Chapter 18: Seeing Good Vibrations: IR Spectroscopy

Students learn that bonds follow vibrational patterns described by Hooke's Law, and that the frequency at which a bond absorbs IR light depends on both the strength of the chemical bond and the masses of the atoms involved. When infrared radiation matches a bond's natural vibrational frequency, absorption occurs and creates observable peaks in an IR spectrum. The chapter explains how IR spectra are displayed with transmittance percentages on the vertical axis and wavenumber measurements in reciprocal centimeters on the horizontal axis, establishing a foundation for interpreting spectroscopic data. The core content focuses on how different functional groups produce characteristic absorption patterns that allow chemists to identify unknown organic compounds. Specific attention is given to distinctive peak patterns such as the broad absorption associated with hydroxyl groups, the characteristic shape of amine stretches, and the sharp, intense absorption of carbonyl functional groups near 1,700 wavenumber units. The chapter also addresses concepts like the fingerprint region of the spectrum, which contains complex overlapping peaks unique to each molecule, and discusses how the magnitude of dipole moment changes during vibration affects peak intensity. Additionally, the material explains infrared-inactive vibrations, where symmetric molecular vibrations do not produce observable spectroscopic signals. A practical reference table of IR absorption frequencies for common functional groups is provided, enabling students to systematically match observed peaks to structural features and develop proficiency in determining molecular structure from spectroscopic evidence.