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Nuclear Magnetic Resonance (NMR): Basics


In NMR chapter, from the GPAT point of view, the quantum numbers, their role and chemical shift phenomenon are the important points. 
so, here I have given details about the same!

 View other instrumental analysis notes on the homepage ⇢ www.gpat360.blogspot.com 
Nuclear Magnetic Resonance (NMR)

- Absorption of radio frequency (electromagnetic radiation) waves induce transition in the molecules.
- Radiofrequency region: 4 MHz to 750 MHz

Principle:

- It is a branch of absorption spectroscopy in which radio frequency waves induce transitions between magnetic energy levels of nuclei of a molecule.
- The nucleus or proton behaves as a spinning magnet and it can align itself or oppose to an external magnetic field.
- It has precessional motion around itself.
- When external magnetic field is applied, this spinning proton either align (lower energy) or oppose (higher energy) to the field.

Precessional frequency (ν) ∝ Strength of external magnetic field (B°)
- In an applied magnetic field, magnetic nuclei like proton precess at a frequency ‘ν’ and when this frequency matches with radiofrequency, the exchange of energy takes place between radio waves and spinning nuclei.
- This leads to transition of nuclei from E1 ⟶  E2.
Source- Nuclear Magnetic Resonance 


Quantum numbers and their role in NMR

Spin quantum number is symbolized as ‘I’.
Quantum number is associated with mass number and atomic number of the nuclei.
The nuclei exhibit NMR phenomenon only if the spin quantum number (I) is greater than zero (0).

Mass Number
Atomic Number
Spin Quantum Number
Examples
Odd
Odd/ Even
½, 3/2, 5/2
136C, 178O
Even
Even
0
126C, 168O
Even
Odd
1, 2, 3
147N


Chemical Shift

What is Chemical Shift in NMR?
The shift in the position of NMR signals compared with standard resulting from the shielding and deshielding by electrons are referred as chemical shift.
The difference between the absorption position of proton and the absorption position of proton of reference compound is known as the chemical shift.
Most commonly employed reference compound: TMS (Tetramethyl silane).
 Chemical shift is a dimensionless property (expressed in ppm).

What is Shielding and Deshielding of protons?
-Each nucleus is having its own magnetic field (secondary field) which arises due to external (applied) magnetic field.

-When the secondary field produced by the circulating electrons oppose the applied (external) field at a particular nucleus in the molecule, it means that effective field experienced by that molecule is less than the applied field. This is known as positive shielding.

-This causes the appearance of peak in the upfield in NMR spectrum. This means that the value of applied field necessary to bring the nucleus into resonance will be greater than that of absence of secondary field.

-If the secondary field produced by the circulating electrons reinforces the applied field, the position of resonance moves downfield. This is known as negative shielding.
NMR shielding and deshielding- chemical shift

-Thus in Shielding, secondary field opposes applied field. Peak appears at upfield. (High energy is required).
- In Deshielding, secondary field reinforces the applied field. Peak appears at downfield. (Low energy is required).


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