11.1 Infrared Spectroscopy

LEARNING GOALS

After   Chapter 11.1,   you will    be  able    to:

Predict the IR  peaks   for common  organic functional  groups, including   ketones,    carboxylic

acids,  and alcohols

Recall  the conditions  in  which   IR  spectroscopy    is  generally   used,   and what    it  is  used    to

measure

Infrared (IR) spectroscopy measures molecular vibrations, which can be seen as bond stretching,
bending, or combinations of different vibrational modes. To record an IR spectrum, infrared light is
passed through a sample, and the absorbance is measured. By determining what bonds exist within
a molecule, we hope to infer the functional groups in the molecule.

INTRAMOLECULAR VIBRATIONS AND ROTATIONS

The infrared light range runs from λ = 700 nm to 1 mm, but the useful absorptions for spectroscopy
occur at wavelengths of 2500 to 25,000 nm. On an IR spectrum, we use an analog of frequency called
wavenumber. The standard range corresponding to 2500 to 25,000 nm is 4000 to 400 cm–1. When
light of these wavenumbers is absorbed, the molecules enter excited vibrational states. Four types
of vibration that can occur are shown in Figure 11.1. Others include twisting and folding.

Figure 11.1. Molecular  Vibrations  Measured    by  Infrared    (IR)    Spectroscopy

Bond    bending and stretching; twisting    and folding can also    occur.