Infrared Spectroscopy

Infrared (IR) spectroscopy is the workhorse technique for identifying functional groups in organic molecules. When IR radiation passes through a sample, bonds with a changing dipole moment absorb specific frequencies that match their natural vibrational modes — stretches (along the bond axis) and bends (deforming the bond angle). The spectrum plots transmittance against wavenumber (ν̃, in cm⁻¹), where higher wavenumber corresponds to higher energy. This lesson covers the principle of vibrational absorption, the selection rule that distinguishes IR-active from IR-silent modes, the diagnostic functional-group region (1500–4000 cm⁻¹), the fingerprint region (400–1500 cm⁻¹), and how to assemble peak evidence into a confident functional-group assignment. We close with the connection to greenhouse-gas absorption — the same physics that lets you identify a ketone in the lab also explains why CO₂ warms the planet.

Spec mapping (AQA 7405): This lesson maps to §3.3.15 (infrared spectroscopy). It builds directly on §3.1.3 lesson 3 (electronegativity → bond dipole → dipole change on vibration → IR activity), and links forward to L0 of this course (mass spectrometry), L2 (proton NMR), and L6/L7 (combined-technique structure determination), where IR is used alongside MS and NMR to identify unknowns. The data booklet supplied in AQA Paper 2 contains a wavenumber correlation table — students are expected to use it rather than memorise every value, but the broad ranges below must be known by heart. Refer to the official AQA specification document for the exact wording of §3.3.15.

Assessment objectives: AO1 (recall) — the wavenumber ranges for O–H, N–H, C–H, C=O, C=C, C–O, C≡N; the requirement that a vibration cause a dipole-moment change to absorb IR; the units of wavenumber. AO2 (application) — interpret a labelled IR spectrum and assign the functional groups present; distinguish between two candidate structures using the spectrum. AO3 (analysis and evaluation) — resolve close cases (aldehyde vs ketone; alcohol vs amine; ester vs carboxylic acid) using subtle wavenumber differences and shape cues, and integrate IR evidence with mass spectrometry and NMR to determine an unknown structure.

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The Vibrational Principle

Bonds as Springs

A covalent bond is not a rigid link — it behaves like a tiny spring holding two atomic masses together. Stretch the spring and release it: the atoms oscillate about the equilibrium bond length at a characteristic frequency. Bend two bonds joined at an atom: the bond angle oscillates about its equilibrium value. To a very good first approximation, the bond obeys Hooke's law, and the vibrational frequency is given by

ν = (1/2π) √(k/μ)

where k is the bond force constant (a measure of stiffness — stronger bonds, stiffer springs) and μ is the reduced mass, μ = m₁m₂/(m₁ + m₂). Two consequences fall straight out of this expression and explain almost everything you need to know about IR wavenumbers:

• Stronger bonds vibrate at higher wavenumber. Triple bonds (C≡N at 2200–2260 cm⁻¹) appear above double bonds (C=O at 1680–1820 cm⁻¹), which appear above single bonds (C–C in the fingerprint region, 1000–1300 cm⁻¹).
• Lighter atoms vibrate at higher wavenumber. O–H, N–H, and C–H stretches all sit above 2800 cm⁻¹ because the hydrogen atom is so light that μ ≈ m_H. Conversely, C–Cl and C–Br stretches sit deep in the fingerprint region because chlorine and bromine are much heavier than carbon.

These two trends — bond order ↑ and mass ↓ both push wavenumber up — let you reason about an unfamiliar spectrum even when the data booklet doesn't list a specific group.

Wavenumber Units

Wavenumber, ν̃, is defined as ν̃ = 1/λ when λ is measured in centimetres, giving units of cm⁻¹. It is proportional to frequency (ν = c × ν̃) and therefore to photon energy (E = hc × ν̃). Chemists use wavenumber rather than wavelength because it scales linearly with energy: a peak at 3000 cm⁻¹ has exactly twice the photon energy of one at 1500 cm⁻¹. The IR region examined in routine spectroscopy spans 400–4000 cm⁻¹, corresponding to wavelengths of 25–2.5 μm and photon energies of roughly 5–50 kJ mol⁻¹ — far less than visible-light photons (200–300 kJ mol⁻¹) but exactly matched to vibrational quanta.

The Selection Rule: Changing Dipole Moment

A vibration absorbs IR radiation only if it causes a change in the molecular dipole moment. The electromagnetic field of the IR photon couples to the oscillating dipole — no oscillating dipole, no coupling, no absorption. This single rule explains the most important pattern in the field:

• Homonuclear diatomics — H₂, N₂, O₂, Cl₂ — are completely IR-silent. The two atoms are identical, the bond has no dipole moment, and stretching it cannot create one. This is why the bulk of our atmosphere (78% N₂, 21% O₂) is transparent to infrared.
• Heteronuclear diatomics — HCl, CO, NO — are strongly IR-active. The bond has a permanent dipole, and stretching it modulates that dipole.
• Polyatomic molecules need case-by-case analysis of each normal mode. Some modes change the dipole (IR-active), others don't (IR-inactive). CO₂ is the classic example — see the dedicated section below.

Key Definition: A vibrational mode is IR-active if and only if it produces a change in the molecule's electric dipole moment during the motion. The magnitude of the absorption (peak intensity) is proportional to the square of the dipole change, ∂μ/∂Q.

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The Functional-Group Region (1500–4000 cm⁻¹)

The upper half of the IR spectrum — from 1500 cm⁻¹ up to about 4000 cm⁻¹ — contains the diagnostic peaks for the common functional groups taught at A-Level. The ranges below must be memorised; the official AQA data booklet supplies a similar table, but you should know it well enough to interpret spectra under exam pressure.

Bond	Compound class	Wavenumber (cm⁻¹)	Shape / notes
O–H	Alcohol	3200–3550	Broad, rounded
O–H	Carboxylic acid	2500–3300	Very broad, often overlaps C–H
N–H	Amine, amide	3300–3500	Medium; primary amine shows two peaks
C–H	sp³ alkane	2850–3000	Medium–strong, just below 3000
C–H	sp² alkene / aromatic	3000–3100	Just above 3000 — diagnostic
C–H	Aldehyde (–CHO)	~2720 and ~2820	Two weak peaks — diagnostic of CHO
C≡N	Nitrile	2220–2260	Sharp, medium
C≡C	Alkyne	2100–2260	Variable; weak for symmetric alkynes
C=O	Aldehyde	1720–1740	Strong, sharp
C=O	Ketone	1705–1720	Strong, sharp
C=O	Ester	1735–1750	Strong, sharp
C=O	Carboxylic acid	1700–1725	Strong, sharp
C=O	Amide	1640–1690	Strong
C=O	Acid anhydride	1800–1850	Two bands
C=O	Acid chloride	1770–1815	Strong, sharp
C=C	Alkene	1620–1680	Medium, often weak
C–O	Alcohol, ether, ester	1000–1300	Strong

Reading the Hydrogen-Stretch Window (2500–3700 cm⁻¹)

The window above 2500 cm⁻¹ is the first thing to scan. Hydrogen's tiny mass (μ ≈ 1) puts every X–H stretch up here, but the four classes — O–H, N–H, C–H, and the special CHO doublet — all have distinct signatures:

• Alcohol O–H appears as a broad, rounded peak between 3200 and 3550 cm⁻¹. The breadth comes from hydrogen bonding: in liquid samples, every O–H is hydrogen-bonded to varying degrees, and each different bond length shifts the wavenumber slightly. The result is a continuous distribution that looks like a smooth hill.
• Carboxylic-acid O–H is so heavily hydrogen-bonded (acids exist as cyclic dimers in the liquid state, with two simultaneous O–H···O=C bridges) that the peak sprawls from about 2500 to 3300 cm⁻¹, and almost always swallows the underlying sp³ C–H peaks at 2900 cm⁻¹. If you see a broad mound stretching well below 3000 cm⁻¹, you are looking at an acid.
• N–H stretches are narrower than O–H because nitrogen is less electronegative and hydrogen-bonding is weaker. Primary amines (–NH₂) show a characteristic two-peak pattern from the symmetric and asymmetric N–H stretches; secondary amines (–NHR) show one peak; tertiary amines (–NR₂) show no N–H peak at all.
• C–H stretches are essentially universal in organic compounds — almost every sample shows a peak near 2900 cm⁻¹. The diagnostic split is above vs below 3000 cm⁻¹: sp³ C–H (alkane) sits just below 3000; sp² C–H (alkene or aromatic) sits just above. An aldehyde additionally shows two weak peaks at roughly 2720 and 2820 cm⁻¹ from the unusual –CHO C–H stretch — a very useful tell.

The C=O Window (1670–1820 cm⁻¹)

The carbonyl peak is the loudest, sharpest, and most diagnostic single feature in most spectra. Its exact position narrows the carbonyl class to within ~30 cm⁻¹:

• 1735–1750 — ester (highest of the common carbonyls; the electronegative ester oxygen withdraws electron density, stiffening the C=O bond)
• 1720–1740 — aldehyde
• 1705–1725 — ketone or carboxylic acid (ketones in pure form sit slightly higher; acids are usually identified by the broad 2500–3300 cm⁻¹ O–H mound)
• 1640–1690 — amide (lowest, because the nitrogen lone pair donates into C=O, weakening it)
• 1800–1850 — acid anhydride (two bands from symmetric and asymmetric stretches)
• 1770–1815 — acid chloride

You won't usually pin the class from C=O alone — you'll combine it with the H-stretch evidence. Acid = broad 2500–3300 + C=O ~1710. Ester = no O–H + C=O ~1740 + C–O ~1200. Aldehyde = CHO doublet at 2720/2820 + C=O ~1730.

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The Fingerprint Region (400–1500 cm⁻¹)

Below 1500 cm⁻¹ the spectrum becomes a dense forest of overlapping peaks from C–C, C–O, C–N stretches and a wide variety of bending and skeletal modes that couple together across the whole molecule. Individual peaks are difficult to assign, but the overall pattern is unique to each compound — like a fingerprint, no two compounds have identical fingerprint regions. Forensic and quality-control labs identify substances by digitally matching the fingerprint region against a reference spectrum from a database (SDBS, NIST WebBook).

Key Definition: The fingerprint region (~400–1500 cm⁻¹) contains overlapping skeletal and bending modes whose collective pattern is unique to the compound. A-Level interpretation focuses on functional groups in the 1500–4000 cm⁻¹ region; the fingerprint region is used for confirmation by database comparison.

A few useful peaks do sit in the fingerprint region — most notably C–O at 1000–1300 cm⁻¹ (alcohols, ethers, esters) and the aromatic C–H out-of-plane bends at 700–900 cm⁻¹ (used to determine the substitution pattern on a benzene ring at undergraduate level). For A-Level you only need to recognise C–O.

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Worked Spectrum Interpretation

Example 1 — Ethanoic acid (CH₃COOH)

An IR spectrum shows:

• Very broad mound stretching from 2500 to 3300 cm⁻¹ (swamping the C–H peaks)
• Strong, sharp peak at 1710 cm⁻¹
• Sharp peak at ~1290 cm⁻¹

The broad 2500–3300 cm⁻¹ feature is the carboxylic-acid O–H dimer mound. The 1710 cm⁻¹ peak is the C=O of the acid. The 1290 cm⁻¹ peak is the C–O stretch. All three together confirm a carboxylic acid. With Mᵣ = 60 from mass spec, only ethanoic acid (CH₃COOH) fits.

Example 2 — Following propan-1-ol → propanal → propanoic acid by IR

Track the same carbon skeleton through two sequential oxidations:

• Propan-1-ol (CH₃CH₂CH₂OH): broad O–H at ~3350 cm⁻¹, C–O at ~1050 cm⁻¹, no C=O. Alcohol signature.
• Propanal (CH₃CH₂CHO): O–H gone, strong C=O at ~1730 cm⁻¹ appears, plus the diagnostic CHO doublet at 2720/2820 cm⁻¹. Aldehyde signature.
• Propanoic acid (CH₃CH₂COOH): broad O–H reappears, but now as the 2500–3300 cm⁻¹ acid mound, with C=O at ~1710 cm⁻¹. Acid signature.

The same spectrum, run before and after a reaction, is one of the most powerful diagnostic uses of IR — you literally watch the functional group transform.

Example 3 — Identifying an ester from IR alone

A liquid sample with fruity smell gives an IR with:

• No broad O–H, no N–H
• Strong C–H at ~2960 cm⁻¹ (sp³ alkyl), all below 3000 cm⁻¹ (no alkene/aromatic)
• Strong, sharp C=O at 1740 cm⁻¹ (note: high — at the ester end of the C=O window)
• Strong peak at ~1175 cm⁻¹ (C–O)

The combination of high-wavenumber C=O (~1740) plus C–O (~1175) plus the absence of any O–H is the textbook ester pattern. Likely candidate: ethyl ethanoate or a small ester — confirm with mass-spec Mᵣ.

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Distinguishing Similar Functional Groups

The questions that separate B from A* candidates are usually "given two plausible structures, use the IR to choose between them." Three classic discriminations follow.

Aldehyde vs ketone

Both have a strong, sharp C=O between 1700 and 1740 cm⁻¹ — the ~20 cm⁻¹ wavenumber gap (aldehyde slightly higher at 1720–1740, ketone slightly lower at 1705–1720) is real but not reliable in a noisy spectrum or a printed exam paper. The clincher is the CHO doublet at 2720 and 2820 cm⁻¹ — two weak peaks just below 3000 that only an aldehyde possesses. Look for those; if absent, it's a ketone.

Carboxylic acid vs ester

Both have a strong C=O around 1710–1750 cm⁻¹. The clincher is the broad 2500–3300 cm⁻¹ O–H mound — present for the acid (because of the dimer), absent for the ester. An ester additionally shows a strong, sharp C–O stretch near 1175 cm⁻¹, slightly more intense than the acid's C–O at ~1290 cm⁻¹, but the H-stretch window is the safer call.

Alcohol vs primary amine

Both have a peak in the 3200–3550 cm⁻¹ window. The clincher is shape and multiplicity: the alcohol O–H is a single broad rounded mound; the primary amine N–H shows a characteristic two-peak split (symmetric + asymmetric stretches of –NH₂) that is narrower overall. If you only see one peak in that window with rounded broad shape, it's an alcohol; if you see a doublet with sharper features, it's a primary amine.

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CO₂, Greenhouse Gases, and the Symmetric Stretch

CO₂ is the textbook example of the dipole-change selection rule, and the same physics that makes IR a laboratory tool makes CO₂ a greenhouse gas.

CO₂ is a linear molecule, O=C=O, with no permanent dipole moment — the two C=O bond dipoles point in exactly opposite directions and cancel exactly. But CO₂ has three vibrational normal modes (linear triatomics have 3N − 5 = 4 modes, with the bend doubly degenerate):

• Symmetric stretch (both C=O bonds lengthen and contract together, in phase): the two bond dipoles remain equal and opposite throughout the motion. The molecular dipole is always zero. ∂μ/∂Q = 0. IR-inactive.
• Asymmetric stretch (one C=O lengthens while the other shortens): the two bond dipoles no longer cancel — one is larger, one is smaller, leaving a net oscillating dipole along the molecular axis. IR-active. Absorbs at ~2349 cm⁻¹.
• Bending mode (the molecule bends out of linearity in either of two perpendicular planes — doubly degenerate): the two bond dipoles, no longer collinear, leave a net dipole perpendicular to the original axis. IR-active. Absorbs at ~667 cm⁻¹.

The Earth's surface emits thermal infrared radiation centred on ~600–700 cm⁻¹ (the peak of a 288 K blackbody). CO₂'s 667 cm⁻¹ bending mode sits squarely in this window, absorbing outgoing infrared and re-emitting it isotropically — including downward, back to the surface. This is the molecular origin of CO₂'s contribution to the greenhouse effect. H₂O (which has both bending and stretching IR-active modes) is the dominant natural greenhouse gas; CH₄ (C–H stretches at ~3000 cm⁻¹ and bends at ~1300 cm⁻¹) is a smaller but per-molecule far more potent anthropogenic contributor. N₂ and O₂, IR-silent as homonuclear diatomics, contribute nothing despite making up 99% of the atmosphere.

Key Point: It is the changing dipole moment that matters, not the permanent dipole moment. CO₂ has zero permanent dipole, yet two of its three vibrational modes are IR-active. The asymmetric stretch and bending mode both create transient dipoles during the motion — that is sufficient for absorption.

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Practical Skills: Recording an IR Spectrum

How a sample is prepared depends on its state:

• Solids are traditionally ground with dry potassium bromide (KBr) and compressed into a transparent disc; KBr is IR-transparent across the working range, leaving only the sample's peaks. Modern instruments more often use ATR (Attenuated Total Reflection) sampling, where the solid is pressed against a diamond or ZnSe crystal and the IR beam interrogates the surface directly via an evanescent wave — no sample preparation at all.
• Liquids are squeezed as a thin film between two NaCl or KBr salt plates (glass absorbs IR and cannot be used as a window).
• Gases are measured in a long-path-length gas cell (typically 10 cm) to give enough absorbing molecules in the beam.
• Solutions are run in a solvent that has minimal absorption in the region of interest — CCl₄ and CS₂ used historically (now restricted), CDCl₃ in research labs.

Modern instruments are Fourier-Transform Infrared (FTIR) spectrometers: the entire IR range is acquired simultaneously via an interferometer (a moving mirror generates an interferogram), and the spectrum is recovered by Fourier transform. FTIR is far faster, more sensitive, and higher-resolution than the older dispersive instruments (which used a prism or grating to scan one wavenumber at a time). A research-grade FTIR can record a spectrum from 4000 to 400 cm⁻¹ in under a second.

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