How 1H NMR Spectroscopy Works
A complete guide to reading proton NMR spectra and determining molecular structure
What Is 1H NMR?
Nuclear Magnetic Resonance (NMR) spectroscopy exploits the fact that hydrogen nuclei behave like tiny magnets. When placed in a strong external magnetic field, these nuclei absorb radio-frequency energy at frequencies that depend on their electronic environment.
The result is a spectrum where each chemically distinct group of hydrogens produces a separate signal. From these signals you can determine three things: the type of hydrogen (chemical shift), the number of neighbors (splitting), and the number of equivalent hydrogens (integration).
Why it matters
NMR is the most powerful routine technique for determining molecular structure in organic chemistry. While IR tells you what functional groups are present, NMR tells you how the carbon skeleton is connected and where the hydrogens are. Together with mass spectrometry, these three techniques can determine the complete structure of most organic molecules.
Chemical Shift
The x-axis of an NMR spectrum shows chemical shift in parts per million (ppm), measured relative to a reference compound called TMS (tetramethylsilane) which is defined as 0 ppm.
The position of a signal on this scale depends on the electron density around the hydrogen. Electrons shield the nucleus from the external magnetic field. More electron density means more shielding, which shifts the signal upfield (toward 0 ppm). Less electron density means less shielding (deshielding), shifting the signal downfield (toward higher ppm).
Shielding (upfield)
Electron-donating groups increase electron density around nearby H atoms, shielding them from the magnetic field. Simple alkyl groups (CH3, CH2) far from electronegative atoms are the most shielded, appearing near 0.8-1.5 ppm.
Deshielding (downfield)
Electronegative atoms (O, N, halogens) and pi systems (C=O, aromatic rings) withdraw electron density, deshielding nearby H atoms. The more electron withdrawal, the further downfield: alkyl (~1 ppm) to aromatic (~7 ppm) to aldehyde (~10 ppm).
Why ppm?
The actual frequency difference between signals is tiny (a few hundred Hz out of millions). Dividing by the spectrometer frequency and multiplying by 10⁶ gives a field-independent scale. A signal at 2.1 ppm is at 2.1 ppm whether measured on a 90 MHz or 400 MHz instrument.
Chemical Shift Regions
Memorizing these approximate regions lets you quickly identify the type of hydrogen producing each signal.
| Region (ppm) | Type of Hydrogen | Examples |
|---|---|---|
| 0.8 - 1.8 | Alkyl (sp3 C-H) | CH3, CH2 in alkanes |
| 1.8 - 2.5 | Alpha to C=O or C=C | COCH3, allylic CH |
| 2.5 - 4.5 | Alpha to O, N, or halogen | OCH3, NCH2, CH2Br |
| 4.5 - 6.5 | Vinyl (alkene =C-H) | =CH2, =CH- |
| 6.5 - 8.5 | Aromatic | Benzene ring H |
| 9.0 - 10.0 | Aldehyde | -CHO |
| 10.0 - 12.0 | Carboxylic acid | -COOH |
Variable signals: OH and NH
Hydroxyl (-OH) and amine (-NH) protons are special cases. Their chemical shift varies dramatically with concentration, temperature, and solvent (typically 1-5 ppm for alcohols, 6-8 ppm for amides, and 10-12 ppm for carboxylic acids). They usually appear as broad signals due to hydrogen bonding and rapid exchange.
Equivalent Hydrogens and Signal Count
Hydrogens in the same chemical environment produce the same NMR signal. The number of distinct signals equals the number of chemically distinct hydrogen environments.
To count environments, look for symmetry elements (mirror planes, rotation axes) that relate one hydrogen to another. If two hydrogens can be interconverted by a symmetry operation, they are equivalent.
Acetone: 1 signal
A mirror plane through the C=O makes both CH3 groups identical. All 6 H atoms are equivalent, giving a single signal.
Ethanol: 3 signals
Three environments: CH3 (3H), CH2 (2H), and OH (1H). The three CH3 hydrogens are equivalent by free rotation around the C-C bond.
Diethyl ether: 2 signals
A mirror plane through the oxygen makes both ethyl groups identical. 10 hydrogens give just 2 signals: CH3 (6H) and O-CH2 (4H).
Tip: The number of signals often gives you more structural information than the chemical shifts. A molecule with only 1 signal must be highly symmetric. A molecule with many signals has low symmetry or many different functional groups.
Integration
The area under each signal is proportional to the number of hydrogens producing it. Modern spectrometers display this as a stepped integration curve or as numerical values.
Integration gives you ratios, not absolute numbers. A spectrum showing two signals with a 3:2 ratio could be 3H:2H, 6H:4H, or 9H:6H. You need the molecular formula (from mass spectrometry) to determine the absolute count.
Ethanol - Integration Ratios
CH₃CH₂OH
Reading integration
Ethanol CH3:CH2:OH = 3:2:1 - confirms three CH3 hydrogens, two CH2, one OH
Toluene ArH:CH3 = 5:3 - confirms monosubstituted benzene with methyl
Acetone: single signal 6H - confirms two equivalent CH3 groups
Spin-Spin Splitting (The n+1 Rule)
Neighboring hydrogen atoms interact through the bonding electrons, causing each signal to split into multiple lines. This is called spin-spin coupling or J-coupling.
The splitting follows a simple rule: a hydrogen with n equivalent neighbors on adjacent carbons splits into n+1 lines.
| Neighbors (n) | Pattern | Abbreviation | Intensity Ratio |
|---|---|---|---|
| 0 | Singlet | s | 1 |
| 1 | Doublet | d | 1:1 |
| 2 | Triplet | t | 1:2:1 |
| 3 | Quartet | q | 1:3:3:1 |
| 4 | Quintet | qn | 1:4:6:4:1 |
| 5 | Sextet | sx | 1:5:10:10:5:1 |
| 6 | Septet | sp | 1:6:15:20:15:6:1 |
Pascal's triangle
The intensity ratios follow Pascal's triangle. Each row is formed by adding adjacent numbers from the row above. The center lines of a multiplet are always the tallest.
Coupling Constants (J Values)
The spacing between lines in a multiplet is called the coupling constant (J), measured in Hz. Unlike chemical shift, J values are independent of spectrometer frequency.
Two groups that are coupled to each other always share the same J value. This is how you confirm which groups are neighbors: if the triplet at 1.2 ppm has J = 7.0 Hz and the quartet at 3.7 ppm also has J = 7.0 Hz, they are coupled to each other.
Typical coupling constants
Exchangeable Protons (OH, NH)
Protons bonded to oxygen (-OH) or nitrogen (-NH, -NH2) behave differently from C-H protons because they can exchange between molecules in solution.
Characteristics
Exchangeable protons typically appear as broad singlets regardless of neighbors. Their chemical shift is variable and depends on concentration, temperature, and solvent.
Why no coupling?
Rapid exchange means the proton jumps between molecules faster than the NMR timescale. The coupling to neighboring H atoms is averaged out. A D2O shake (adding heavy water) replaces OH/NH with OD/ND, making their signals disappear - a useful diagnostic test.
The Aromatic Ring Current
Aromatic protons appear at 6.5-8.5 ppm - much further downfield than you might expect for sp2 C-H. This is not just electronegativity; it is caused by the ring current effect.
When an aromatic ring is placed in a magnetic field, the circulating pi electrons generate their own local magnetic field. At the positions of the ring protons (outside the ring), this induced field reinforces the external field, so the protons experience a stronger effective field and resonate at higher frequency (further downfield).
Substituent effects on aromatic shift
Electron-donating groups (OH, NH2, OCH3) increase electron density in the ring, slightly shielding the aromatic protons and moving them upfield (~6.7-7.2 ppm). Electron-withdrawing groups (CHO, NO2, COOH) deshield them further downfield (~7.4-8.5 ppm).
Recognizing Common Patterns
With practice, you will recognize these coupling patterns instantly.
Ethyl group (-CH2CH3)
Triplet + quartet with J ~ 7 Hz. The classic pattern: CH3 is a triplet (2 neighbors) and CH2 is a quartet (3 neighbors). Integration ratio 3:2.
Isopropyl group (-CH(CH3)2)
Doublet + septet with J ~ 6 Hz. The CH3 is a doublet (1 neighbor) integrating for 6H and the CH is a septet (6 neighbors) integrating for 1H.
Isolated singlet
A singlet means no neighbors on adjacent carbons. Common for: CH3 next to C=O (acetyl), OCH3 (methyl ester), or CH3 on quaternary carbon. Integration tells you how many H.
Monosubstituted benzene
5H multiplet in the aromatic region (6.5-8.5 ppm). The ortho, meta, and para protons couple to each other, creating a complex multiplet that is characteristic of monosubstitution.
Aldehyde (-CHO)
A 1H signal far downfield at 9.4-10.0 ppm. If attached to a CH group, it shows as a doublet; if attached to an aromatic ring or quaternary carbon, a singlet.
A Strategy for Interpreting Spectra
When you encounter an unknown 1H NMR spectrum, follow this approach:
- 1. Count the signals: This tells you the number of distinct hydrogen environments and gives clues about molecular symmetry.
- 2. Read the integration: Determine the ratio of hydrogens for each signal. With the molecular formula, convert ratios to absolute numbers.
- 3. Note chemical shifts: Assign each signal to a region (alkyl, alpha to EWG, aromatic, etc.) to identify the type of hydrogen.
- 4. Analyze splitting: Use the n+1 rule to determine how many neighbors each group has. Match coupling constants to connect coupled groups.
- 5. Look for diagnostic patterns: Ethyl (t+q), isopropyl (d+sp), monosubstituted benzene (5H multiplet), aldehyde (far downfield singlet/doublet).
- 6. Assemble the structure: Connect the fragments. Each coupled pair must be on adjacent carbons. Check that your proposed structure accounts for all signals, integrations, and splittings.
Ready to practice?
Apply what you have learned by interpreting NMR spectra with interactive 3D molecules.