Lecture 11: Infrared Spectroscopy Part 1

List the expected stretching frequency ranges for peaks in an infrared spectrum that indicate the presence of: (b) an alkene

=C-H stretch 3340-3250 cm⁻¹, C=C stretch 1680-1620 cm⁻¹ (if the alkene is not symmetrical)

Briefly explain the events that result in an infrared spectrum.

Absorption of an infrared proton results in the excitation of the molecule to a higher vibrational quantum state.

The infrared absorption peaks of most common functional groups have characteristic features (two absorptions, broad peak,etc.) in addition to the photon energies. Briefly describe the important examples of this in the five zone analysis.

Alcohol, Amine: The O-H and N-H stretches are usually broadened due to hydrogen bonding Carboxylic acid: Must have two peaks: broad O-H stretch in zone 2 and C=O stretch in zone 4 Aldehyde: Must have two sets of sp² C-H stretching peaks: ~2900 and ~2700 cm⁻¹ in zone 2 and C=O stretch in zone 4 Alkyne: A peak caused by the triple bond stretch is not observed when the alkyne is symmetric or nearly so Carbonyl: Often the most intense peak in the entire spectrum Benzene ring: Must have peaks at about 1600 cm⁻¹ and another at about 1500 cm⁻¹ Since most benzene ring compounds have at least one hydrogen atom attached to the benzene ring, an sp² C-H stretch peak is also seen in zone 2

List the expected stretching frequency ranges for peaks in an infrared spectrum that indicate the presence of: (a) terminal alkyne

C-H stretch 3340-3250 cm⁻¹ and CtripleC stretch 2260-2000 cm⁻¹

List the expected stretching frequency ranges for peaks in an infrared spectrum that indicate the presence of: (d) ketone

C=O stretch 1750-1705 cm⁻¹ (almost always very strong)

What molecular structure features control the intensity of an infrared absorption?

For a stretching vibration, a photon is absorbed only if that vibration results in a change in bond dipole. The bond dipole is a product of the bond length and charge difference of the bonded atoms. As a bond vibrates, the bond length changes, so this criterion is met. The charge difference is determined by the EN of the bonded atoms. If atoms are even slightly different, then a small change in bond dipole occurs, along with corresponding absorption of an infrared photon. Asymmetrical bonds give IR absorptions whereas symmetrical bonds do not

List the expected stretching frequency ranges for peaks in an infrared spectrum that indicate the presence of: (c) alcohol

O-H stretch 3650-3200 cm⁻¹ (usually strong and broad)

Explain why similar functional groups absorb infrared photons of similar energies.

The energy of the photon necessary to excite the molecule to a higher vibrational quantum level is controlled by the masses of the attached atoms and the bond strength. Similar functional groups are made up of similar atoms. This is why we can use IR spectroscopy to determine what functional groups are present or absent in a compound by simply looking at the positions of the IR peaks

What is the fingerprint region of the infrared spectrum? Why is it usually ignored?

The region from about 1450 to 400 cm⁻¹ is called the fingerprint region. It's often complex because it contains many peaks. Ignored because difficult to make precise assignments. However, fingerprint regions of two IR spectra of same compound are identical. Thus, this region of an unknown compound can be compared with the region of a known sample.

Briefly explain the five-zone approach for analysis of an infrared spectrum.

We divide the IR spectrum into 5 zones, based upon the energy of photons absorbed by twelve common functional groups