# Analytical Techniques and Spectroscopy
Modern chemists use instrumental analysis to identify unknown compounds. At A-Level, you need to interpret data from mass spectrometry, infrared spectroscopy, and NMR spectroscopy, and combine evidence from multiple techniques to deduce structures.
1. Mass Spectrometry (MS)
How It Works
- Sample is vaporised and ionised (usually by electron impact)
- Ions are accelerated and separated by mass-to-charge ratio ()
- Detector measures abundance of each value
Key Features
- Molecular ion peak (): highest significant value = molecular mass
- Base peak: most abundant ion (tallest peak, set to 100%)
- Fragmentation pattern: molecule breaks into characteristic pieces
Common Fragments
| loss | Fragment lost |
|---|---|
| 15 | CH₃ |
| 17 | OH |
| 29 | CHO or C₂H₅ |
| 31 | CH₃O |
| 45 | CHO₂ or C₂H₅O |
2. Infrared Spectroscopy (IR)
Bonds in molecules absorb IR radiation at specific frequencies, causing vibrations (stretching/bending).
Key Absorptions
| Bond | Wavenumber (cm⁻¹) | Appearance |
|---|---|---|
| O−H (alcohol) | 3200–3550 | Broad |
| O−H (carboxylic acid) | 2500–3300 | Very broad |
| N−H | 3300–3500 | Medium |
| C−H | 2850–3100 | — |
| C=O | 1680–1750 | Strong, sharp |
| C−O | 1000–1300 | — |
| C=C | 1620–1680 | — |
Fingerprint Region (below ~1500 cm⁻¹)
The region below ~1500 cm⁻¹ is unique to each molecule — like a fingerprint. It's used to confirm identity by comparison with a database.
Applications
- Identifying functional groups in unknown compounds
- Monitoring reactions (appearance/disappearance of absorptions)
- Breathalyser: detecting ethanol via C-H and O-H absorptions
- Detecting atmospheric pollutants (CO₂, greenhouse gases)
3. Nuclear Magnetic Resonance (NMR) Spectroscopy
¹H NMR (Proton NMR)
¹H NMR tells you about the hydrogen atoms in a molecule.
Chemical shift (): Position on the x-axis (ppm). Depends on the electronic environment of the hydrogen.
| Environment | Chemical Shift (ppm) |
|---|---|
| R-CH₃ (alkyl) | 0.7–1.6 |
| R-CH₂-R | 1.2–1.8 |
| R-CH₂-Cl/Br | 3.0–4.0 |
| R-O-CH | 3.3–4.3 |
| R-CHO (aldehyde) | 9.0–10.0 |
| Ar-H (aromatic) | 6.5–8.0 |
| R-COOH | 10.0–12.0 |
| R-OH | 0.5–6.0 (variable) |
Interpreting ¹H NMR
- Number of peaks = number of different hydrogen environments
- Chemical shift = type of environment
- Integration (area under peak) = relative number of H in each environment
- Splitting pattern = number of H on adjacent carbons (n+1 rule)
Splitting (Spin-Spin Coupling)
| Adjacent H atoms | Splitting Pattern |
|---|---|
| 0 | Singlet (s) |
| 1 | Doublet (d) |
| 2 | Triplet (t) |
| 3 | Quartet (q) |
n+1 rule: A peak is split into (n+1) peaks, where n is the number of equivalent H on adjacent carbons.
Example: Ethanol (CH₃CH₂OH)
- CH₃: δ ~1.2, triplet (3H) — split by 2 adjacent H on CH₂
- CH₂: δ ~3.7, quartet (2H) — split by 3 adjacent H on CH₃
- OH: δ ~2.6, singlet (1H) — no splitting (rapid exchange)
D₂O Shake
Adding D₂O to a sample causes OH and NH peaks to disappear (H is exchanged for D). This identifies OH/NH protons.
4. ¹³C NMR
- Each peak represents a different carbon environment
- No splitting (in routine spectra)
- Simpler than ¹H NMR
- Number of peaks = number of different carbon environments
5. Combined Techniques
To identify an unknown compound, combine evidence:
- MS: molecular mass ( peak) and formula
- IR: functional groups present (e.g. C=O, O-H, N-H)
- ¹H NMR: number and type of H environments, integration ratios, splitting
- ¹³C NMR: number of different C environments
6. Practice Questions
- A mass spectrum shows and fragments at = 45, 43, 31, 29, 15. The IR shows a broad O-H and strong C=O. Deduce the structure.
- A ¹H NMR spectrum shows: singlet at δ 2.1 (3H) and singlet at δ 3.7 (3H). Suggest a structure.
- Explain how a D₂O shake helps identify O-H groups in NMR.
- An IR spectrum shows a strong absorption at 1715 cm⁻¹ but no broad O-H. What functional group is present?
- Compound X has formula C₃H₆O. Its ¹H NMR shows a quartet and a triplet. Its IR shows C=O. Identify X.
Want to check your answers and get step-by-step solutions?
Summary
- Mass spectrometry: molecular mass () and fragmentation pattern
- IR spectroscopy: functional groups from absorption frequencies
- ¹H NMR: number of H environments, integration, splitting (n+1 rule)
- ¹³C NMR: number of different C environments
- Combined techniques are needed to fully identify unknown compounds
- D₂O shake removes OH/NH peaks from NMR spectra