THE NMR PHENOMENON :-
The spinning nucleus :-
The nucleus of hydrogen atom - it's proton behaves as a tiny spinning bar magnet, and it does so because it possesses both electric mechanical spin and electric charge. Any type of spinning charged body will be generate magnetic field and the nucleus of hydrogen is no exception.
The effect of external magnetic field :-
All bar magnets, the proton will respond to the influence of an external magnetic field, and it will tend to align itself with that field, in the manner of a compass needle in the earth magnetic field. Because of the quantum mechanics of restrictions with apply to nuclei but not to the compass needles, the proton can only adopting two orientation with respect to external magnetic field, either aligned with the field lower energy state, or opposed to the field the higher energy state. We can describes these orientation as a parallel with or anti-parallel with the applied field.
Precessional motion :-
The proton is behaves as a spinning magnet, not only can it align itself with or oppose an external magnetic field, but it will move in a characteristic ways and under the influence of the external magnet. Like a top the proton it will precess arrounding the axis of an applied external magnetic field. It can precess in two principle orientation, either aligned with the field or opposed to the field. This is represented in the below the figure, where " B₀" is the external magnetic field.
Precessional frequency : The spinning frequency of the nucleus doesn't change, but the speed of the precession does it. The Precessional frequency υ, directly proportional to the strength of the external magnetic field is
" B₀ " that is
υα β₀
For example, a proton exposed to external magnetic force of 1.4 T ( = 14000 gauss) will precess ɑ60 million times per seconds, so that υ = 60 MHz. For external field 2.3T, √ is ɑ 100 MHz.
Energy transitions :-
When a proton is external magnetic field is 1.4T, it will be precessing at frequency of ɑ60 MHz. It's capable of taking up any one of the two positions are aligned orientation, it can absorb energy and pass into the opposed orientation : subsequently it can lose with extra energy and relaxing back into the aligned position. If we irradiate the precessing nuclei with a beam of radio frequency energy and move to the highest energy state. The precessing proton only absorbed energy from the radio frequency source, if the precessing frequency is same as the frequency of the radio frequency beam: when this occurred, the nucleus and radio frequency beam, are said to be in resonance, Hence that term is " Nuclear Magnetic Resonance"
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