A Level Chemistry - How to Solve: NMR Spectroscopy (Chemical Shift, Integration, Splitting Patterns) – Complete Guide

How to Solve: NMR Spectroscopy (Chemical Shift, Integration, Splitting Patterns) – Complete Guide

Introduction

"Master NMR spectroscopy, and you’ll crack 10–15% of your A-Level Chemistry exam—identifying unknown molecules, predicting structures, and scoring top marks on organic synthesis questions."

(For GCSE/IGCSE: "NMR is the key to understanding how chemists ‘see’ molecules—like a fingerprint for organic compounds. Nail this, and you’ll ace questions on structure determination.")

WHAT YOU NEED TO KNOW FIRST

Before diving into NMR, you must understand: 1. Proton environments – How hydrogen atoms in different parts of a molecule experience different magnetic fields. 2. Electronegativity & shielding – How nearby atoms (e.g., O, Cl) pull electron density away from protons, changing their chemical shift. 3. Spin-spin coupling – How neighbouring protons split signals into multiplets (n+1 rule).

(If you’re shaky on these, pause and review them first—NMR won’t make sense without them!)

KEY TERMS & FORMULAS

1. Chemical Shift (δ, in ppm)

• Definition: The position of a signal on the NMR spectrum, measured in parts per million (ppm).
• Reference: TMS (tetramethylsilane, Si(CH₃)₄) is the standard (δ = 0 ppm).
• Key ranges (MEMORISE THIS):
• 0–2 ppm: Alkyl (CH₃, CH₂, CH) protons (shielded).
• 2–4 ppm: Protons next to electronegative atoms (e.g., -OH, -Cl, -Br).
• 4–6 ppm: Alkenes (C=C-H).
• 6–8 ppm: Aromatic (benzene ring) protons.
• 9–10 ppm: Aldehydes (R-CHO).
• 10–12 ppm: Carboxylic acids (R-COOH).

2. Integration (Peak Area)

• Definition: The area under a peak tells you the relative number of protons in that environment.
• Formula:
• Integration ratio = Number of protons in environment A : Number of protons in environment B
• Example: If a peak at 1.2 ppm has an integration of 3H, and a peak at 3.5 ppm has 2H, the ratio is 3:2.

3. Splitting Patterns (n+1 Rule)

• Definition: Peaks split into multiplets due to spin-spin coupling with neighbouring protons.
• Formula (MEMORISE THIS):
• Number of peaks = n + 1
  • n = number of equivalent neighbouring protons (within 3 bonds)
• Common patterns (MEMORISE THIS):
• Singlet (s): 0 neighbours (e.g., -OH, -CHO, isolated CH₃).
• Doublet (d): 1 neighbour (e.g., CH next to CH).
• Triplet (t): 2 neighbours (e.g., CH₂ next to CH₂).
• Quartet (q): 3 neighbours (e.g., CH₃ next to CH₂).
• Multiplet (m): 4+ neighbours (complex splitting).

STEP-BY-STEP METHOD

Follow these 5 steps for every NMR question:

Step 1: Count the number of signals

• Each unique proton environment gives one signal.
• How to check: If two protons are in identical chemical environments (e.g., the 6 H in (CH₃)₂C), they give one peak.

Step 2: Read the chemical shifts (δ, ppm)

• Use the key ranges to guess possible groups.
• Example: A peak at 7.2 ppm → likely aromatic (benzene ring).

Step 3: Check integration (peak area)

• The area under each peak tells you the relative number of protons.
• Example: If a peak at 1.2 ppm has an integration of 6H, it’s likely two identical CH₃ groups.

Step 4: Analyse splitting patterns (n+1 rule)

• Look at how many peaks each signal splits into.
• Example: A quartet (4 peaks) means 3 neighbouring protons (n=3 → 3+1=4).

Step 5: Piece together the structure

• Combine chemical shift, integration, and splitting to deduce the molecule.
• Example: If you see:
• Triplet (3H) at 1.2 ppm → CH₃ next to CH₂.
• Quartet (2H) at 3.5 ppm → CH₂ next to CH₃. → Likely CH₃-CH₂- group (ethyl group).

WORKED EXAMPLES

Example 1 – Basic: Ethanol (CH₃CH₂OH)

NMR Spectrum: - Peak 1: 1.2 ppm, triplet, integration = 3H - Peak 2: 3.7 ppm, quartet, integration = 2H - Peak 3: 2.6 ppm, singlet, integration = 1H

Step-by-Step Solution: 1. Count signals: 3 peaks → 3 proton environments. 2. Chemical shifts:
- 1.2 ppm (triplet, 3H) → CH₃ next to CH₂ (alkyl).
- 3.7 ppm (quartet, 2H) → CH₂ next to CH₃ (deshielded by -OH).
- 2.6 ppm (singlet, 1H) → -OH (no neighbours, broad singlet). 3. Splitting:
- Triplet (3H) → 2 neighbours (CH₂) → CH₃-CH₂-.
- Quartet (2H) → 3 neighbours (CH₃) → -CH₂-CH₃.
- Singlet (1H) → 0 neighbours → -OH. 4. Structure: CH₃-CH₂-OH (ethanol).

What we did and why: - Used chemical shift to identify groups. - Used integration to confirm proton counts. - Used splitting to link groups together.

Example 2 – Medium: 1-Bromopropane (CH₃CH₂CH₂Br)

NMR Spectrum: - Peak 1: 1.0 ppm, triplet, integration = 3H - Peak 2: 1.8 ppm, sextet, integration = 2H - Peak 3: 3.4 ppm, triplet, integration = 2H

Step-by-Step Solution: 1. Count signals: 3 peaks → 3 proton environments. 2. Chemical shifts:
- 1.0 ppm (triplet, 3H) → CH₃ next to CH₂ (alkyl).
- 1.8 ppm (sextet, 2H) → CH₂ with 5 neighbours (unusual—check splitting).
- 3.4 ppm (triplet, 2H) → CH₂ next to Br (deshielded). 3. Splitting:
- Triplet (3H) → 2 neighbours → CH₃-CH₂-.
- Sextet (2H) → 5 neighbours → CH₂ with 5H next door? No—actually 2 neighbours (n=5? No!) → Correction: A sextet means 5 peaks → n=4 neighbours (but that’s impossible for CH₂).
- Realisation: This is a pentet (5 peaks) from 4 neighbours (e.g., CH₂ between CH₃ and CH₂Br).
- Triplet (2H) → 2 neighbours → -CH₂-Br. 4. Structure: CH₃-CH₂-CH₂-Br (1-bromopropane).

What we did and why: - Recognised sextet = 5 peaks → n=4 neighbours (but adjusted for realistic structure). - Used deshielding (3.4 ppm) to place Br on the end.

Example 3 – Exam-Style: Unknown Compound (C₃H₆O)

NMR Spectrum: - Peak 1: 2.1 ppm, singlet, integration = 3H - Peak 2: 2.4 ppm, quartet, integration = 2H - Peak 3: 9.8 ppm, singlet, integration = 1H

Step-by-Step Solution: 1. Count signals: 3 peaks → 3 proton environments. 2. Chemical shifts:
- 2.1 ppm (singlet, 3H) → CH₃ next to C=O (ketone/aldehyde).
- 2.4 ppm (quartet, 2H) → CH₂ next to CH₃ (deshielded by C=O).
- 9.8 ppm (singlet, 1H) → Aldehyde (-CHO). 3. Splitting:
- Singlet (3H) → 0 neighbours → CH₃-C=O.
- Quartet (2H) → 3 neighbours → CH₂-CH₃.
- Singlet (1H) → 0 neighbours → -CHO. 4. Structure: CH₃-CH₂-CHO (propanal).

What we did and why: - Used 9.8 ppm to confirm aldehyde. - Linked CH₃ (singlet) and CH₂ (quartet) to form CH₃-CH₂-. - Combined to get CH₃-CH₂-CHO.

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP (Night Before the Exam)

"Right, listen up—NMR in 60 seconds. First, count the peaks—each one is a different proton environment. Next, check the chemical shift: 0–2 ppm is alkyl, 6–8 ppm is benzene, 9–10 ppm is aldehyde. Then, look at integration—the area under the peak tells you how many protons are in that group. Now, splitting: singlet (0 neighbours), doublet (1), triplet (2), quartet (3). Use the n+1 rule—number of peaks = neighbours + 1. Finally, piece it together like a puzzle. If you see a triplet at 1 ppm and a quartet at 3.5 ppm, that’s an ethyl group (CH₃-CH₂-). And watch out for OH/NH—broad singlets that disappear with D₂O. That’s it—go smash that exam!"