IR spectroscopy is an analytical method used primarily in chemistry to identify and characterize chemical substances. The technique works by shining infrared light through a sample and measuring the specific frequencies of energy that the material absorbs. Every molecule absorbs this radiation uniquely based on its atomic structure and bonds. This absorption pattern creates a distinct molecular fingerprint, allowing researchers to confirm a compound’s identity or determine the structural features of an unknown material. The modern Fourier Transform Infrared (FT-IR) spectrometer provides fast and accurate data by simultaneously collecting information across the entire infrared spectrum.
The Physics of Molecular Vibration
The foundation of IR spectroscopy is that molecular bonds behave like springs connecting two masses. These bonds are constantly in motion, vibrating at specific natural frequencies. A molecule absorbs energy only if the frequency of the infrared radiation precisely matches one of its natural vibrational frequencies. This absorption excites the molecule, moving it from a lower to a higher vibrational energy state.
Energy transfer requires a change in the molecule’s net dipole moment, which represents the separation of positive and negative charges. Symmetric molecules like nitrogen (\(\text{N}_2\)) and oxygen (\(\text{O}_2\)) do not absorb IR radiation because their motions do not alter their zero dipole moment. Conversely, molecules like carbon monoxide (\(\text{CO}\)) and water (\(\text{H}_2\text{O}\)) are active in the IR spectrum because their vibrations change the charge distribution.
Molecular vibrations are categorized into two types: stretching and bending. Stretching involves a continuous change in the distance between two atoms along the bond axis. These motions can be symmetric (both bonds lengthen and contract simultaneously) or asymmetric (one lengthens while the other contracts).
Bending vibrations involve a change in the angle between bonds and require less energy than stretching motions. These angular movements are subdivided into four types:
- Scissoring.
- Rocking.
- Wagging.
- Twisting.
Scissoring and rocking occur in the molecular plane, while wagging and twisting occur out of the plane. The precise frequency depends on the masses of the atoms and the stiffness of the bond connecting them.
Key Components of an IR Spectrometer
The modern FT-IR spectrometer uses specialized components to generate and measure the infrared beam. The process begins with the infrared radiation source, which must produce a broad range of frequencies. A common source is the Globar, a silicon carbide rod heated to high temperatures to emit intense infrared light. This light beam is then directed toward the instrument’s core.
The heart of the FT-IR spectrometer is the Michelson interferometer, which encodes the spectral information. This device splits the incoming infrared beam into two paths using a beamsplitter. One path reflects off a fixed mirror, while the other reflects off a moving mirror, creating a path difference.
The two beams are recombined at the beamsplitter, where they interfere before passing through the sample compartment. The remaining energy then strikes the detector. Common detectors include the deuterated triglycine sulfate (DTGS) for routine measurements or the mercury cadmium telluride (MCT) for specialized applications. The detector converts the measured light intensity into an electrical signal.
Generating and Analyzing the Spectrum
The operational sequence relies on controlling the path difference of the infrared beams within the interferometer. As the moving mirror travels, the two beams constructively and destructively interfere, producing a complex wave containing information about all infrared frequencies simultaneously. This raw data signal, measured as light intensity versus the position of the moving mirror, is called an interferogram.
The interferogram does not resemble the final spectrum; it is a signal in the time or distance domain. The detector records this rapidly changing interference pattern, representing the entire infrared range in seconds. This ability to measure all frequencies at once is an advantage of FT-IR over older, dispersive instruments.
To transform this raw signal into the final, interpretable spectrum, the computer applies the Fourier Transform (FT). The FT algorithm deconvolutes the complex interferogram, separating the signal into its individual frequency components. This conversion shifts the data from the time or distance domain into the frequency domain, resulting in the final infrared spectrum. The output plots light intensity against wavenumber, a unit of frequency proportional to energy, typically expressed in inverse centimeters (\(\text{cm}^{-1}\)).
Reading the Output: Functional Groups
The resulting infrared spectrum is typically displayed as a plot of percent transmittance (vertical axis) against wavenumber (horizontal axis). Transmittance refers to the amount of light that passes through the sample. The dips or troughs in the plot represent the frequencies of infrared light absorbed by the material, and a strong absorption peak indicates an active functional group or bond.
Interpretation of the spectrum is divided into two main regions. The first is the functional group region (4000 to \(1500\text{ cm}^{-1}\)), used to identify specific chemical groups. For instance, a strong, broad peak around \(3300\text{ cm}^{-1}\) indicates an \(\text{O-H}\) (alcohol or acid) bond, while a sharp, intense peak near \(1700\text{ cm}^{-1}\) suggests a \(\text{C=O}\) (carbonyl) bond.
The second region is the fingerprint region, found below \(1500\text{ cm}^{-1}\). This area contains complex peaks arising from the bending and stretching of single \(\text{C-C}\), \(\text{C-O}\), and \(\text{C-N}\) bonds. While these peaks are difficult to assign individually, the overall pattern is unique to every molecule, similar to a human fingerprint. Matching this region of an unknown sample to a reference spectrum is the most definitive way to confirm identity.