NUCLEAR MAGNETIC

RESONANCE

SPECTROSCOPY

*Nuclear magnetic resonance spectroscopy is a powerful analytical technique

used to characterize organic molecules by identifying carbon-hydrogen

constitution within molecules.

*Two common types of NMR spectroscopy are used to characterize organic

structure: 1H NMR is used to determine the type and number of H atoms in a

molecule; 13C NMR is used to determine the type of carbon atoms in the

molecule.

*The source of energy in NMR is radio waves which have long wavelengths,

and thus low energy and frequency.

*When low-energy radio waves interact with a molecule, they can change the

nuclear spins of some elements, including 1H and 13C.

*The most commonly studied nuclei are 1H and 13C,other nuclei such as 2H,6Li, 10B, 14N, 29Si, 31P,etc. are also studied.

*A spinning charge creates a magnetic moment, so these nuclei can be

thought of as tiny magnets.

*If we place these nuclei in a magnetic field, they can line up with or

against the field by spinning clockwise or counter clockwise.

S

A spinning nucleus with it's magnetic field aligned with the magnetic field of a magnet

- spin state,favorable,lower energy

N

S

N

N

S - spin state,unfavorable,higher energy

A spinning nucleus with it's magnetic field aligned against the magnetic field of a magnet

S

N

*Alignment with the magnetic field (called α) is at lower energy than

against the magnetic field (called β). The energy difference depends on

the strength of the magnetic field.

*For example, we place CH4 molecule in a magnetic field.

*We find the energy difference of the α - and β - state of the protons by

irradiating them with EM radiation of just the right energy.

*In a magnet of 7.05 Tesla, it takes EM radiation of about 300 MHz

(radio waves).So, if we bombard the molecule with 300 MHz radio

waves, the protons will absorb that energy and we can measure that

absorbance.

*In a magnet of 11.75 Tesla, it takes EM radiation of about 500 MHz

(stronger magnet means greater energy difference between the α - and

β - state of the protons)

*Data is instead reported using the “chemical shift” scale as described

later.

E

Bo

E = h x 300 M Hz E = h x 500 MHz

7.05 T 11.75 T

proton spin state (lower energy)

proton spin state (higher energy)

Graphical relationship between

magnetic field (Bo) and frequency (

for 1H NMR absorptions

at no magnetic field,there is no difference beteen- and - states.

0 T

*Video illustrating hydrogen NMR

Schematic diagram for NMR spectroscopy:-

*Sampling:-Tetramethylsilane is the accepted internal standard for

calibrating chemical shift for 1H, 13C and 29Si NMR spectroscopy in organic

solvents (where TMS is soluble).

* In water, where it is not soluble, sodium salts of DSS, 2,2-dimethyl-2-

silapentane-5-sulfonate, are used instead.

*Because of its high volatility, TMS can easily be evaporated, which is

convenient for recovery of samples analyzed by NMR spectroscopy

*There are mainly two types of NMR spectroscopy instruments:-

(1)Fourier transform NMR instrument

(2)Continuous wave NMR instrument

*1>Fourier Transform NMR Instrument : In Fourier Transform NMR

Instrument small energy change takes place in the magnitude , present in

NMR and hence the sensitivity of this instrument is very less. The

sensitivity in Fourier Transform NMR is increased by adding the square root

of recorded spectra’s together .This process takes place by a continuous wave

instrument . Simultaneous irradiation of frequency occurs in a spectrum

having a radio frequency pulse and then the nuclei returns back to thermal

equilibrium or its normal state .

*2>Continuous Wave NMR Instrument : The Continuous Wave NMR

follows the principle of optical spectrometers .The frequency of the given

source of products is scanned by placing it in a magnetic field .Sometimes in

some products the source is kept constant and the magnetic field is scanned

and changed .Previously experiments were done using Continuous Wave

NMR but now Fourier transform NMR is high prefered .

*The most abundant form of carbon is 12C. It does not give NMR.

* The less abundant carbon isotope 13C and its spin is 1/2, so it can be used in

NMR.

*Carbon NMR is simpler than proton NMR because of the obvious reason that

carbon NMR has large chemical shift ranging as much as 200 ppm; while for

proton NMR, it is not more than 10ppm.

*On the other side in contrast to 1H NMR, the intensities of the signals are not

normally proportional to the number of equivalent 13C atoms and are instead

strongly dependent on the number of surrounding spins.

*Rest in all means, the C NMR is similar to H NMR.

*Medicine

*Chemistry(to detect compound)

*Purity determination (w/w NMR)

*Non-destructive testing

*Acquisition of dynamic information

*Data acquisition in the petroleum industry

*Flow probes for NMR spectroscopy

*Process control

*Earth's field NMR

*Zero Field NMR

*Quantum computing

*Magnetometers

Courtsey:-

GOOGLE

YOUTUBE

BAHL AND BAHL

WILLEY