Nuclear Magnetic Resonance (NMR) in Organic Compounds

Nucleus and Magnetic Field

• A nucleus with protons (charged particles in motion) develops a magnetic field, behaving like a tiny bar magnet.
• In the absence of a magnetic field, nuclei are randomly oriented, but when a field is applied, they align with or against the applied field.

NMR (Nuclear Magnetic Resonance) Spectroscopy

• NMR is a spectroscopic technique used for structure elucidation (determining the number and location of H or C atoms in a compound) and purity assessments of organic compounds.
• NMR only detects atoms with spin quantum number > 0, such as 1H and 13C.
• Atoms with even atomic number and mass number, like 12C, have a spin quantum number of 0.

Magnetic and Nonmagnetic Nuclei

• Nuclei with an odd number of protons (e.g., 1H) or neutrons (e.g., 13C) are magnetic.
• Nuclei with even numbers of both protons and neutrons are nonmagnetic.

NMR Principle

• When a proton in a magnetic field is irradiated with radio waves of energy corresponding to the energy difference between two spin states, absorption of these radio waves occurs, and a "spin-flip" from low energy state to high energy state occurs.
• The frequency of the nucleus and the frequency of the absorbed radiation are "in resonance".
• The proton loses energy, returning to the ground state, and releases a photon of specific frequency, which is monitored.

NMR Analysis

• To analyze a sample using NMR, you need:
  • A sample
  • A magnetic field
  • Electromagnetic radiation (EMR), radio waves with long wavelength and low energy and frequency

Shielding and De-shielding

• High electron density around a nucleus shields the nucleus from the external magnetic field, resulting in signals upfield in the NMR spectrum.
• Lower electron density around the nucleus de-shields it from the external magnetic field, resulting in signals downfield in the NMR spectrum.
• Electronegative groups attached to a CH system decrease electron density around protons, causing de-shielding and a "compound chemical shift".

Tetramethylsilane (TMS)

• TMS is used to find a peak of zero in the NMR spectrum.
• TMS is used as a standard in NMR because:
  • Its 12 hydrogen atoms are in the same environment, resulting in only one signal in the NMR spectrum.
  • The 1H atoms in TMS are highly shielded by the silicon atom, resulting in a peak on the spectrum at the extreme right-hand side (shielded, upfield).

Chemical Shift

• Chemical shift is the difference between the resonance frequency of the observed proton and that of TMS, measured in parts per million (PPM).
• The resonance frequency of a nucleus in a magnetic field is influenced by its chemical environment, which arises from the shielding or deshielding of the nucleus by the surrounding electron cloud.

NMR Spectrometer

• Components of an NMR spectrometer include:
  • Sample holder
  • Glass tube
  • Permanent magnet
  • Magnetic coil
  • Radio frequency source
  • Detector
  • Readout system
• Types of samples include:
  • Both liquid and solid samples
  • Solvents for liquid samples: chloroform (CHCl3), carbon tetrachloride (CCL4), and carbon disulfide (CS2)
  • Solid samples: crystals, microcrystalline powders, gels, and protein fibers