicking on any of the absorption peaks in the spectrum you can see a molecular animation of the vibrational motion excited when radiation of that frequency is absorbed by the molecule. Click through the absorption features of propane, paying close attention to the molecular motions which give rise to the absorption features. You can zoom in on specific regions on the spectrum by right clicking and opening a box around the area you wish to see in better detail. To zoom out, right click the mouse and select zoom out from the menu. Note the frequency in the bottom left corner of the spectrum box. The C- C stretch for propane is found at 878 cm - 1. Look at this molecular motion. Table V.1: Peaks in the IR and Spectra for Propane, C 3 H 8 (Gaussian Spectra) 764 Y N 1520 Y Y(v. small) 878 Y Y 1521 Y Y(v. small) 923 N Y (v. small) 1529 Y Y(v. small) 944 Y Y (v. small) 1535 Y Y 1084 Y (v. small) Y 3027 Y Y(v. small) 1188 Y Y 3029 Y Y 1226 Y (v. small) Y 3037 Y Y 1315 N Y 3074 Y(v. small) Y 1379 Y(v. small) Y(v. small) 3112 N Y(v. small) 1426 Y Y(v. small) 3117 Y Y 1441 Y Y(v. small) 3122 Y Y 1513 N Y 3128 Y Y

7 19. Open the propene.log link by following steps The C=C stretch in propene is both IR and active. This means that you will find a peak in the IR and spectra for this motion. Use this information to determine the frequency associated with the C=C stretch in propene. Table V.2: Peaks in the IR and Spectra for Propene, C 3 H 6 (Gaussian Spectra) 599 Y Y 1517 Y Y 926 Y(v. small) Y 1522 Y Y 980 Y Y(v. small) 1698 Y Y 984 Y(small) Y(small) 3038 Y Y 1025 Y Y(v. small) 3113 Y Y 1082 Y(v. small) Y(small) 3133 Y Y 1191 Y Y 3147 Y Y 1339 N Y 3169 Y Y 1436 Y Y 3253 Y Y 1466 Y Y 20. Repeat steps with the propyne.log link and look for the C C stretch. This motion shows as a small peak in the IR and is active. What is the frequency associated with the C C stretch? Table V.3 Peaks in the IR and Spectra for Propyne, C 3 H 4 (Gaussian Spectra) 692 Y Y 2206 Y(small) Y 943 Y(v. small) Y(small) 3043 Y Y 1075 Y(small) Y(v. small) 3130 Y Y 1453 Y Y 3494 Y Y 1510 Y Y

8 Follow- Up Questions: Note the differences between the three molecules. At what frequency does the C to C stretching occur in each molecule? How does this frequency relate to the bond strength? Are all peaks observed in the spectra found in the IR spectra? Why does propane have a more complicated spectrum than propyne? PART B: Thioformaldehyde and Formaldehyde Spectra Procedure: 1. Repeat steps 3 7 from Part A with the thioformaldehyde.log and formaldehyde.log files 2. Open the thioformaldehyde link by following steps from Part A. 3. Use the animation feature to observe the molecular motions that occur in a molecule of thioformaldehyde. In particular, look for the region in the spectra where the C=S stretch can be found. What is the frequency for the C=S Stretch in thioformaldehyde? 4. Open the formaldehyde.log link by following steps from Part A. Use the animation feature to observe the molecular motions that occur in a molecule of formaldehyde. What is the frequency of the C=O stretch in formaldehyde? Follow- Up Questions: 1. Is the C=S stretch in thioformaldehyde more active in the IR or spectra? 2. Is the C=O stretch in formaldehyde more active in the IR or spectra? 3. Is the frequency of oscillation of the C=O stretch in formaldehyde greater than or less than the frequency of the C=S stretch in thioformaldehyde? How does this frequency difference relate to the mass of the atom double bonded to the carbon? Are all peaks observed in the spectra found in the IR spectra for formaldehyde? Classroom Activity: IR and Scans of hexane, 1- hexene, and 1- hexyne

9 Due Date: Grade: EXERCISE 1: Representing molecular vibrations with spring oscillations Name: Date of Exercise: Team Members: Observations and Data: Small Stopper Mass: Large Stopper Mass: Spring Set Stopper Size Spring Set Displacement (cm) Time for 10 Oscillations (s) 1 Spring Small Test 1: 2 Springs Small Test 1: 3 Springs Small Test 1: 1 Spring Large Test 1: 2 Springs Large Test 1: 3 Springs Large Test 1: Average Period (s) Average Frequency (Hz)

10 1. How is the spring displacement related to the strength of the spring set? 2. A spring is a useful model for a covalent chemical bond. Chemists have seen evidence that bonds are always oscillating. In fact they never come to rest. How would the frequency of oscillation correlate with the bond strength? 3. Consider the C- C, C=C and C C bonds. Predict the relative strengths of these three bonds, that is, which is strongest and which is weakest. Then predict the relative oscillation frequencies of these three bonds which has the highest frequency and which the lowest - - based on your observations in this activity. Explain your reasoning 4. What is the effect of different attached masses on frequency of vibration for a given spring? (Compare results small and large stoppers.) 5. Consider the three bonds, C - C, C - H, and C - O. Assume, for purposes of this discussion, that their bond strengths are approximately equal. Based on your observations of the springs with different attached masses, what do you predict to be the relative oscillation frequencies of these three bonds? Explain you reasoning.