Proton NMR spectroscopy: principles, interpretation

🧪 Proton NMR is like a dance floor where each hydrogen atom (proton) is a dancer. The magnet pulls them into a line, and when we give them a little “push” (radiofrequency pulse), they wiggle and send back a signal. By listening to how they wiggle, we learn where each proton sits in the molecule.

🔬 Magnetic Field Alignment
In a strong magnetic field $B_0$, protons align either with or against the field. The energy difference is tiny, but with a radiofrequency pulse we can flip protons into the higher energy state. When they relax back, they emit a signal.

📐 Chemical Shift ($δ$)
The frequency at which a proton resonates depends on its electronic environment. We express this as a chemical shift: $$δ = \frac{ν - ν_{ref}}{ν_{spectrometer}} \times 10^6 \;\text{ppm}$$ where $ν$ is the proton’s frequency and $ν_{ref}$ is that of a reference (usually TMS). The smaller the shielding, the larger the $δ$ value.

🧩 Coupling (J‑coupling)
Protons that are close (usually 3 bonds apart) interact, splitting each signal into a multiplet. The splitting pattern tells us how many neighbouring protons there are.

🧪 Spectrum features:

• δ 1.2 ppm – triplet, integration 3H → CH₃ of ethyl group
• δ 2.5 ppm – quartet, integration 2H → CH₂ next to CH₃
• δ 3.7 ppm – singlet, integration 3H → O‑CH₃ of acetate
• δ 7.2 ppm – singlet, integration 3H → CH₃ of acetyl group (no splitting because no neighbours)

?? The pattern matches the structure CH₃COOCH₂CH₃.

🔍 Always start with integration. It gives the proton count for each signal.

🔍 Use the n+1 rule. A doublet means one neighbour, a triplet two neighbours, etc.

🔍 Check for symmetry. Equivalent protons give the same signal.

🔍 Remember the chemical shift ranges. They are your first clue to the proton type.

💡 Practice with real spectra. The more you see, the faster you’ll recognise patterns.