Akhand-IR Worksheet PDF

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EXPERIMENT 4:

Name: Leila Akhand

INFRARED SPECTROSCOPY

TA: Khaustav Khatua Chem : 237

Section:502 Date: 2/25/2021

Infrared Spectroscopy Worksheet Open the IR Tutor program. Initially, a screen will open telling you to change your monitor resolution. DO NOT change the screen resolution, just hit continue! Once you have opened the IR Tutor program, use the program to answer the following questions. Each question can be answered with the information found in the program. 1. Where is the IR region relative to visible light? a. near the red b. near the violet 2. How did Hershel first detect IR light? Herschel blackened a thermometer bulb and placed it in the path of IR light; this allowed the thermometer to warm above room temperature. Then he later put a sample of water in front of IR light and compared the reference and sample temperatures. This is the measure of the absorption of IR light. 3. Is all infrared light the same wavelength? No. 4. What does λ stand for? It stands for wavelength, or the length of one oscillation cycle. 5. What does υ stand for? It stands for frequency, or the number of cycles that pass a certain point in one second. 6. What does υ(bar) stand for? It stands for wavenumber, or the inverse of the wavelength. 7. Write an equation that relates Energy(E) to wavelength.

E= hc/λ 8. From your equation, what happens to energy as the wavelength increases? As wavelength increases, energy decreases; they are inversely proportional. 9. Write an equation that relates Energy and wavenumbers.

E=hcν 10. From this new equation, what happens to wavenumbers as energy increases? As energy increases, wavenumbers increase; they are directly proportional. 11. What range of wavenumbers represents the mid IR region? 4000-400 cm^(-1)

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12. Why are lenses not used in IR instruments? Lenses are not used because the lens material would absorb IR light and the reading would be inaccurate. 13. Which type of IR instruments measures all wavelengths of IR at once? a. Classical b. Fourier Transform 14. What happens to the potential energy of a spring if it is stretched from its equilibrium length? If a spring is stretched, then a greater force pulls each end towards the other. The spring exerts a restoring force to bring the spring back to its equilibrium length. 15. What happens to the potential energy of a spring if it is compressed from its equilibrium length? If a spring is compressed, then a lesser force pushes each end away from the other. The spring exerts a restoring force to bring the spring back to its equilibrium length. 16. What happens to the frequency of a vibration if you increase the force constant? The larger the force constant (the stronger the spring) leads to a higher frequency; they are directly proportional. 17. What happens to the frequency of a vibration if you increase the mass? The larger the mass, the lower the frequency; they are inversely proportional. 18. The vibrations of molecular bonds are not as simple as a symmetric parabolic graph. What is happening to the bond in a molecule as the energy levels out? (this is on the right side of the graph) When the molecules are pulled far enough apart, the bonds break and the energy levels out and this is represented by an anharmonic potential graph. 19. What is the change in energy associated with an overtone? 2hv

20. What must happen to the dipole for a specific vibration to show up in IR Spectroscopy? The dipole must change when a transition occurs to absorb IR light; this is why H2 can’t be viewed using IR light, because their net dipole is always zero. 21. What are the three stretches that a water molecule has? bend

symmetric stretch

antisymmetric stretch

22. Draw these three vibrations using arrows to show the direction of the stretch or bend.

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bend

symmetric stretch

asymmetric stretch

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23. Which takes more energy, a bend, or a stretch? (look at where they are in on the spectrum in wavenumbers, and then look at your equation from question 9) stretch

24. Draw the four molecular vibrations for CO2 using arrows to show the direction of the stretch or bend. Label which one(s) will result in IR absorption.

Symmetric stretch

antisymmetric stretch

in-plane bending

out-of-plane bending

25. What is the range in wavenumbers of the fingerprint region (remember that we need a high, and a low, not just “anything below …”) High of 1400 and a low of 600

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Learning to Interpret IR Spectra

First peak to the far left: CH3 antisymmetric sketch; CH3 symmetric stretch Next peak to the right: CH3 antisymmetric bend

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Next peak to the far right: CH3 umbrella bend

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First peak to the far left: CH2 antisymmetric stretch Next peak to the right: CH3/CH2 antisymmetric stretch Next peak to the right: C-C double bond stretch Next peak to the right: CH3 antisymmetric bend Next peak to the right: CH2 out-of-plane twist Peak to the far right: CH2 out-of-plane bend

First peak to the far left: triple bond C-H stretch Infrared Spectroscopy Worksheet Page 7

Next peak to the right: CH3/CH2 antisymmetric and CH3/CH2 symmetric stretch Next peak to the right: triple bond C-C stretch Next peak to the right: CH3 antisymmetric bend Last peak to the right: triple bond C-H bend overtone 26. Notice that while 1-heptyne has a peak at 3300 cm-1, but it is not there in 4-Octyne. Why is it not there in 4-Octyne? There is not a peak at 3300 cm^(-1) in 4-Octyne like there is in 1-heptyne because 1heptyne has a hydrogen attached while 4-Octyne is between two carbons. This means that C-H stretch does not show so there is no peak there for 4-Octyne.

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27. Notice also that the peak at 2100 cm-1 is also only in 1-heptyne, and not 4-Octyne. Why did this peak disappear? This pea disappeared for 4-Octyne because the triple C-C stretch is symmetric, so this cancels out this bond’s dipole so there is no peak for 4-Octyne.

First peak to the left: CH3/CH2 antisymmetric and CH3/CH2 symmetric stretch Next peak to the right: triple bond C-N stretch Last peak to the right: CH3 antisymmetric bend and CH2 scissoring bend

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First two arrows to the left: unsaturated C(sp2)-H stretch and CH3 antisymmetric/CH3 symmetric stretch Next peaks to the right: overtone bonds Next peak to the right: symmetric ring stretch Last peak to the right: in-plane ring stretch 28. How can we see the difference between sp2 and sp3 C-H stretches when looking at an IR? Sp3 C-H stretch means that there are only single-bonded carbons while sp2 C-H stretch means that the carbon has double bonds. The different single and double bonds change degrees of unsaturation, but it could also be due to rings.

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First peak to the left: OH stretch Next peak to the right: CH2 symmetric stretch Next peak to the right: CH3 antisymmetric and CH2 scissoring bend Last peak to the right: C-O stretch coupled with C-C stretch

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First peak to the left: NH2 antisymmetric/symmetric stretch Next peak to the right: CH3/CH2 symmetric stretch

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First peak to the left: CH2 symmetric stretch Next peak to the right: CHO stretch/bend Last peak to the right: double bond C-O stretch

29. List the wavenumbers where the carbonyl peak will be for each of these compounds. O

O

O H

H

H

1725 cm^(-1)

1705 cm^(-1)

1715 cm^(-1)

30. What happens to the position of the carbonyl peak as resonance of the carbonyl increases? As the resonance of the carbonyl increases, this shifts the position of the C=O stretch to lower frequencies.

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First peak to the left: CH2 antisymmetric stretch Next peak to the right: double bond C-O stretch 31. List the wavenumbers where the carbonyl peak will be for each of these compounds.

1815 cm^(-1)

1780 cm^(-1) 1745 cm^(-1) 1715 cm^(-1) 1705 cm^(-1)

32. What is causing the change in where the carbonyl peak is for the different rings? The change in where the carbonyl peak is for the different rings is caused by the vibration shifting to a higher frequency by coupling to the stretch of the adjacent C-C bonds.

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First peak on far left: OH stretch of COOH dimer Next peak to the right: CH2 antisymmetric stretch Last peak to the right: C=O stretch

First peak on the far left: CH3/CH2 stretch Next peak to the right: C=O stretch Infrared Spectroscopy Worksheet Page 15

Last peak to the right: O-C(O)-C stretch 33. How could you tell the difference between an ethyl acetate and diethyl ether if you used only IR spectroscopy? (Hint: what peak would ethyl acetate have that diethyl ether would not) You could tell the difference between an ethyl acetate and a diethyl ether based on where their peaks are. For instance, there is a peak for ethyl acetate at 1745 cm^(-1) (C=O stretch) and 1200 cm^(-1) (C-O interaction). Diethyl ether, on the other hand, only has a peak at 1050 cm^(-1) (C-O stretch; there is no C=O stretch).

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First peak to the left: CH3 and CH2 stretches Next peaks to the right: double bond C-O symmetrically coupled stretch and double bond C-O antisymmetric coupled stretched Last peak to the right: C-O stretch

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