Nuclear magnetic resonance is the study of molecules by measuring the interaction of
radiofrequency electromagnetic radiations in the region 4 to 900 MHz with the nuclei of
molecules placed in a strong magnetic field.
It is used to characterize organic molecules by identifying carbon-hydrogen frameworks within
the molecules and also used in quality control and research for determining the content and
purity of a sample as well as its molecular structure. The protons present in the molecule will
behave differently depending on the surrounding chemical environment, making it possible to
elucidate their structure.
Types of NMR-
Two types of NMR spectroscopy are used
1. 1 H NMR- used to determine the type and number of H atoms in a molecule.
2. 13 C NMR- used to determine the type of carbon atoms in a molecule
The rules for determining the net spin of a nucleus are as follows;
If the number of neutrons and the number of protons are both even, then the nucleus has NO spin.
If the number of neutrons plus the number of protons is odd, then the nucleus has a half-integer spin
(i.e. 1/2, 3/2, 5/2)
• If anexternalmagneticfieldis applied,thenumberofpossibleorientationscalculatedby(2I+1).
PRINCIPLE OF NMR
•The principleis basedonthe-
spinning of nucleus and
generating amagnetic field.
– field nuclear spinarerandom
•With Bo ,nuclei align
themselves eitherwith oragainst
field of external
• If anexternalmagneticfieldis applied,anenergytransfer(ΔE) is possiblebetweengroundstatetoexcited
RULES FOR SPIN-SPIN COUPLING
• Chemicallyequivalentprotonsdonotshowspin-spincoupling &Onlynonequivalent protonscouple.
X Hb Hc Hd H
Ha C C C C H
Hc &Hd donotcouplebecausetheyareequivalent
H H H H
• Protonsonadjacentcarbonsnormallywill couple.
• Protonsseparatedbyfourormorebondswill notcouple.