Identifies functional groups by absorption of IR radiation causing molecular vibrations.
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Infrared spectroscopy measures the absorption of infrared radiation, usually between 4000 and 400 cm-1, by molecules whose bonds vibrate at the same frequency as the light. A vibration is IR active only when it changes the molecular dipole moment, so polar bonds such as C=O and O-H usually give strong absorptions. Stretching and bending vibrations produce peaks whose positions depend on the strength and reduced mass of the bond, following a Hooke's law relation in which frequency is proportional to the square root of k/mu. Because specific functional groups absorb at characteristic positions, IR is a fast way to identify them in an unknown. The spectrum is usually plotted as transmittance against wavenumber.
Key ideas: Wavenumber (cm-1) is 10^4 divided by wavelength in micrometres. Hooke's law: wavenumber = (1/2 pi c) sqrt(k/mu). Stronger bonds and lighter atoms vibrate at higher wavenumber, so triple bonds exceed double bonds, which exceed single bonds. Prepare solids as KBr discs or use ATR. Water and CO2 in air add interfering peaks, so run a background.
Common mistakes and connections: Students expect every bond to appear, but symmetrical vibrations that do not change dipole moment (such as the C=C of symmetrical alkenes or N2) are IR inactive. Intensity varies with bond polarity. Do not assign peaks from the fingerprint region alone. Read the spectrum from the high wavenumber end, starting with the O-H, N-H, C-H and C=O regions. Related to Raman, symmetry and selection rules.
An acetone-like ketone spectrum is dominated by the strong C=O stretch near 1715 cm-1, with C-H stretches near 2960 cm-1. The interactive lesson shows this as a live animation or chart.
A strong peak near 1715 cm-1 indicates a carbonyl group, most often a ketone.
Identifies functional groups by absorption of IR radiation causing molecular vibrations.
Infrared Spectroscopy is a intermediate topic in Analytical, Lab & Spectroscopy. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Analytical, Lab & Spectroscopy first: Beer-Lambert Law, Colorimetry, Calibration Curve.
The next topics in Analytical, Lab & Spectroscopy are Characteristic IR Absorptions and Fingerprint Region.
Yes. The interactive lesson includes a self-check question on Infrared Spectroscopy, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Infrared Spectroscopy is topic 41 of 99 in Analytical, Lab & Spectroscopy.