Interpreting Infrared Spectra
Start with the big picture
IR-active vibrations produce absorptions when they change a molecule’s dipole moment, while symmetric non-polar stretches may be absent. Spectra are reported in wavenumbers (cm⁻¹): higher values generally correspond to stronger bonds or lighter atoms. To interpret a spectrum, first look for prominent diagnostic absorptions. Broad O–H peaks, N–H bands, carbonyl absorptions, and characteristic triple-bond regions can help narrow functional-group possibilities. Peak position and shape also matter: conjugation, ring strain, and hydrogen bonding can shift absorptions, while broad peaks may indicate hydrogen bonding. The region below 1500 cm⁻¹ contains a complex fingerprint pattern that can support identity matching. Sample preparation and solvent choice may obscure parts of a spectrum, so interpretation benefits from attention to measurement conditions and confirmation with NMR or mass spectrometry.
What you'll learn
- Explain which bond vibrations are IR-active and how wavenumber relates to bond properties.
- Recognize common O–H, N–H, C–H, carbonyl, and triple-bond absorption patterns.
- Describe how conjugation, ring strain, and hydrogen bonding can affect peak positions or shapes.
- Use diagnostic peaks and the fingerprint region as complementary evidence.
- Identify sample-preparation or solvent effects that may obscure spectral regions.
Continue your study
Work through the complete notes and reinforce the topic with the study tools available in the full lesson.
