Topic overview

Start with the big picture

Molecular vibrations produce quantized energy levels. In the harmonic oscillator model, levels are evenly spaced and fundamental transitions follow Δv = ±1; anharmonicity reduces the spacing and allows weak overtones and combination bands. A vibration is infrared active when it changes dipole moment, while Raman activity requires a change in polarizability. Vibrational frequency increases with bond force constant and decreases with reduced mass, so stronger bonds and lighter atoms tend toward higher wavenumbers. Spectra also offer diagnostic clues: group-frequency bands and the fingerprint region help identify molecular features, while amide bands inform analysis of protein structure. Hydrogen bonding broadens and shifts X–H stretching bands. The broader lesson also introduces phenomena and techniques that extend vibrational spectroscopy’s analytical uses.