NMR spectroscopy

 NMR

Nuclear magnetic resonance spectroscopy (NMR) is basically another form of absorption spectroscopy. Under appropriate condition in a magnetic field a sample can absorb electromagnetic radiation in the radiofrequency region, as frequencies being governed by the characteristic sample. Absorption is a function of certain nuclei in the molecule. A plot of the frequencies of the avsorption peaks versus peak intensities constitutes an NMR spectrum. NMR is a powerful tool for the investigation of molecular structure.

A hydrogen nucleus may have a clockwise (+1/2) or anticlockwise (-1/2) spin and the nuclear magnetic moment μ in the two cases are in the opposite direction. In an applied magnetic field, all protons have their magnetic moment either aligned with the field or opposed. The two spin state of hydrogen atom +1/2 is at lower energy since it aligned with the applied magnetic field while the spin state -1/2 is of higher energy state since it opposes the applied magnetic field.

Reasons for taking tetramethylsilane (TMS) as a reference compound
1. TMS has 12 equivalent protons and gives an intense single signal.
2. The electronegativity of silicon is very low as compared to carbon as a result of this the shielding of equivalent proton in TMS is more than that of almost all organic compound.
3. TMS is chemically inert and has a very low boiling point (300K) so that it can easily be evaporated after the spectrum has been recorded.
4. TMS is not soluble in aqueous solution.
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NMR