Infrared Spectroscopy

How are Wave number and Wavelength related?

(W) = 1/λ -the higher the wavelength the lower the wavenumber

Disadvantages of IR

-Cannot be used to examine single atoms. Species must contain a chemical bond. -Homonuclear species (N2,O2...) -It is not useful in complex mixtures. -As water is a strong absorber in the IR aqueous solutions are difficult to analyze.

What limitations are there on a solvent used in IR?

-Must be transparent in the region to be studied -Water and alcohols are avoided because of the broad O-H band -Must be chemically inert to not react with the analyte or the cell

How might we find IR more useful than UV?

-universal technology used on liquids, solids, and gases -rich source of structural info -fast (<5 min) -easy to use -small sample size -inexpensive (<$10,000) -nondestructive

Provide 5 advantages of Infrared Spectroscopy.

-universal technology used on liquids, solids, and gases -structural info -fast (<5 min) -easy to use -small sample size -inexpensive (<$10,000) -nondestructive

Decide which structure bests fits the spectrum. Briefly explain why the other two structures were excluded.

1-Methoxy-2-butyne

Assign one of the following structures to the IR spectra below. You must justify your answer.

2-methyl-2-propen-1-ol

How many bending and how many stretching possibilities are there? Will all of these be IR active?

4 bending -Scissoring (In-plane) -Rocking (In-plane) -Wagging (Out of Plane) -Twisting (Out of Plane) 2 stretching -Symmetric -Asymmetric -Asymmetric stretching and bending IS IR active -Symmetric stretching and bending is NOT IR active

What region of the spectrum is used for IR spectroscopy?

500-4000 cm-1

How are absorption and % transmittance related?

A = log (1/T) -the higher the transmittance the lower the absorbance

In each of the following pairs of bonds, select the one that stretches at the highest frequency. A. C-O or C=O B. C-O or C-Cl C. C=C or C≡C D. C-C or C-O E. C-H or O-H

A. C=O B. C-O C. C≡C D. C-C E. O-H

Are these at a high or low energy: A. higher wavenumber B. higher frequency C. long wavelength

A. high energy B. high energy C. low energy

Rank the following bond stretches in order of expected absorption intensity and stretching energy: C-F, C-Cl, C-Br, and C-I.

Absorption intensity depends on polarity and number of bonds present. C-F > C-Cl > C-Br > C-I

How does the absorption of IR affect a molecule?

Absorption of IR changes in the vibrational motions of a molecule. Different vibrational modes include stretching and bending.

Simply by exchanging an atom of hydrogen (C-H) for deuterium (C-D) we notice a significant change in the stretching vibration. Use the above formula to explain this behavior.

As the atomic mass increases the stretching frequency decreases. Since deuterium is heavier than hydrogen it will have a lower frequency.

Which compound would be expected to show intense IR absorption at 3300 cm-1? A. CH3-CH2-CH3 B. CH3-C≡C-H C. CH3-CH=CH2 D. CH3-CH2-CH2-F

B. CH3-C≡C-H

Which of the following bonds would show the strongest absorption in the IR? A. carbon-hydrogen B. oxygen-hydrogen C. sulfer-hydrogen

B. oxygen-hydrogen O has the highest polarity in relation to H

What problems are caused by carbon dioxide in an IR spectrum? Why is this frequently a problem? How is this problem solved?

Broad peak that are very large. Lab techs breathe into the IR spec releasing CO2. Hold breath when preparing to use the IR spec.

Explain which has a lower characteristic stretching frequency, the C-H or C-D bond?

C-H has a higher stretching compared to C-D. ----higher mass=lower stretching

Rank the following bonds in order of increasing stretching frequency (cm-1) in IR spectroscopy: C-H, C≡C, C-O, and C=O

C-O < C=O < C≡C < C-H

Which of the following compounds gives the following IR spectra? Justify your answers.

C. CH3-CH2-CH=CH2

Explain which has a lower characteristic stretching frequency, the C=O bond or the C-O bond?

C=O has a higher stretching compared to C-O. ----stronger bonds=higher stretching

Why is the hydrogen bond such a broad band in the IR?

Compound has a high number of individual molecules and each may be H-bonded to a different extent. The IR spectrum is acquiring each IR absorptions at varying frequencies for each bond => IR peak appears broadened because it is an average of all these slightly different absorptions.

In order for a vibration to be observed in the Infrared Spectra, the vibration must change the _____ of the molecule.

Dipole

What is the region of the IR spectra called which contains the most complex vibrations?

Fingerprint region

It appears at first glance that CO2 would not be IR. However, CO2 is IR active. Explain this seeming contradiction.

Symmetrical stretching doesn't result in a dipole and is IR inactive. However, asymmetrical stretching produces a strong dipole that is IR active.

When recording an IR spectrum in the lab, we take no special precautions to account for the presence of oxygen, nitrogen, or argon in the air. What are the consequences of not compensating for these gases?

There are no consequences for a lack of compensation because O2, N2, and Ar are all non-polar species and their dipole moments don't change during vibration.

Explain the following series: Bond Stretch cm-1 CH 3000 CC 1200 CO 1100 CCl 750 CBr 650

This is an example of mass effect, where the greater the mass the lower the frequency.

What differences do we see on the absorption of energy in the IR versus the UV?

UV/Vis energy is absorbed: we see an electron transition and excitation of ground state electrons. IR is absorbed: stretching and bending of covalent bonds which can reveal info about functional groups

Why are salts such as NaCl, NaBr, KBr used in IR analysis?

Used as salt plates because: -inexpensive -easy to polish -transparent

When does the absorption of water and carbon dioxide in the IR become an issue?

When the sample under study has the same absorption bands at the same wavenumbers => sample bands being masked by water and carbon dioxide => sample detection problems.

What is the relationship between frequency and wavelength?

c= ν λ so there is a indirect relationship

From the following IR and MS, identify the compound from the following examples. You must justify your answer by identifying the defining characteristics from these spectra.

methyl 2,2-dimethylpropanoate

In some detail explain the following formula and what practical implications this formula has in Infrared Spectroscopy. What affect will changes in k and m have on an IR peak. v=1/2pi*sqrt(k/mr)

v = Frequency (cm-1) k = Spring stiffness (Bond Strength) mr = Reduced mass -Frequency increases with decreasing atomic mass -Frequency increases with increasing bond energy

Why is water almost never used as an IR solvent?

water is a strong absorber in the IR and produces a very large broad peak