Nuclear Magnetic Resonance — Interactive Learning Resource -

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NMR Visualizer

A structured introduction to Nuclear Magnetic Resonance spectroscopy — from fundamental physics to practical structure elucidation. Expand each section to explore.

Part A NMR: What, How & Why

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What is NMR?

Nuclear Magnetic Resonance (NMR) spectroscopy is the single most powerful analytical tool available to chemists for determining the structure of organic molecules in solution. It exploits the magnetic properties of certain atomic nuclei — most importantly ¹H (proton) and ¹³C — to produce detailed information about the connectivity, environment, and quantity of each atom in a molecule.

NMR is non-destructive: the sample is fully recovered after analysis, unlike mass spectrometry or combustion analysis.

The Physics: Nuclear Spin

Nuclei with an odd mass number or odd atomic number possess a property called nuclear spin (quantum number I ≠ 0). A spinning charged particle generates a tiny magnetic dipole moment — the nucleus behaves like a microscopic bar magnet.

For ¹H and ¹³C, I = ½, giving two allowed spin states: α (spin +½, low energy, aligned with field) and β (spin −½, high energy, opposing field).

ΔE = γ·ℏ·B₀

where γ is the gyromagnetic ratio (unique per nucleus), ℏ is the reduced Planck constant, and B₀ is the applied magnetic field strength.

How NMR Works: The Experiment

▸	Step 1 — Alignment: The sample is placed in a strong external magnetic field B₀ (typically 4.7–23 T). Nuclei align either with or against the field.
▸	Step 2 — Excitation: A radiofrequency (RF) pulse at the Larmor frequency tilts the net magnetization into the transverse plane.
▸	Step 3 — Detection (FID): As nuclei precess and relax back to equilibrium, they emit an RF signal — the Free Induction Decay (FID).
▸	Step 4 — Fourier Transform: The FID (time domain) is converted by FT into the NMR spectrum (frequency domain) showing peaks at characteristic chemical shifts.

Why NMR?

NMR provides information that no other single technique can match:

NMR tells you

▸	Which functional groups are present
▸	How many H or C in each environment
▸	How atoms are connected (coupling)
▸	3D geometry and stereochemistry
▸	Purity and ratio of components

Other techniques

▸	IR: functional groups only
▸	MS: molecular mass & fragments
▸	UV-Vis: conjugation / chromophores
▸	X-ray: full 3D (needs crystals)
▸	Elemental: atom ratios only

Modern NMR instruments operate at 300–1000 MHz (for ¹H), corresponding to B₀ fields of 7–23 Tesla — stronger than MRI machines used in hospitals.

Part B ¹H-NMR — Proton NMR

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Chemical Shift (δ)

The chemical shift δ (in ppm) describes where a peak appears on the frequency axis, relative to TMS (tetramethylsilane, set at 0.00 ppm). It reflects the electronic environment of the proton: electron-withdrawing groups reduce shielding → higher δ (downfield shift); electron-donating groups increase shielding → lower δ (upfield).

Chemical shift scale — ¹H-NMR (0–12 ppm)

12

10

9

7

5

3

2

0 ppm

● ~10–12 ppm — COOH, CHO

● ~6–8 ppm — Ar–H, vinyl

● ~3–5 ppm — O–CH, N–CH, CCl

● ~0–3 ppm — alkyl, alicyclic

Proton type	δ range (ppm)	Example
TMS reference	0.00	(CH₃)₄Si
Alkyl CH₃	0.7–1.3	Ethane, propane
Allylic / propargylic	1.6–2.5	Toluene CH₃
α to carbonyl	2.0–2.7	Acetone, acetaldehyde
N–CH	2.2–2.9	Amine, amide
O–CH₃, O–CH₂	3.3–4.0	Methanol, ether
Vinyl (C=C–H)	4.5–6.5	Styrene CH=CH₂
Aromatic Ar–H	6.5–8.5	Benzene, toluene
Aldehyde CHO	9.4–10.0	Benzaldehyde
Carboxylic COOH	10.5–12.5	Acetic acid

Integration

In ¹H-NMR, the area under each peak is proportional to the number of equivalent protons producing it. Integration is expressed as relative ratios. For ethanol (CH₃CH₂OH): the ratio is approximately 3:2:1 for CH₃:CH₂:OH protons.

Multiplicity — The n+1 Rule

Protons on adjacent carbons couple with each other through bonds, splitting peaks into multiplets. A proton with n equivalent neighbors appears as an n+1 multiplet:

Singlet (s) · Doublet (d) · Triplet (t) · Quartet (q) · Quintet · Sextet · Septet · Multiplet (m)

Classic example — ethanol:

▸	CH₃ has 2 CH₂ neighbors → triplet (2+1=3)
▸	CH₂ has 3 CH₃ neighbors → quartet (3+1=4)
▸	OH in CDCl₃ → usually broad singlet (fast exchange)

J (coupling constant) = separation between lines, in Hz

Typical vicinal (³J) coupling: 6–8 Hz for freely rotating sp³ C–C; aromatic ortho H: 7–9 Hz; trans alkene: 12–18 Hz; cis alkene: 6–12 Hz.

Solvents

NMR solvents must not contain ¹H. Most common: CDCl₃ (residual peak at δ 7.26 ppm), DMSO-d₆ (δ 2.50), D₂O (δ 4.79), CD₃OD (δ 3.31).

Part C ¹³C-NMR — Carbon NMR

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Why ¹³C?

Carbon is the backbone of every organic molecule, yet ¹²C (99% natural abundance) is NMR-silent (I = 0). The ¹³C isotope (I = ½, 1.1% abundance) is NMR-active but its low abundance and smaller gyromagnetic ratio (γ) make it ~6000× less sensitive than ¹H. Modern instruments compensate with signal averaging and the Nuclear Overhauser Effect (NOE).

Wider Chemical Shift Range (0–220 ppm)

¹³C shifts span a much broader range than ¹H, making peaks less likely to overlap. The range reflects the carbon's bonding and oxidation state:

Carbon type	δ range (ppm)	Example
Alkyl C (sp³)	0–50	Cyclohexane (~27), ethane
C–halogen, C–O (sp³)	30–90	CHCl₃, ethanol C–O
Alkyne C≡C	65–90	Phenylacetylene
Aromatic C (sp²)	110–160	Benzene (~128)
Alkene C=C	100–150	Styrene (~113, ~137)
Nitrile C≡N	115–120	Acetonitrile (~117)
Ester / carbamate C=O	155–175	Ethyl acetate (~171)
Carboxylic acid C=O	175–185	Acetic acid (~178)
Aldehyde CHO	190–205	Benzaldehyde (~190)
Ketone C=O	195–215	Acetone (~206), cyclohexanone

Broadband-Decoupled ¹³C-NMR

In routine ¹³C spectra, all H–C couplings are eliminated by simultaneously irradiating all ¹H frequencies (broadband decoupling). This produces clean singlets for every chemically distinct carbon — one peak = one unique carbon environment. No integration ratios are directly meaningful (NOE enhancement varies).

A ¹³C spectrum of benzene shows a single peak at ~128 ppm — all 6 carbons are equivalent.

DEPT (Distortionless Enhancement by Polarization Transfer)

DEPT experiments distinguish C, CH, CH₂, and CH₃ by the phase and presence/absence of signals:

▸	DEPT-135: CH and CH₃ point up (positive); CH₂ points down (negative); quaternary C absent.
▸	DEPT-90: Only CH carbons appear.
▸	DEPT-45: All C–H carbons appear (CH, CH₂, CH₃) with positive phase.

By comparing DEPT-135 and the broadband-decoupled spectrum, quaternary carbons (e.g. C=O, C quaternary in rings) can be identified as peaks present in the full spectrum but absent in DEPT.

Part D Comparison & Applications

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¹H vs ¹³C: Side-by-Side

Property	¹H-NMR	¹³C-NMR
Natural abundance	99.98%	1.1%
Relative sensitivity	1 (reference)	~1/6000
Chemical shift range	0–15 ppm	0–220 ppm
Integration	Quantitative (direct)	Non-quantitative (routine)
Multiplicity (routine)	Yes — n+1 rule	No (broadband decoupled)
Peaks per compound	# unique H environments	# unique C environments
Key info	H count, coupling, stereochem	C skeleton, carbonyl type
Best for	Connectivity, quantity of H	Carbon framework, C=O type

Structure Elucidation Workflow

▸	Step 1: Obtain molecular formula (from MS). Calculate Hydrogen Deficiency Index (HDI = (2C+2+N−H−X)/2).
▸	Step 2: Examine ¹³C — how many unique carbons? Any carbonyl peaks (155–220 ppm)?
▸	Step 3: Use DEPT to classify each carbon (CH, CH₂, CH₃, C).
▸	Step 4: Read ¹H shifts for functional group identification (aromatic? aldehyde? OH?).
▸	Step 5: Use coupling patterns and J values to establish connectivity (which protons are on adjacent carbons).
▸	Step 6: Use integration to count protons per environment.
▸	Step 7: Assemble fragments and confirm against spectral data.

Real-World Applications

Pharmaceutical Industry Identity and purity testing of APIs; polymorph characterization; metabolite identification; in vivo MRI contrast agents.	Food & Agriculture Adulteration detection (e.g. honey, olive oil); metabolomics of plant extracts; quality control of beverages.
Materials Science Polymer structure and degree of polymerization; solid-state NMR for catalysts and minerals.	Natural Products Complete structure determination of complex alkaloids, terpenoids, and antibiotics — the gold standard method.

Select a compound

Select a compound from the list to view its ¹H-NMR spectrum.

Select a compound

Select a compound from the list to view its ¹³C-NMR spectrum.