Theory — NMR Spectroscopy
1. The basic NMR experiment
Nuclei with non-zero spin (¹H, ¹³C, ¹⁹F, ³¹P) align with or against a strong magnetic field. An RF pulse flips them; as they relax, they emit a signal at a frequency that depends on the local electronic environment. Different chemical environments give different frequencies, so each unique proton (or carbon) appears at a different position in the spectrum. Modern NMR magnets are 7-21 Tesla (300-900 MHz for ¹H).
2. Chemical shift (δ)
Reported in parts per million (ppm) on the δ scale. TMS (Si(CH₃)₄) is the reference at δ = 0. Higher ppm = "downfield" (left); lower ppm = "upfield" (right). Electronegative atoms or pi systems deshield protons (move them downfield); alkyl environments shield them (keep them upfield).
| ¹H environment | δ (ppm) | Notes |
|---|---|---|
| Saturated alkyl C-H (CH₃, CH₂, CH) | 0.5-2.0 | CH₃ ~0.9, CH₂ ~1.3, CH ~1.5 |
| Alpha to C=O, allyl, benzyl | 2.0-2.6 | -CH₂-CO-, ArCH₂-, etc. |
| C-H next to N, S, halogen | 2.5-4.0 | Cl/Br ~3.5; N ~2.5 |
| C-H next to O (-CH₂-O-) | 3.5-4.5 | Ester, ether, alcohol |
| Vinyl C-H (alkene) | 4.5-6.5 | |
| Aromatic C-H | 6.5-8.5 | Benzene ~7.3 |
| Aldehyde C-H | 9.0-10.5 | Very distinctive |
| Carboxylic acid O-H | 10-13 | Very broad; often missing in CDCl₃ |
| Alcohol/amine O-H, N-H | 0.5-5 (variable) | Broad; exchange in D₂O |
3. Integration: how many protons?
The area under each peak is proportional to the number of protons in that environment. Integration tells you relative numbers, e.g., 3:2:1 for ethanol (CH₃CH₂OH). Modern NMR shows step curves or numerical ratios.
4. Multiplicity: the n+1 rule
If a proton has n equivalent neighbours, its signal is split into n+1 peaks. Intensities follow Pascal\'s triangle.
| Neighbours | Multiplicity | Pattern | Common in |
|---|---|---|---|
| 0 | singlet (s) | 1 | Isolated CH₃; OH; tert-butyl |
| 1 | doublet (d) | 1:1 | One adjacent H |
| 2 | triplet (t) | 1:2:1 | CH₃ in ethyl group |
| 3 | quartet (q) | 1:3:3:1 | CH₂ in ethyl group |
| 4 | quintet | 1:4:6:4:1 | Middle CH₂ in -CH₂-CH₂-CH₂- |
| 5 | sextet | 1:5:10:10:5:1 | CH₂ with 5 neighbours (e.g., propyl bromide CH₂) |
| 6 | septet | 1:6:15:20:15:6:1 | CH in isopropyl group |
Ethyl group (-CH₂-CH₃): CH₃ has 2 neighbours \u2192 triplet (3 peaks). CH₂ has 3 neighbours \u2192 quartet (4 peaks). Classic pattern in ethyl esters, ethyl ethers, ethylbenzene.
5. Coupling constants (J)
The spacing between peaks of a multiplet is J (in Hz). Reveals geometry between coupled nuclei.
| Coupling type | Typical J (Hz) |
|---|---|
| Vicinal ³J (free rotation, sp³) | 6-8 |
| Vicinal cis (alkene) | 6-12 |
| Vicinal trans (alkene) | 12-18 |
| Aromatic ortho | 7-10 |
| Aromatic meta | 2-3 |
| Aromatic para | 0-1 |
6. Common diagnostic patterns
- Ethyl group (-CH₂CH₃): triplet (3H, ~1.0-1.4 ppm) + quartet (2H, varies).
- Isopropyl ((CH₃)₂CH-): doublet (6H, ~0.9-1.3) + septet (1H, varies).
- tert-Butyl ((CH₃)₃C-): singlet (9H, ~1.2-1.5).
- Methyl ester (-OCH₃): singlet (3H, ~3.6-3.7).
- Aromatic: 4-5 H in 6.8-7.5. Mono-substituted: complex multiplet ~7.3. Para-disubstituted: AA\'BB\' pattern looks like two doublets.
- OH/NH: often broad singlets at variable position; disappear with D₂O shake.
7. Brief introduction to ¹³C NMR
¹³C NMR shows one peak per unique carbon. Wider chemical shift range (0-220 ppm) gives better resolution. Spectra are typically broadband proton-decoupled (no C-H splitting), so each carbon is a singlet.
| ¹³C environment | δ (ppm) |
|---|---|
| Saturated alkyl C | 10-50 |
| C-N | 30-60 |
| C-O (alcohol, ether) | 50-90 |
| Alkene C | 100-150 |
| Aromatic C | 120-150 |
| Carbonyl C (ester) | 165-175 |
| Carbonyl C (acid) | 170-185 |
| Carbonyl C (aldehyde, ketone) | 190-220 |
Aliphatic CH₂: ~1.2-1.5
Allyl/benzyl/α-carbonyl CH: 2.0-2.6
-OCH₃ (methyl ester): ~3.65 (singlet)
-OCH₂- (in ester R-O-CH₂-): ~4.1 (often quartet)
Aromatic H: 6.5-8.5
Aldehyde H: 9-10
Acid OH: 10-13
Combined with integration + multiplicity, these landmarks identify ~80% of common organic compounds.
8. Systematic NMR analysis approach
- Count signals. How many distinct chemical environments are there?
- Read chemical shifts. What functional groups do they suggest?
- Read integrations. How many protons in each environment? Express as ratio.
- Read multiplicities. How many neighbours does each have? Use n+1 rule.
- Assemble fragments. Match patterns to ethyl, isopropyl, t-butyl, methyl ester, aromatic, etc.
- Combine with MS/IR. NMR gives the framework; MS gives the molecular weight; IR gives functional group confirmation.
Instructions
The Simulation has four parts. Complete in order.
Prerequisite: Familiarity with organic functional groups and the IR + Mass Spec labs is helpful. NMR is the third member of the spectroscopy quartet.
Simulation
Four interactive parts. Use the ↺ Reset Simulation button to clear all answers and start over.
Eight chemical-shift identification cases. For each: (a) chemical shift region; (b) expected multiplicity; (c) which proton type.
Six unknown samples. Click Acquire Spectrum, identify the compound from the ¹H NMR. Peak labels appear after you answer.
Eight harder puzzles: distinguishing isomers, full structural deduction.
Round 1 — SDS interpretation
Four common NMR solvents. Each has 4 questions.
Round 2 — Microscale sample prep
Six sample-prep scenarios.
Team Questions
Discuss with your team before answering.
Example Lab Notebook Entry
Use the format below as a template.
NMR Spectroscopy — Lab Notebook Entry
Submitted by: [Student Name]
Course: Organic Chemistry I · Section: 201-A · Date: May 9, 2026
Objective
To learn the systematic interpretation of ¹H NMR spectra: chemical shift, integration, multiplicity, and coupling constants. To recognise common diagnostic patterns (ethyl, isopropyl, tert-butyl, methyl ester, aromatic, OH/NH). To deduce structure by combining NMR data with mass spectrometry and IR. To understand sample preparation requirements: deuterated solvents, TMS reference, lock signal.
Instrument bench results (Section II)
| Sample | Compound | Key ¹H NMR signals |
|---|---|---|
| 1 | Ethanol | Triplet 1.20 (3H), quartet 3.70 (2H), broad singlet 2.6 (1H, OH) |
| 2 | Acetone | Singlet 2.17 (6H, two equivalent methyls) |
| 3 | Ethyl acetate | Singlet 2.04 (3H, COCH₃), quartet 4.12 (2H, OCH₂), triplet 1.26 (3H, CH₃) |
| 4 | Toluene | Singlet 2.36 (3H, ArCH₃), multiplet 7.2 (5H, aromatic) |
| 5 | 1-Bromopropane | Triplet 1.04 (3H, CH₃), sextet 1.86 (2H, central CH₂), triplet 3.40 (2H, CH₂Br) |
| 6 | p-Xylene | Singlet 2.30 (6H, two equivalent ArCH₃), singlet 7.04 (4H, aromatic) |
Discussion
NMR spectroscopy answers four questions about each unique proton: where (chemical shift), how many (integration), how many neighbours (multiplicity), and what coupling (J). The chemical shift indicates the chemical environment: aliphatic CH₃ ~0.9, allyl/benzyl/α-carbonyl CH₂ ~2-2.6, -OCH₂- ~3.5-4.5, aromatic 6.5-8.5, aldehyde 9-10, acid 10-13. The shift moves downfield with proximity to electronegative atoms or pi systems.
Integration tells you the relative number of protons in each environment. For ethanol (CH₃CH₂OH), the integration ratio is 3:2:1 corresponding to the 3 H of CH₃, 2 H of CH₂, and 1 H of OH. The ratios are reliable; absolute counts require comparison to a known reference.
Multiplicity follows the n+1 rule: if a proton has n equivalent neighbours, its signal splits into n+1 peaks with intensities matching Pascal\'s triangle. The classic ethyl pattern (-CH₂CH₃) shows the CH₃ (with 2 neighbours) as a triplet, and the CH₂ (with 3 neighbours) as a quartet. The isopropyl pattern shows a doublet (the 6 equivalent methyls) and a septet (the lone CH); the t-butyl group is a singlet integrating for 9H.
The instrument bench (Section II) demonstrated these patterns on six samples. Ethanol shows the alcohol triplet/quartet/broad-OH; acetone shows a single peak (all 6 H equivalent); ethyl acetate shows the diagnostic ester pattern (singlet OCH₃ or COCH₃, quartet OCH₂, triplet CH₃); toluene shows ArCH₃ singlet + aromatic multiplet; 1-bromopropane shows a triplet/sextet/triplet pattern characteristic of -CH₂-CH₂-CH₂X; para-xylene shows two singlets (high symmetry: two CH₃ equivalent + 4 aromatic H equivalent).
For sample preparation, the key requirement is a DEUTERATED solvent. Regular CHCl₃ would dominate the spectrum with its ¹H signal; CDCl₃ replaces ¹H with ²H (deuterium, which doesn\'t appear in ¹H NMR) and provides a "lock signal" that the spectrometer uses to maintain field stability. TMS (tetramethylsilane) is added at 0 ppm as the chemical shift reference. The choice of solvent depends on the sample: CDCl₃ for most organics; DMSO-d₆ for polar compounds and amides; D₂O for very polar/charged compounds; methanol-d₄ for OH/NH-containing compounds where exchange info is wanted.
D₂O shake is a useful diagnostic: adding a drop of D₂O to the sample exchanges OH and NH protons with deuterium, removing them from the ¹H spectrum. The remaining peaks (CH only) confirm which were the exchangeable protons. Alcohols and acids lose their OH peaks; amines lose NH peaks.
Conclusion
NMR spectroscopy is the most powerful single technique for organic structure determination, providing direct mapping of the carbon-hydrogen framework with information about connectivity through coupling. Combined with mass spectrometry (molecular weight) and IR (functional groups), NMR completes the spectroscopic identification toolkit. The next lab in this Spectroscopy quartet (UV/Vis) covers electronic transitions and is the fourth and final member.
References
1. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Vyvyan, J. R. Introduction to Spectroscopy, 5th ed., Cengage, 2015, Ch 3-7.
2. Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric Identification of Organic Compounds, 8th ed., Wiley, 2014, Ch 3-5.
3. SDBS — Spectral Database for Organic Compounds, AIST Japan: https://sdbs.db.aist.go.jp.
4. Friebolin, H. Basic One- and Two-Dimensional NMR Spectroscopy, 5th ed., Wiley-VCH, 2010.
Practice Questions
Work through each before peeking at the hint.