Spectroscopy: NMR, IR and Mass Spectrometry

A-Level Chemistry · Analytical Techniques

Identifying organic molecules

Chemists combine several analytical techniques to work out an unknown structure. Each gives a different piece of the puzzle.

Mass spectrometry

The molecular ion peak (M⁺) — the peak with the highest m/z — gives the relative molecular mass of the compound.
Fragmentation produces smaller peaks; the mass lost between peaks identifies fragments (e.g. loss of 15 = CH₃, loss of 29 = CHO or C₂H₅).
The M+1 peak (from carbon-13) and isotope patterns (e.g. a 1:1 pair 2 apart indicates bromine; 3:1 indicates chlorine) give extra clues.

Infrared (IR) spectroscopy

Bonds absorb IR radiation at characteristic frequencies (making them vibrate). Key absorptions (wavenumber, cm⁻¹):

O–H (alcohol): broad, ~3230–3550
O–H (carboxylic acid): very broad, ~2500–3300
C=O (carbonyl): strong, sharp, ~1680–1750
C–H: ~2850–3100

The fingerprint region (below ~1500 cm⁻¹) is unique to each molecule and can confirm identity by comparison with a database. IR is also used to monitor greenhouse gases.

Nuclear magnetic resonance (NMR)

NMR uses the magnetic properties of nuclei in a strong magnetic field; TMS is the reference standard (0 ppm).

¹³C NMR

The number of peaks = the number of chemically different carbon environments.
The chemical shift (ppm) indicates the type of carbon environment.

¹H (proton) NMR — three pieces of information

1. Number of peaks = number of different hydrogen environments.

2. Integration (peak area) = the ratio of hydrogens in each environment.

3. Splitting (n+1 rule): a peak is split into n+1 lines by n hydrogens on the adjacent carbon (a doublet = 1 neighbour, triplet = 2, quartet = 3). This reveals which groups are next to each other.

Combining techniques

Use mass spec for the molecular mass/formula, IR for functional groups, and NMR for the carbon/hydrogen environments and connectivity — together they pin down the structure.

Worked example

An ¹H NMR spectrum shows a triplet and a quartet in a 3:2 ratio. What group does this suggest?

A triplet (2 neighbours) + quartet (3 neighbours) in a 3:2 ratio is the classic pattern of an ethyl group (CH₃CH₂–). ✓

Common mistakes

Reading the molecular mass from the wrong peak — it's the M⁺ (highest m/z), not the base peak.
Confusing IR broad O–H (alcohol) with the very broad carboxylic-acid O–H.
Applying the n+1 rule to hydrogens on the same carbon rather than adjacent carbons.

Exam tips

Mass spec → molecular mass (M⁺) and fragments (mass differences).
IR → match absorptions to functional groups (especially C=O and O–H).
¹H NMR → count environments (peaks), read ratios (integration), use n+1 splitting for neighbours.

Key facts to remember

Mass spec: M⁺ peak = molecular mass; fragmentation and isotope patterns (Br 1:1, Cl 3:1) give clues.
IR: characteristic absorptions (C=O ~1700, O–H alcohol broad ~3300); the fingerprint region identifies the molecule.
NMR: number of peaks = number of environments; integration = H ratio; n+1 rule splitting shows adjacent hydrogens.