Infrared (IR) spectroscopy is an analytical method that provides a unique molecular fingerprint of a chemical compound. The technique measures how molecules absorb infrared light, causing the bonds within the molecule to vibrate more vigorously. Each type of chemical bond possesses specific vibrational frequencies. When the light’s frequency matches a natural bond frequency, energy is absorbed. By recording which frequencies are absorbed, scientists gain insights into the presence of specific structural units, or functional groups, within the compound.
Deciphering the Spectrum Layout
The infrared spectrum is presented as a graph. The horizontal axis, or X-axis, represents the frequency of light and is measured in units called wavenumbers (\(\text{cm}^{-1}\)). Wavenumbers are inversely proportional to wavelength, and the scale is typically drawn decreasing from left to right, ranging from approximately 4000 \(\text{cm}^{-1}\) down to 400 \(\text{cm}^{-1}\).
The vertical axis, or Y-axis, commonly displays the percent transmittance (\(\%T\)). High transmittance means most light passed through without absorption, corresponding to the baseline at the top of the graph. A downward spike, referred to as a “peak” or “absorption band,” indicates the molecule absorbed energy at that wavenumber, resulting in lower transmittance. The lower the transmittance percentage, the stronger the absorption.
The full range is divided into two main areas. The higher wavenumber region (4000 \(\text{cm}^{-1}\) to 1500 \(\text{cm}^{-1}\)) is the functional group region, containing diagnostic peaks for initial identification of major functional groups. The lower wavenumber section holds information used primarily for confirmation.
Identifying Characteristic Functional Groups
The functional group region (4000 \(\text{cm}^{-1}\) to 1500 \(\text{cm}^{-1}\)) is used for initial analysis because it reveals the presence of major functional groups. The position of a peak is determined by the masses of the atoms involved and the stiffness of the bond connecting them. Stronger bonds and lighter atoms vibrate at higher frequencies, leading to higher wavenumbers.
One recognizable peak is the O–H stretch, which appears as a broad, medium-to-strong absorption in the 3200–3600 \(\text{cm}^{-1}\) range for alcohols. The wide, rounded shape of this band is characteristic and results from the hydrogen bonding between molecules. The N–H stretch of amines and amides is found between 3300 and 3500 \(\text{cm}^{-1}\), but it is sharper and less intense than the O–H peak.
The C–H stretching vibrations appear just below and just above the 3000 \(\text{cm}^{-1}\) mark. Peaks below 3000 \(\text{cm}^{-1}\) (2850–2960 \(\text{cm}^{-1}\)) indicate C–H bonds on saturated \(\text{sp}^3\) carbons (alkanes). Conversely, peaks above 3000 \(\text{cm}^{-1}\) (3020–3100 \(\text{cm}^{-1}\)) signal C–H bonds attached to \(\text{sp}^2\) carbons, characteristic of alkenes or aromatic rings.
The carbonyl group (C=O) produces a distinct signal, appearing as a strong, sharp peak between 1650 and 1750 \(\text{cm}^{-1}\). The exact position differentiates between ketones, aldehydes, esters, and carboxylic acids. For instance, a ketone or aldehyde absorbs around 1700–1740 \(\text{cm}^{-1}\), while conjugation shifts the peak to a lower wavenumber. Triple bonds, such as C\(\equiv\)C (alkynes) or C\(\equiv\)N (nitriles), are revealed by weak to medium sharp peaks in the 2100–2260 \(\text{cm}^{-1}\) region.
Interpreting the Fingerprint Region
The fingerprint region is the area below 1500 \(\text{cm}^{-1}\), extending down to 400 \(\text{cm}^{-1}\). This section is complex and characterized by a dense pattern of absorption bands. These peaks arise mainly from the bending vibrations of \(\text{C-C}\), \(\text{C-O}\), and \(\text{C-N}\) single bonds, which are unique to the overall structure of the molecule.
The complexity makes it difficult to assign individual peaks in this area to specific, simple bonds. The pattern of peaks in the fingerprint region acts like a molecular barcode—it is distinct for every compound. Even two molecules with the same functional groups but different overall connectivity, such as structural isomers, will exhibit different patterns in this region. Therefore, this area is primarily used for the confirmation of a compound’s identity by comparing the spectrum to a reference spectrum of a known substance.
A Step-by-Step Interpretation Strategy
A systematic approach begins with the functional group region. The first step is to check the 3200–3600 \(\text{cm}^{-1}\) range for signs of O–H or N–H bonds, noting whether the peak is broad (O–H) or sharp (N–H). Following this, the region around 3000 \(\text{cm}^{-1}\) should be analyzed to determine the hybridization of \(\text{C-H}\) bonds, looking for peaks above or below this boundary.
The second major step involves scanning the 1650–1750 \(\text{cm}^{-1}\) area for the strong, characteristic \(\text{C=O}\) carbonyl stretch. The intensity and sharp nature of this peak provides information about the type of compound present. If a carbonyl is present, its exact position helps refine the structural assignment, such as distinguishing an ester from a ketone.
Next, attention should shift to the 2000–2500 \(\text{cm}^{-1}\) range to check for the presence of triple bonds, which appear as weak to medium, sharp peaks. The final step involves looking for other double bond stretches, like \(\text{C=C}\) (alkene) near 1650 \(\text{cm}^{-1}\), and \(\text{C-O}\) single bond stretches in the 1000–1300 \(\text{cm}^{-1}\) region.
Once major functional groups are identified, the fingerprint region (400–1500 \(\text{cm}^{-1}\)) is used for final confirmation. This comparison with a known reference spectrum ensures the proposed structure matches the unique molecular vibrations of the compound. Always use a reliable correlation table to confirm the precise wavenumber ranges for all observed peaks.