Infrared Spectroscopy (Interpretation)

A broad, strong absorption centred around 3200–3550 cm⁻¹ is characteristic of the O–H bond. In alcohols, hydrogen bonding between molecules broadens this peak significantly — a free O–H (in dilute solution) appears sharper near 3600 cm⁻¹. In carboxylic acids, hydrogen bonding is even stronger, producing a very broad absorption that can span from roughly 2500 cm⁻¹ to 3300 cm⁻¹, often described as a 'broad hump'. Carboxylic acids also show a strong C=O stretch near 1700–1725 cm⁻¹, so the combination of both absorptions confirms –COOH.

The carbonyl group (C=O) produces one of the most diagnostic and reliable peaks in IR spectroscopy: a strong, sharp absorption near 1630–1850 cm⁻¹, the exact position depending on the molecular environment. Aldehydes appear at approximately 1720–1740 cm⁻¹ and also show two weak C–H stretches near 2700–2850 cm⁻¹ (the Fermi doublet). Ketones absorb near 1705–1725 cm⁻¹. Esters show a higher-wavenumber absorption around 1735–1750 cm⁻¹ and also a strong C–O stretch near 1000–1300 cm⁻¹. Amides absorb at lower wavenumbers, around 1630–1690 cm⁻¹, because the nitrogen lone pair delocalises into the carbonyl, reducing the bond order and thus the stretching frequency. Conjugation with a C=C also lowers the C=O frequency by about 30–40 cm⁻¹.

N–H bonds absorb in the region 3300–3500 cm⁻¹, similar to O–H but generally narrower and less broad. Primary amines (–NH₂) show two peaks in this region (symmetric and asymmetric N–H stretches), whereas secondary amines (–NH–) show only one. Primary amides also display two N–H peaks near 3300 and 3500 cm⁻¹ combined with the amide C=O around 1650–1690 cm⁻¹. Tertiary amines and tertiary amides lack N–H absorptions altogether, making their absence diagnostically useful.

Virtually all organic molecules show C–H stretching absorptions between 2850 and 3000 cm⁻¹ (sp³ C–H), around 3000–3100 cm⁻¹ (sp² C–H in alkenes and arenes), and near 3300 cm⁻¹ (sp C–H in terminal alkynes). Because these appear in almost every organic spectrum, they are less useful for differentiation unless combined with other evidence. Below 1500 cm⁻¹, the fingerprint region contains C–C, C–O, and C–N single-bond stretches plus a multitude of bending vibrations. This region is unique to each molecule: comparing it with a reference library spectrum confirms identity beyond doubt, which is how forensic scientists and pharmaceutical quality-control labs authenticate compounds.

When interpreting an IR spectrum, follow a logical sequence: (1) Check 3200–3600 cm⁻¹ for broad O–H or N–H stretches. (2) Check 1630–1850 cm⁻¹ for a strong C=O absorption and note its precise position to distinguish aldehyde, ketone, ester, carboxylic acid, or amide. (3) Look for the Fermi doublet near 2700–2850 cm⁻¹ to confirm an aldehyde. (4) Check for C≡N near 2200–2260 cm⁻¹ or C≡C near 2100–2260 cm⁻¹. (5) Examine 1000–1300 cm⁻¹ for strong C–O single-bond stretches (alcohols, esters, ethers). (6) Use the fingerprint region to confirm identity against a known spectrum.

• \tilde{\nu} — Wavenumber (cm⁻¹): The number of wave cycles per centimetre; the x-axis unit in an IR spectrum.
• c — Speed of light (cm s⁻¹): Speed of electromagnetic radiation, approximately 3 × 10¹⁰ cm s⁻¹.
• k — Force constant (N m⁻¹): A measure of bond stiffness; triple bonds have larger k than double bonds, which in turn are larger than single bonds, explaining why triple bonds absorb at higher wavenumbers.
• \mu — Reduced mass (kg): Defined as μ = m₁m₂/(m₁+m₂); lighter atoms vibrate at higher frequencies, so O–H absorbs at much higher wavenumber than O–D.

Problem. An unknown colourless liquid gives an IR spectrum with the following absorptions: a broad strong trough centred at 3300 cm⁻¹, a strong sharp trough at 1715 cm⁻¹, and a trough at 2700–2850 cm⁻¹ (weak, double peak). There is no broad absorption below 3000 cm⁻¹. Identify the functional groups present and suggest a possible compound class.

Answer. The spectrum indicates the presence of an aldehyde functional group (C=O at 1715 cm⁻¹ confirmed by the Fermi doublet near 2700–2850 cm⁻¹) and an alcohol hydroxyl group (broad O–H near 3300 cm⁻¹). The compound is a hydroxyaldehyde.

• NMR Spectroscopy (Introduction) — NMR and IR spectroscopy are complementary techniques: IR identifies functional groups while NMR reveals the carbon–hydrogen framework, and together they can fully elucidate an unknown structure.
• Mass Spectrometry — Mass spectrometry provides molecular mass and fragmentation patterns; combining it with IR data greatly narrows down possible molecular structures.
• Functional Groups in Organic Chemistry — IR spectroscopy is primarily used to detect functional groups; a solid understanding of carbonyl, hydroxyl, amine, and other groups is essential for correct spectral interpretation.
• The Electromagnetic Spectrum — Understanding where infrared radiation sits in the electromagnetic spectrum — and why photon energy must match vibrational energy gaps — underpins the physical basis of IR spectroscopy.
• Carbonyl Compounds (Aldehydes and Ketones) — The C=O stretch is the most diagnostically useful IR absorption; understanding the chemistry of aldehydes, ketones, esters, and carboxylic acids explains the variation in C=O frequency.