Infrared (IR) spectroscopy is an analytical technique that measures the absorption of infrared radiation by molecules, providing information about molecular vibrations and functional groups. It is essential for identifying chemical bonds, confirming compound identity, and monitoring reactions.
Principle of IR Absorption
Molecules absorb infrared radiation when the frequency of the radiation matches the natural vibrational frequency of a chemical bond, causing the bond to stretch, bend, or rock. For a vibrational mode to be IR-active, the molecular dipole moment must change during the vibration. The spectrum is plotted as transmittance (%) versus wavenumber (cm-1), with the mid-IR region spanning 4000-400 cm-1.
Key Spectral Regions
The IR spectrum is divided into two key regions. The Functional Group Region (4000-1500 cm-1) contains characteristic absorptions for specific bonds such as O-H (3200-3600 cm-1), N-H (3300-3500 cm-1), C=O (1700-1750 cm-1), and C≡N (2200-2260 cm-1). The Fingerprint Region (1500-400 cm-1) provides a complex pattern of absorptions unique to each molecule, used for compound identification by spectral matching.
Instrumentation
A Fourier Transform Infrared (FT-IR) spectrometer uses an interferometer to collect all wavelengths simultaneously, producing a spectrum via Fourier transformation. The Attenuated Total Reflectance (ATR) accessory allows direct measurement of solid or liquid samples without preparation. For transmission mode, sample preparation involves KBr pellets or Nujol mulls for solids and thin films for liquids.
Sample Handling
Solids can be ground with KBr and pressed into a transparent pellet, or measured directly using ATR. Liquids are placed between NaCl or KBr salt plates, which are transparent to IR radiation. Gases are measured in specialized gas cells with long path lengths to enhance absorption.
Applications
IR spectroscopy is used for identification of functional groups in organic synthesis and natural product chemistry, confirmation of polymer composition and degradation products, forensic analysis of paints, fibers, and drugs, and monitoring reaction progress and detecting intermediates in real time.
