Infrared Spectroscopy

Spec Mapping — OCR H432 Module 4.2.4(a)-(b) — Infrared spectroscopy, covering bond vibration as the physical basis of IR absorption (stretching and bending modes); the wavenumber unit (cm⁻¹) and the right-to-left axis convention; the selection rule that only vibrations producing a change in dipole moment absorb IR; the OCR data sheet correlation table for the diagnostic functional groups (O-H broad 3200-3550 alcohol, O-H very broad 2500-3300 carboxylic acid, N-H 3300-3500, C-H 2850-3100, C=O 1620-1820, C=C 1620-1680, C-O 1000-1300, C≡C 2100-2260, C≡N 2220-2260); the distinction between the functional-group region (>1500 cm⁻¹) and the fingerprint region (<1500 cm⁻¹) used for compound matching against reference spectra; the environmental application of IR absorption to greenhouse-gas physics; combined use of IR with mass spectrometry (Lesson 10) for structure determination of unknowns (refer to the official OCR H432 specification document for exact wording).

Infrared (IR) spectroscopy is one of the two routinely-examined analytical techniques in OCR A-Level Chemistry — the other is mass spectrometry, developed in the next lesson. IR exploits a beautifully direct piece of physics: every covalent bond behaves like a tiny harmonic oscillator with a natural vibrational frequency determined by the bond force constant and the reduced mass of the two atoms it joins. When the molecule is exposed to infrared electromagnetic radiation, photons whose energy exactly matches the vibrational energy spacing of a particular bond are absorbed and converted into extra vibrational motion, producing a characteristic dip in the transmitted IR beam at that frequency. Because the C=O bond of a carbonyl is stiffer than the C-O bond of an alcohol, and because the O-H bond of a carboxylic acid is strongly hydrogen-bonded into a dimer (broadening its absorption dramatically), each functional group leaves a recognisable signature in the IR spectrum. The OCR data sheet provides the correlation table; the A-Level student's job is to read off which functional groups are present and combine that information with mass spectrometry, NMR, and the molecular formula to identify the unknown compound. This lesson develops the physical principle (Hooke's-law analogy + dipole-change selection rule), the IR correlation table for the key functional groups, the distinction between the functional-group region (4000-1500 cm⁻¹) and the fingerprint region (1500-400 cm⁻¹), the environmental application to greenhouse-gas warming (IR-active polyatomic gases absorb terrestrial outgoing radiation; symmetric homonuclear diatomics like N₂ and O₂ are IR-inactive and do not warm), and four worked examples on identifying unknowns and following reactions by IR.

Key Definition: Infrared absorption occurs when a molecular vibration (a bond stretch or bend) increases the molecule's dipole moment and the photon energy matches the spacing between vibrational quantum levels of that mode. The absorbed energy is converted to extra vibrational motion; the transmitted IR beam shows a dip at the corresponding wavenumber. Wavenumber $\tilde{\nu}$ν~ in cm⁻¹ is the reciprocal of wavelength in cm; higher wavenumber means higher photon energy.

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1. The Principle — Bonds Vibrate

Every covalent bond behaves like a tiny spring connecting two masses. It can stretch (the two atoms moving closer together and further apart along the bond axis) and bend (the bond angle changing). Each type of vibration has a characteristic natural frequency, which depends on:

• The masses of the two atoms (heavier atoms vibrate more slowly).
• The strength of the bond (stiffer spring → higher frequency).

Mathematically, the frequency of a simple stretch is given by:

$\nu = \frac{1}{2\pi}\sqrt{\frac{k}{\mu}}$ν=2π1​μk​​

where k is the bond force constant and μ is the reduced mass. You do not need this equation for the exam but it explains the trends: strong bonds (C=O, C≡C) absorb at high wavenumbers, weak bonds or heavy atoms (C–Cl, C–Br) absorb at low wavenumbers.

1.1 What Is "Absorption"?

When a bond is exposed to infrared radiation (wavelengths roughly 2.5–25 μm), it can absorb a photon whose energy exactly matches the spacing between its vibrational energy levels. The photon's energy is converted into extra vibrational motion. This shows up as a dip in the transmitted IR signal at that frequency.

The horizontal axis of an IR spectrum is wavenumber in cm⁻¹ (a weird but useful unit: wavenumber = 1 / wavelength in cm, so higher wavenumber = higher frequency = higher energy). The axis runs from 4000 cm⁻¹ on the left to 400 cm⁻¹ on the right — opposite to most other axes.

1.2 Selection Rule: Dipole Change

A bond only absorbs IR if its vibration causes a change in dipole moment. Perfectly non-polar bonds (H–H, symmetrical O=C=O stretch in one specific mode) are "IR silent". In practice, almost every bond in an organic molecule has some dipole and shows up in the IR.

Useful consequence: Homonuclear diatomics like N₂ and O₂ are IR-inactive, which is why they don't trap IR in the atmosphere. CO₂ is IR-active in its bending and asymmetric stretching modes — that is what makes it a greenhouse gas.

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2. The Correlation Table — Key Functional Groups

The core skill at A-Level is reading off which functional groups are present from the position of absorption peaks. You must be able to use the OCR data sheet correlation table — or reproduce the key values from memory:

Bond	Location in molecule	Wavenumber / cm⁻¹	Appearance
O–H	Alcohol (free)	3550–3700	Sharp
O–H	Alcohol, H-bonded	3230–3550	Broad
O–H	Carboxylic acid	2500–3300	Very broad, often stretches most of the high-ν region
N–H	Amine, amide	3300–3500	Medium, sometimes two peaks (NH₂)
C–H	Alkane, alkene, arene	2850–3100	Medium
C≡C	Alkyne	2100–2260	Weak/medium
C≡N	Nitrile	2220–2260	Medium, sharp
C=O	Aldehyde, ketone, acid, ester, amide	1630–1820	Strong, sharp — very diagnostic
C=C	Alkene	1620–1680	Weak/medium
C–O	Alcohol, ester, ether	1000–1300	Strong

2.1 The Three Most Diagnostic Peaks

In an A-Level exam question, the peaks to look for first are:

1. C=O around 1700 cm⁻¹ — strong, sharp. If present, you have an aldehyde, ketone, acid, ester or amide. If absent, rule all five out.
2. Broad O–H somewhere between 2500 and 3550 cm⁻¹ — if narrow and around 3500, it is an alcohol. If very broad and stretching down to 2500, it is a carboxylic acid.
3. C–H just below 3000 cm⁻¹ — present in almost every organic molecule, so not very diagnostic on its own, but indicates the presence of CH bonds.

graph TD
  A[Look at spectrum] --> B{C=O around 1700?}
  B -->|Yes| C{O-H broad 2500-3300?}
  B -->|No| D[No carbonyl - look for alcohol/amine/alkene/CN]
  C -->|Yes| E[Carboxylic acid]
  C -->|No| F{O-H 3230-3550?}
  F -->|Yes| G[No - inconsistent, could be ester with residual moisture]
  F -->|No| H[Aldehyde, ketone, ester or amide]
  D --> I{O-H 3200-3600?}
  I -->|Broad| J[Alcohol]
  I -->|Two peaks around 3300-3500| K[Primary amine]
  I -->|Absent| L[Hydrocarbon or haloalkane]

2.2 Distinguishing Acid from Alcohol

Both contain –OH, but the carboxylic acid O–H peak is very broad and stretches from ~3300 down to ~2500 cm⁻¹, often overlapping and burying the C–H region. The alcohol O–H is narrower and confined to 3230–3550 (broad) or 3550–3700 (sharp, rare). The extra broadening in acids comes from strong hydrogen bonding in dimer form.

2.3 Distinguishing Aldehyde from Ketone

Both show C=O around 1700, but aldehydes have a characteristic weak, doubled C–H stretch around 2700–2900 cm⁻¹ from the H on the carbonyl carbon. If you see that, it is an aldehyde.

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3. The Fingerprint Region

Below about 1500 cm⁻¹ lies the fingerprint region — a dense forest of small peaks from bending vibrations and C–C, C–N, C–O skeletal vibrations. Unlike the functional group region (1500–4000 cm⁻¹), the fingerprint region is:

• Too complex to interpret peak-by-peak at A-Level.
• Unique to each compound — it is the molecule's "fingerprint".

3.1 How It Is Used

If you have a known reference spectrum of pure compound X, and an unknown spectrum that matches peak-for-peak in the fingerprint region, the unknown is X. Commercial IR databases (Sadtler, NIST) contain hundreds of thousands of reference spectra, and software matches them automatically.

Key use: The fingerprint region confirms a compound's identity by matching against a known reference spectrum. The functional group region (above 1500 cm⁻¹) is used to identify which functional groups are present.

graph LR
  A["IR spectrum<br/>4000 to 400 cm-1"] --> B["Functional group region<br/>4000-1500 cm-1"]
  A --> C["Fingerprint region<br/>1500-400 cm-1"]
  B --> D["Identifies bonds and functional groups<br/>O-H, C=O, C-H, etc"]
  C --> E["Unique to each compound<br/>used for identification by matching to reference"]

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4. Environmental Application: IR and Greenhouse Gases

The same physics explains why certain atmospheric gases are greenhouse gases. The Sun's peak emission is in the visible, but the Earth re-radiates absorbed energy as IR (~10 μm wavelength, around 1000 cm⁻¹). Any molecule with a bond absorbing in that region traps outgoing radiation.

• CO₂ absorbs strongly in the bending mode around 667 cm⁻¹ and in the asymmetric stretch around 2349 cm⁻¹.
• H₂O absorbs strongly across the fingerprint region.
• CH₄ has strong C–H stretches.
• CFCs and HFCs absorb in the C–F and C–Cl region.

All of these are IR-active because their vibrations change their dipole moment. By contrast, O₂ and N₂ are symmetric diatomics and do not absorb IR — that is why they are not greenhouse gases despite being the main atmospheric constituents.

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5. Worked Example — Identifying an Unknown

An unknown organic compound has the molecular formula C₃H₆O₂ and gives the following key peaks in its IR spectrum:

• A very broad peak from 2500–3300 cm⁻¹
• A strong, sharp peak at 1715 cm⁻¹
• A strong peak at 1280 cm⁻¹

Identify the compound and draw its structural formula.

Answer:

• The very broad 2500–3300 cm⁻¹ is a carboxylic acid O–H.
• The strong, sharp 1715 cm⁻¹ is a C=O.
• Together, these two confirm a carboxylic acid.
• The 1280 cm⁻¹ peak is a C–O stretch, consistent with an acid.

With three carbons, two oxygens, and six hydrogens, plus the need for –COOH, the compound is propanoic acid, CH₃CH₂COOH.

(Check: 3 C + 6 H + 2 O = C₃H₆O₂ ✓.)

Ethanoic acid would only have 2 carbons, and 2-hydroxypropanal would lack the broad acid O–H. Propanoic acid is the fit.

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6. Worked Example — Distinguishing Isomers

You have two unknown samples, each with formula C₃H₆O: one is propanal, the other is propan-2-one (propanone, acetone). Explain how infrared spectroscopy could distinguish them.

Answer:

Both compounds show a strong C=O peak close to 1710–1720 cm⁻¹, so the C=O alone does not distinguish them.

Look for:

1. Aldehyde C–H "doublet" around 2700–2900 cm⁻¹. Propanal has an H on the carbonyl carbon and shows this weak doubled peak. Propanone has no such H and does not show it.
2. Fingerprint region. The two compounds have distinct fingerprint patterns; matching against library spectra would give an unambiguous identification.

Therefore an absorption in the 2700–2900 cm⁻¹ region is the most diagnostic difference in the functional group region.

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7. Worked Example — Following a Reaction by IR

A student oxidises propan-1-ol to propanoic acid by refluxing with acidified potassium dichromate(VI). Describe the changes they would expect to see in the IR spectrum of the sample at the start and end of the reaction.

Answer:

Start (propan-1-ol):

• Broad O–H 3230–3550 cm⁻¹ (alcohol).
• C–H ~2900 cm⁻¹.
• No C=O around 1700 cm⁻¹.
• C–O ~1050 cm⁻¹.

End (propanoic acid):

• Very broad O–H 2500–3300 cm⁻¹ (acid), possibly overlapping C–H.
• Strong, sharp C=O around 1715 cm⁻¹ (new peak — diagnostic of successful oxidation).
• C–O ~1280 cm⁻¹.

The key change is the appearance of a strong C=O peak at ~1715 cm⁻¹ that was not present in the starting alcohol, and the broadening of the O–H peak to extend down below 3000 cm⁻¹. Together these confirm conversion to the carboxylic acid.

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8. IR Spectrum Structure

A typical IR spectrum for an A-Level exam has:

• x-axis: wavenumber in cm⁻¹, running right to left from 4000 to about 400.
• y-axis: percentage transmittance (T), running from 0 % at the bottom to 100 % at the top.
• Peaks are drawn as downward dips (low transmittance = high absorption).
• The most useful region at A-Level is 1500-4000 cm⁻¹ (functional groups); below 1500 is the fingerprint region.

4000 3000 2000 1500 500 wavenumber / cm-1