IR vs NMR Spectroscopy

Infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy are the two techniques organic chemists use most to find out what they’ve made. They’re often taught side by side, and students sometimes treat them as interchangeable. They’re not. IR answers “which functional groups are here?” quickly and cheaply; NMR answers “how are the atoms connected?” in far more detail. Knowing what each can and can’t tell you is the key to solving structure problems efficiently.

Quick comparison

Feature	Infrared (IR)	NMR
Radiation	infrared (wavenumbers about 4000–400 cm⁻¹)	radio waves, with the sample in a strong magnetic field
What absorbs	bonds that change dipole as they vibrate	nuclei with spin, most often ¹H and ¹³C
Main information	functional groups present	number and type of H and C environments, their ratio and connectivity
Typical output	% transmittance against wavenumber; downward peaks	signals against chemical shift (ppm)
Sample	solid, liquid or gas; often measured directly (ATR)	dissolved in a deuterated solvent, a few mg
Time per spectrum	about a minute	minutes (¹H) to longer (¹³C)
Instrument cost	relatively low; benchtop and portable models common	high; superconducting magnets need liquid helium (cheaper benchtop models exist)
Destructive?	no	no
Distinguishes isomers?	sometimes (via fingerprint region matching)	usually, clearly

What each actually measures

IR: bond vibrations

Covalent bonds behave a little like springs. They stretch and bend at natural frequencies that depend on the bond strength and the masses of the atoms. When IR radiation of the same frequency hits the molecule, the bond absorbs it and vibrates more strongly, provided the vibration changes the molecule’s dipole moment.

So an IR spectrum is a list of which bond types are present:

• O–H: broad, 3200–3550 cm⁻¹ (alcohols); very broad, 2500–3300 cm⁻¹ (carboxylic acids)
• N–H: 3300–3500 cm⁻¹
• C=O: strong, 1680–1750 cm⁻¹
• C≡N: about 2200–2260 cm⁻¹

See infrared spectroscopy.

NMR: nuclei in a magnetic field

Some nuclei, including ¹H and ¹³C, behave like tiny magnets. In a strong external magnetic field, they can line up with or against it, and radio waves of exactly the right frequency flip them between these states. That frequency depends slightly on the electron environment around each nucleus, which is measured as the chemical shift.

A ¹H NMR spectrum tells you:

• how many hydrogen environments (number of signals)
• what kind of environment (chemical shift)
• how many hydrogens in each (integration ratio)
• how many neighbouring hydrogens (splitting, n + 1 rule)

See NMR explained.

Strengths and blind spots

IR strengths

• Instantly spots key functional groups, especially O–H and C=O.
• Fast, cheap and needs almost no sample preparation.
• Excellent for monitoring reactions: watching an O–H band disappear as an alcohol is oxidised to a ketone, for example.
• The fingerprint region (below about 1500 cm⁻¹) allows identity checks against a reference spectrum.
• Works on solids, liquids, gases and even surfaces.

IR blind spots

• Says little about the carbon skeleton or how groups are connected.
• Symmetrical bonds with no dipole change (such as C=C in some symmetrical alkenes) may be weak or invisible.
• Hard to interpret mixtures, since bands overlap.
• Not very quantitative in routine use.

NMR strengths

• Maps the connectivity of the molecule: which groups are next to which.
• Distinguishes most isomers clearly.
• Integration makes ¹H NMR quantitative; it can measure the ratio of components in a mixture.
• Two-dimensional NMR techniques can solve very complex structures, including natural products and proteins.

NMR blind spots

• Needs a soluble sample and deuterated solvent.
• Expensive instruments and maintenance.
• Less sensitive: needs milligrams, where some techniques need micrograms.
• Some functional groups have no hydrogens to see directly in ¹H NMR (e.g. C=O in a ketone); ¹³C NMR or IR fills the gap.

Worked example 1: IR distinguishes, NMR confirms

Is a product propan-1-ol or propanal?

• IR: propan-1-ol shows a broad O–H band near 3300 cm⁻¹ and no C=O; propanal shows a strong C=O near 1730 cm⁻¹ and no broad O–H. IR settles it in one glance.
• NMR would also distinguish them (propanal has an aldehyde H near 9.8 ppm), but IR is quicker and cheaper for this question.

Worked example 2: IR struggles, NMR decides

Is a product propan-1-ol, CH₃CH₂CH₂OH, or propan-2-ol, (CH₃)₂CHOH?

• IR: both show a broad O–H band and C–H and C–O bands. The fingerprint regions differ, but you’d need reference spectra to be sure.
• ¹H NMR:
  • propan-1-ol: four signals (CH₃ triplet, CH₂ multiplet, CH₂–O triplet, OH)
  • propan-2-ol: three signals (6H doublet, 1H septet, OH)
• ¹³C NMR: propan-1-ol has 3 carbon environments; propan-2-ol has 2.

NMR settles it immediately.

Worked example 3: the two together

A compound C₄H₈O₂ shows a strong IR band at 1740 cm⁻¹ and no O–H. Its ¹H NMR shows a 3H singlet at 2.0 ppm, a 2H quartet at 4.1 ppm and a 3H triplet at 1.2 ppm.

• IR → ester (C=O at 1740, no O–H).
• NMR → CH₃C(=O)– (singlet at 2.0) and –OCH₂CH₃ (quartet at 4.1, triplet at 1.2).
• Structure: ethyl ethanoate. Neither technique alone would be as convincing as the two together.

More practice: spectroscopy practice questions.

Which to use first?

In most labs, the practical order is:

1. IR (and often mass spectrometry) as a quick first look: which groups? what mass?
2. NMR to determine the full structure and check purity.
3. Other techniques if needed: mass spectrometry for molecular mass and formula; elemental analysis for composition; X-ray crystallography for the 3D structure of crystals.

For routine quality control of a known compound, IR alone is often enough: a matching fingerprint confirms identity. For a new compound, NMR is essential.

Reading the two spectra side by side

A practical habit when you have both spectra: write two short lists before trying any structure.

• From IR: groups present (e.g. “C=O yes, O–H no, N–H no”).
• From NMR: fragments (e.g. “ethyl group, CH₃ next to C=O, no aromatic H”).

Then look for a structure that uses every fragment and every group, and nothing else. This stops you from building a structure around one striking signal while ignoring the rest.

Everyday and industrial uses

• IR: identifying plastics for recycling, checking raw materials in pharmaceutical factories, breath alcohol analysers, forensic fibre and paint analysis, measuring CO₂ in air.
• NMR: drug discovery, determining protein structures, checking the purity of pharmaceuticals, food authenticity (for example, detecting adulterated honey or fruit juice), and, through the same physics, MRI scanning in hospitals, which images hydrogen nuclei in the water of the body.

Key takeaways

• IR measures bond vibrations and identifies functional groups; NMR measures nuclear spin transitions and maps connectivity.
• IR is fast, cheap and works on almost any sample; NMR is more expensive but far more informative about structure.
• IR easily distinguishes different functional groups; NMR easily distinguishes isomers with the same groups.
• The two are complementary and usually used together, often with mass spectrometry.
• For a quick identity check, IR; for a new structure, NMR. See the spectroscopy overview.