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Organic Chemistry 2 · Spectroscopy

Nuclear Magnetic Resonance Spectroscopy

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Nuclear magnetic () places a molecule in a strong magnetic field and pulses it with radio waves, making certain nuclei (¹H, ¹³C) absorb energy at their resonance frequency. Each signal encodes chemical shift (environment), (), and splitting (neighbor count) — together they reveal the carbon-hydrogen skeleton.

Why this matters

MRI is NMR applied to hydrogen nuclei of water and fat in tissue, using field gradients to map signal by location. In the lab, NMR confirms drug-candidate structure, verifies purity, and studies proteins and metabolites — including metabolomics that fingerprints disease biomarkers.

The college version

1. Nuclear spin, the external magnetic field, and resonance

Nuclei with odd mass number (¹H, ¹³C) have , a magnetic moment. In an (B₀), a spin-½ nucleus splits into two levels (aligned α, opposed β). Resonance occurs when a radiofrequency pulse supplies exactly the gap ΔE = hν, flipping nuclei to make a signal. This underlies both proton NMR (¹H) and carbon-13 NMR (¹³C).

2. Chemical equivalence and the number of signals

Chemically equivalent protons (interchangeable by symmetry or rotation) occupy one environment and give one signal, so the equals the number of distinct environments. In n-propane (CH₃CH₂CH₃), a mirror plane through C2 equates the two CH₃ groups, giving two ¹H signals.

3. Chemical shift, shielding, and integration

Chemical shift (δ, ppm) reports position and reflects : electrons make local fields opposing B₀, so shielded nuclei resonate upfield (lower δ) and deshielded nuclei (near electron-withdrawing groups) downfield (higher δ). come from circulating π electrons: aromatic protons sit at 6.5–8 ppm, alkyne protons near 2–3 ppm. Integration measures peak area, proportional to proton count — ethanol's 3:2:1 areas give the relative proton numbers.

How it works

  1. The sample (in a deuterated solvent) sits in a strong magnet.
  2. Nuclei align into spin states split by B₀.
  3. A radio pulse excites them; as they relax they emit a decaying signal (FID).
  4. A Fourier transform converts the FID into a spectrum.
  5. Peaks are referenced to TMS (0 ppm), integrated, and analyzed.
  6. ¹H, ¹³C, and data combine with MS and IR to give the structure.

Common confusions

Do not confuseWithDifference
Chemical shift (δ, ppm)Frequency (Hz)δ is field-independent; Hz depends on magnet
IntegrationSignal heightIntegration is area, not height
Number of signalsNumber of protonsOne signal can represent many equivalent protons
Equivalent neighborsNon-equivalent neighborsClean n+1 vs. complex multiplets
Coupling constant JChemical shiftJ = line spacing (Hz); δ = center (ppm)
DEPT¹³C chemical shiftDEPT adds H-count on top of shifts

Memory aids

"C-N-I-S: Count, Neighbors, Integrate, Shift." Read any ¹H signal in four passes: Count signals, Neighbors give (n+1), Integrate for proton count, Shift tells the environment. For DEPT: "C-135: CH₃/CH up, CH₂ down, quaternary gone."

Quick review

Topic Recap

NMR detects resonance of spin-bearing nuclei (¹H, ¹³C) in an external magnetic field. A proton spectrum yields number of signals (symmetry), chemical shift (environment, with anisotropic effects), integration (proton count), and (multiplicity via the n+1 rule and coupling constants). Carbon-13 NMR adds the framework, and DEPT sorts carbons by attached hydrogens. Working signals → shifts → integration → splitting, mindful of exchange, equivalence, and overlap traps, gives the complete structure.

Knowledge Check

  1. A ¹H signal integrates to 2H and is a quartet. How many neighbors, and what group?
  2. Why do the two CH₃ groups of propane give one signal instead of two?
  3. A ¹³C spectrum shows four signals; DEPT-135 shows two up, one down, one absent. Describe the carbons.
  4. Why are O–H protons often broad and unreliable for integration or splitting?
  5. Two protons couple with J = 7 Hz. What confirms they are on adjacent carbons?

Answers and Rationales

  1. Three neighbors (an ethyl CH₂). Quartet = n+1 = 4, so n = 3; a 2H quartet near 2–3 ppm is a CH₂ next to a CH₃.
  2. Chemical equivalence by symmetry. A mirror plane through C2 interchanges the CH₃ groups, so they occupy one environment; equivalent protons also do not split each other.
  3. Two CH/CH₃ (up), one CH₂ (down), one quaternary (absent). DEPT-135 shows CH₃ and CH in one phase, CH₂ in the other; quaternary carbons vanish.
  4. Proton exchange. O–H/N–H protons exchange rapidly, averaging their environments, so the signal broadens and gives no clean integration or splitting.
  5. Matching coupling constants. A partner with the same J = 7 Hz spacing indicates the protons couple (bonded to adjacent atoms).
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Some nuclei act as tiny spinning magnets. Inside the NMR magnet, each lines up "with" or "against" the field, like a compass needle. A radio pulse of exactly the right energy flips them — that flip is the signal. Where a signal appears depends on how much surrounding electrons shield the nucleus, which differs for every environment.

The comparison "where it stops being exact": the "flipping magnets" picture is a cartoon — the real process is quantum mechanical, and the machine pulses the sample and Fourier-transforms the resulting decay into the peaks you see.

Simple Example

Ethanol (CH₃CH₂OH) gives three signal groups — CH₃, CH₂, and OH protons each occupy a different environment, so each appears at a different chemical shift, with 3:2:1 areas.

Worked example

Determining a structure from NMR data:

  1. Count signals. List distinct ¹H environments (and ¹³C environments) — the number of signals equals the symmetry-distinct nuclei.
  2. Read chemical shifts with chemical-shift ranges: CH₃ ~0.9, CH₂ ~1.3, CH ~1.7, X–CH ~3–4, O–CH₃ ~3.3, alkene ~5–6, aromatic ~6.5–8, aldehyde ~9–10, acid ~10–12 ppm.
  3. Read integrations. Convert areas to whole-number proton counts; their sum equals the formula's hydrogens.
  4. Analyze splitting with the n+1 rule: a proton with n equivalent neighbors splits into n+1 lines (multiplicity) — one → doublet, two → triplet, three → quartet. Spin-spin splitting makes these multiplets; the line spacing is the coupling constant J (Hz). Non-equivalent neighbor sets give complex splitting (doublet of doublets); unresolved overlap gives broad multiplets.
  5. Assemble fragments. Pieces with matching J are bonded to each other; connect them into a skeleton consistent with shifts and integration.
  6. Check with ¹³C and DEPT. Carbon-13 shifts span ~0–220 ppm (C=O 160–220, aromatic 110–160, alkene 100–150, sp³ 0–90). DEPT sorts carbons by attached H count: CH/CH₃ up, CH₂ down, quaternary absent in DEPT-135.
  7. Verify and note limits. Cross-check MS (formula) and IR (groups). Watch limitations/traps: exchangeable O–H/N–H protons give broad, drifting, unintegrated signals; equivalent nuclei do not split each other; overlapping peaks hide signals.

Key takeaways

  • High yield: Number of signals = number of symmetry-distinct environments; equivalent protons do not split each other.
  • High yield: Integration areas give the proton-count ratio (e.g., 3:2:1).
  • High yield: n+1 applies only to equivalent neighbors; non-equivalent sets cause complex splitting.
  • High yield: Mutually coupled partners share equal J — matching J connects fragments.
  • High yield: Shift windows: alkyl 0–2, heteroatom-adjacent 2–4.5, alkene 5–6, aromatic 6.5–8, aldehyde 9–10, acid 10–12 ppm.
  • High yield: DEPT-135: CH₃/CH one phase, CH₂ the other, quaternary absent.
  • O–H/N–H protons exchange (broad, drifting, no clean integration/splitting); ¹³C NMR shows no ¹H splitting (decoupling) and no clean integration.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Explain how nuclear spin and an external magnetic field give rise to resonance, the basis of NMR.
  • Distinguish proton NMR from carbon-13 NMR and interpret chemical equivalence, signals, shift, and integration.
  • Apply the n+1 rule and coupling constants to predict splitting patterns (multiplicity, multiplets, complex splitting).
  • Use DEPT and a structure-determination workflow to assemble a molecule while recognizing NMR limitations and traps.

Key vocabulary

NMR
Magnetic resonance of nuclei
Nuclear spin
Nuclear angular momentum
External magnetic field
B₀ from the magnet
Resonance
Radio absorption at Larmor
Proton NMR (¹H NMR)
NMR of hydrogen nuclei
Carbon-13 NMR (¹³C NMR)
NMR of ¹³C nuclei
Chemical equivalence
Symmetry-related nuclei
Number of signals
Count of distinct environments
Chemical shift (δ)
Position in ppm
Shielding
Electrons reduce felt field
Deshielding
Electron withdrawal raises field
Anisotropic effects
Fields from π electrons
Integration
Peak area
Proton count
Equivalent proton count
Spin-spin splitting
Neighbors split a signal
Multiplicity
Lines in a signal
n+1 rule
n neighbors → n+1 lines
Coupling constant J
Spacing in Hz
Complex splitting
Splitting by non-equivalent sets
Multiplets
Overlapping lines
Chemical-shift ranges
δ windows by proton type
Carbon-13 shifts
δ ~0–220 ppm
DEPT
¹³C by attached H
Structure-determination workflow
Signals→shifts→integration→splitting
Limitations/traps
Exchange/equivalence/overlap pitfalls

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