IR Spectroscopy Concepts

Question 1

Infrared Active

Answer 1

Any molecular vibration that leads to a substantial change in dipole moment is observed in an IR spectrum.

Question 2

Steps for solving infrared spectral problems

Answer 2

Step 1: Check the region around 3000 cm^-1. Absorption in this region is caused by C-H stretching. Absorption is generally to the right of 3000 for sp³ C-H alkane stretching and to the left of 3000 for sp² C-H alkene/aromatic ring stretching.

Step 2: Check for a strong, broad -OH band in the region of 3500. If there is also a peak around 1700 then there may be a carboxyl group, if not it is an alcohol. One or two peaks in the 3500 region at somewhat lower frequency than the -OH may indicate a 2º or 1º amine, respectively.

Step 3: Sharp peak at 1630-1820 indicates a C=O. Aldehyde C-H stretch will be at 2720. Ester will have C-O stretch at 1000-1250.

Question 3

What are the axis labels on an infrared spectrum?

Answer 3

The x-axis : wavenumbers (cm^-1) Read: inverse centimeters

The y-axis: transmittance; 0% at the origin 100% at the top

Question 4

What effect does the weakening/strengthening of a particular bond have on the frequency observed on an IR spectrum?

Answer 4

Weakening the bond will shift the frequency lower (to the right), whereas strengthening the bond will shift the frequency higher (to the left).

Question 5

What can cause the weaking/strengthening of a particular bond in such a way that the IR spectrum frequency is shifted?

Answer 5

Resonance will cause a weaker bond if a contributing structure has a single bond rather than a double bond; this will shift the frequency lower (to the right), a double bond extending from a ring will shift to a higher frequency as ring strain increases due to a decrease in the size of the ring (when comparing two different ring compounds) - this is also due to poor overlap of the p-orbitals due to the smaller angle of the ring bonds - more s character = stronger bond.