NMR Spectroscopy Notes

Introduction to NMR Spectroscopy

• Two main types of NMR:
  1. 13C NMR
  2. 1H (Proton) NMR
• 13C NMR:
  • ~1% of C13 in organic molecules.
  • Modern machines are sensitive enough for full spectra.
  • Simpler spectrum compared to 1H NMR.
• 1H NMR:
  • Intensity of signals (integration value) proportional to the number of equivalent H atoms.

Equivalent Atoms

Carbon Atoms (13C NMR)

• Each set of equivalent carbon atoms gives one signal/peak.

Hydrogen Atoms (1H NMR)

• Each set of equivalent hydrogen atoms gives one signal.
• Intensity related to the number of equivalent H atoms.

Solvents

• Samples dissolved in inert solvents without 1H atoms (e.g., CCl4, CDCl3).
• Solvents don’t show peaks in 1H NMR spectrum.

Calibration and Chemical Shift

• TMS (Tetramethylsilane) used for calibrating the spectrum:
  • One signal, strong signal, non-toxic, inert, easy to remove.
  • Calibration based on chemical shift (δ), measured in ppm.
• Chemical shift indicates field shift from TMS.

Spin-Spin Coupling in 1H NMR

• Splitting: Number of neighboring inequivalent Hs + 1.
• High-resolution 1H NMR shows further line splitting due to neighboring Hs.
• Peak types based on neighbor Hs: singlet, doublet, triplet, quartet.

Spectra Examples

• Ethanol:
  • Three peaks due to three different groups of hydrogens.
  • Group a (triplet), shift: 0.7-1.2 ppm, integration: 3.
  • Group b (quartet), shift: 3.7-4.1 ppm, integration: 2.
  • Group c (singlet), shift: 0.5-5.0 ppm, integration: 1.

Analysis of Molecular Structure

1. Empirical Formula:
   • Calculated from elemental analysis.
2. Molecular Formula:
   • Derived from mass spectrum m/z value.
   • Example: C₈H₁₆O₂ as an ester based on IR spectra (C=O present, no O-H).
3. NMR Detailed Analysis:
   • Determine carbon chain details from number and type of peaks.

Example Structure and Analysis

• Using IR and NMR spectra:
  • Identifies functional groups (e.g., ester).
  • Peaks and integration traces help deduce the structure.
  • Example: CH₂COOCH₂CH₃ with noted peaks and their areas.