Nuclear Magnetic Resonance (NMR) in Organic Compounds

Study Notes

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

Description

This quiz covers the principles of NMR, including the magnetic field of nucleus, spin and alignment of protons, and its application in structure elucidation and purity assessments of organic compounds.