Molecular Vibrations as Chemical Fingerprints
Infrared (IR) spectroscopy probes the vibrational motions of molecules. When infrared radiation passes through a sample, specific frequencies are absorbed, causing bonds to stretch, bend, twist, or rock. The frequencies at which absorption occurs depend on the masses of the atoms involved, the strength of the bonds connecting them, and the overall geometry of the molecule. Because different functional groups absorb at characteristic frequencies, an IR spectrum serves as a powerful tool for identifying the chemical groups present in a compound.
The technique is fast, requires minimal sample preparation, and can analyze solids, liquids, and gases. It has been a cornerstone of chemical identification since the 1940s and remains indispensable in organic chemistry, materials science, forensic science, and environmental monitoring.
Principles of IR Absorption
A molecule absorbs infrared radiation when the frequency of the radiation matches the natural vibrational frequency of a bond, and when the vibration causes a change in the molecule's dipole moment. This second requirement is the selection rule for IR absorption: symmetric vibrations of nonpolar molecules (such as the symmetric stretch of O2 or N2) are IR-inactive because they produce no change in dipole moment.
The vibrational frequency of a bond can be approximated using the harmonic oscillator model: v = (1/2pi) x sqrt(k/mu), where k is the force constant (bond strength) and mu is the reduced mass of the two atoms. Stronger bonds vibrate at higher frequencies, and lighter atoms vibrate at higher frequencies. The O-H bond, which is both strong and involves light hydrogen atoms, absorbs near 3300-3500 cm^-1. The C-C single bond, weaker and involving heavier atoms, absorbs near 800-1200 cm^-1.
A nonlinear molecule with N atoms has 3N - 6 vibrational modes (a linear molecule has 3N - 5). Water, with three atoms, has three vibrational modes: symmetric stretch, asymmetric stretch, and bending. Larger molecules have many more modes, producing complex spectra with dozens or hundreds of absorption bands.
Functional Group Identification
The IR spectrum is conventionally divided into two regions. The functional group region (4000-1500 cm^-1) contains absorptions from stretching vibrations of specific bonds, making it the most diagnostic part of the spectrum for identifying functional groups:
- O-H stretch: Broad absorption centered near 3200-3550 cm^-1 (hydrogen bonding causes broadening). Alcohols show a broad band; carboxylic acids show a very broad, intense absorption from 2500-3300 cm^-1.
- N-H stretch: Medium absorption near 3300-3500 cm^-1. Primary amines show two bands; secondary amines show one.
- C-H stretch: Absorptions near 2850-3000 cm^-1 (sp3 C-H) and 3000-3100 cm^-1 (sp2 C-H).
- C=O stretch: Strong, sharp absorption near 1650-1800 cm^-1. This is often the most prominent band in the spectrum. Ketones absorb near 1715 cm^-1, esters near 1735 cm^-1, amides near 1650 cm^-1.
- C=C stretch: Medium absorption near 1600-1680 cm^-1.
- C-triple-bond-N stretch: Medium absorption near 2210-2260 cm^-1.
The Fingerprint Region
The region below 1500 cm^-1 is called the fingerprint region because the complex pattern of overlapping bending, wagging, and rocking vibrations is unique to each molecule. While individual bands in this region are difficult to assign, the overall pattern can be matched against reference spectra in databases such as the NIST WebBook or SDBS. Even molecules with identical functional groups (such as structural isomers) produce different fingerprint patterns, allowing definitive identification.
Sample Preparation and Instrumentation
Modern IR spectroscopy almost universally uses Fourier transform infrared (FTIR) instruments, which employ a Michelson interferometer rather than a dispersive grating. All wavelengths reach the detector simultaneously, and a mathematical Fourier transform converts the time-domain interferogram into a frequency-domain spectrum. FTIR offers dramatic advantages in speed (a complete spectrum in one second), sensitivity, and signal-to-noise ratio compared to older dispersive instruments.
Samples can be measured in several ways. Attenuated total reflectance (ATR) is the most popular for solids and liquids: the sample is pressed against a crystal (diamond, germanium, or zinc selenide), and the IR beam undergoes total internal reflection within the crystal. The evanescent wave penetrates a few micrometers into the sample, producing an absorption spectrum with minimal preparation. Liquids can also be measured in transmission cells with NaCl or KBr windows (which are transparent to IR). Gases are analyzed in long-path gas cells.
Real-World Applications
IR spectroscopy is used to identify unknown compounds in forensic investigations, verify the identity and purity of pharmaceutical raw materials, monitor industrial processes in real time, and analyze atmospheric gases. Environmental scientists use FTIR to measure concentrations of greenhouse gases like CO2, CH4, and N2O with parts-per-billion sensitivity. Art conservators use portable FTIR instruments to identify pigments, binders, and varnishes in paintings without removing samples. Polymer scientists use IR to determine the composition and degree of crosslinking in plastics and coatings.