Answer

Understanding ¹H NMR Spectra: A Detailed Guide

1. Introduction to ¹H NMR Spectroscopy

¹H NMR (proton nuclear magnetic resonance) spectroscopy provides information about the types and environments of hydrogen atoms in a molecule. Key features analyzed include chemical shifts, signal multiplicity, integration, and the number of unique signals.

2. Reference Standard: Tetramethylsilane (TMS)

• TMS is used as a reference compound in NMR and is assigned a shift of 0 ppm.
• It provides a sharp, singular peak and helps calibrate the chemical shift scale.

3. Case Study: 1,4-Dimethylbenzene (p-Xylene)

Molecular Formula: C₈H₁₀

This compound consists of a benzene ring with two methyl groups at the para positions (1 and 4), creating a plane of symmetry.

4. Chemical Environments and NMR Signals

Each unique proton environment gives rise to a distinct NMR signal.

• Methyl Protons (–CH₃): Two equivalent methyl groups (3 protons each) produce one signal.
• Aromatic Protons: Four equivalent protons on the benzene ring produce another signal.

Total Signals: 2

5. Observed Chemical Shifts

6. Integration and Ratio

The area under each peak corresponds to the number of protons in that environment:

• 2.3 ppm → 6 H from methyl groups
• 7.0 ppm → 4 H from aromatic ring

Integration Ratio: 6:4, simplified to 3:2

7. Importance of Symmetry

Symmetry in the molecule reduces the number of distinct proton environments, leading to fewer signals in the spectrum. In asymmetric compounds, more environments would be present, increasing signal count.

✅ Conclusion