Module 3: The Anatomy of a ¹H NMR Spectrum — the essential skill for reading and interpreting real NMR data.

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📊 Module 3: The Anatomy of a ¹H NMR Spectrum

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🎯 Learning Objective:

By the end of this module, you'll be able to:

Read and interpret the peaks on a proton NMR spectrum, using chemical shift, integration, splitting patterns (multiplicity), and coupling constants.

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🧭 1. Chemical Shift (δ)

📌 What is it?

• A measure of the electronic environment around a proton.

• Reported in parts per million (ppm) on the X-axis of the spectrum.

🧠 Why is it Useful?

It tells you what kind of functional group or neighboring atoms are near a proton.

🌈 Key Ranges:

Type of Proton	δ Range (ppm)
Alkane (R–CH₃, R–CH₂–)	0.8 – 1.5
Next to electronegative atom (O, N, Cl)	3 – 4.5
Alkene (=C–H)	4.5 – 6.5
Aromatic ring (Ph–H)	6 – 8.5
Aldehyde (–CHO)	9 – 10
Carboxylic Acid (–COOH)	10 – 13 (very broad)

🧲 Chemical Shift is influenced by:

• Electron density (shielding vs. deshielding)

• Nearby electronegative atoms

• π systems (aromatic rings, alkenes)

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📏 2. Integration

📌 What is it?

• The area under each peak corresponds to the number of protons giving that signal.

• Not in absolute numbers, but ratios (e.g., 3:2:1)

📐 How to Use It:

• Compare the integrals to deduce how many H atoms are in each environment.

• Multiply by molecular formula (if known) to determine actual proton counts.

💡 Example: A 3:2:1 ratio → might represent CH₃ (3H), CH₂ (2H), and CH (1H)

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🌊 3. Multiplicity (Splitting Patterns)

📌 What is it?

• Each proton’s signal can be split into multiple peaks due to spin-spin coupling with nearby non-equivalent protons (typically 2–3 bonds away).

📚 The (n+1) Rule:

• A proton with n neighboring equivalent protons splits into n + 1 peaks.

# Neighbors (n)	Splitting Pattern	Name
0	1	Singlet
1	2	Doublet
2	3	Triplet
3	4	Quartet
4	5	Quintet
5	6	Sextet
Mixed / Complex	Multiple peaks	Multiplet

🔀 Example (Ethanol):

CH₃CH₂OH

• CH₃– next to CH₂ → sees 2 H → triplet

• –CH₂– next to CH₃ → sees 3 H → quartet

• –OH → no regular neighbors → singlet (often broad)

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🧮 4. Coupling Constant (J)

📌 What is it?

• The distance between the peaks in a multiplet (in Hz).

• Indicates how strongly protons are coupled.

📏 Key Info:

• Reported in Hz (not ppm)

• Measured between adjacent peaks in a split signal

• Identical J-values suggest coupled protons

🔧 Typical Values:

Coupling Type	J Value (Hz)
Vicinal (3-bond, H–C–C–H)	6–8 Hz
Geminal (2-bond, H–C–H)	12–16 Hz
Aromatic (ortho)	6–10 Hz
Trans alkene	11–18 Hz
Cis alkene	6–12 Hz

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🔎 5. Example Spectrum: Ethanol (CH₃CH₂OH)

Group	Shift (δ ppm)	Integration	Multiplicity	Explanation
–CH₃	~1.2	3H	Triplet	2 neighbors on CH₂
–CH₂–	~3.6	2H	Quartet	3 neighbors on CH₃
–OH	~4.5	1H	Singlet (broad)	Exchangeable; usually not split

🔁 The triplet and quartet have the same J-value, confirming they are coupled to each other.

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🧠 Recap: 4 Essential Features to Analyze

Feature	What It Tells You
Chemical Shift (δ)	Type of proton and its environment
Integration	Number of protons in that environment
Multiplicity	Number of neighboring protons (n+1 rule)
Coupling Constant (J)	Distance and strength of coupling

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🧪 Tips for Interpreting Spectra

1. Count number of signals → unique proton environments

2. Check integrations → total protons match molecular formula?

3. Identify splitting → use n+1 rule

4. Check δ values → assign functional groups

5. Cross-reference J-values → determine coupling relationships

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Would you like:

• A visual annotated spectrum of ethanol?

• A practice set of unknown NMR spectra to solve?

• Flashcards for the key ranges and splitting patterns?

Let me know how you'd like to reinforce this module before moving to Module 4: Chemical Environments & Shifts!