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  Nuclear Magnetic Resonance Spectroscopy,

Introduction:

Nuclear magnetic resonance (NMR) spectroscopic technique is depends on the magnetic properties of indeed atomic nuclei. Magnetic properties are simply revealed via molecules, which contain either atoms through odd mass number or an uneven number of electrons. The nucleus of atoms through odd mass number has both spin and magnetic properties. A spinning completely charged nucleus possesses a magnetic moment that is capable of interacting via an outwardly applied field. Nuclear magnetic resonance (NMR) spectroscopy is a technique that is depends on quantization of the spin angular momentum of the nucleus.

Basic Principle of NMR

In order to generate an NMR spectrum, the nucleus must contain a net spin. For a nucleus to have spin, it must contain an odd mass number. Therefore, nuclei these as 1H, 13C, 9F and 31P are appropriate, whereas atoms by even mass number, like 12C and 16O aren't appropriate for NMR. Hydrogen is the most significant material, which displays NMR properties then pursued via Carbon. Therefore NMR studies allow the finding of the positions of such atoms inside a molecule. This is feasible since the magnetic properties of diverse atomic nuclei differ and therefore tend to find out their energy absorption in a magnetic field, whenever irradiated through electromagnetic radiation in the radio frequency region.

The Source of NMR Spectra

Atomic nuclei possess charge. Several isotopes these as 1H act as a small spinning magnet. This is since they possess together electric charge and mechanical spin. A spinning charge produces a magnetic field. The nuclei of certain isotopes of atoms have spin quantum number (l) which is ± 1/2. Hence, they can have either of 2 spin states: +1/2 or -1/2 analogous to the magnetic moment.

In the presence of an external field, the proton can contain 2 orientations; either aligned through the field termed parallel orientation or against the field described antiparallel orientation. The parallel orientation happens at lower energy state while antiparallel at higher energy. In totaling to this, the proton will as well precess around the axis of applied magnetic field. The spinning frequency of the proton doesn't transform but precessional frequency is straight proportional to the strength of the outside field (H0). For instance, a proton exposed to an external magnetic force of 14000 gauss will precess about 60 million times per second so that V = 60 MHz

294_Proton precessing in a magnetic field H0.jpg

Fig: Proton precessing in a magnetic field H0

Whenever the precessing proton is irradiated by a beam of radio frequency, the low energy nuclei might absorb this energy and shift to a higher energy state. This is feasible only if the precessing frequency is equivalent to the frequency of the radio frequency radiation for example if the 2 are in resonance. Therefore, the term is described nuclear magnetic resonance.

NMR spectrum

In NMR experiments, a solution of substance under investigation is placed in a strong magnetic field. Then the solution is irradiated with radio frequency energy of appropriate frequencies. The energy absorbed by the protons is recorded as NMR spectrum.

Pattern of the Spectrum

The environment of each nucleus in a molecule is dissimilar depending on the orbital and bonding electrons. As well the absorption of energy by the nucleus is in accordance through its environment. This enables the hydrogens in a methyl group (-CH3) to be differentiated from those in the methylene group (-CH2), and hydroxyl group (-OH). Instance, consider the dissimilar environments of the hydrogen atoms in the ethanol molecule, CH3CH2OH. The frequency at that absorption takes place for the hydrogen atoms in the -CH3 groups is diverse from those in the -CH2 groups that in turn differs from those in the -OH group. As a consequence the NMR spectrum of ethanol shows three absorption peaks, caused via the hydrogen in each of these 3 groups. The spectrum is calibrated against a standard that is generally tetramethylSilane (TMS) and then interpreted.

1474_tetramethylsilane.jpg

The NMR spectrometer

The essential components of an NMR spectrometer are as given:

  • Strong magnet
  • Radio frequency oscillator
  • Radio frequency receiver
  • Recorder with calibrator and integrator.
  • Sample holder
  • Temperature control

Operational procedure

For a non viscous liquid about 0.5 ml is used, while for solid samples 10 - 50 mg/0.5 ml solvent is utilized. The sample solution is filtered to eliminate any particles. About 0.5 ml is put into a glass sample tube through internal diameter of about 0.5cm. Few drops of the internal standard added. The example is situated in the model holder and left for several minutes for the temperature to equilibrate. The tube is spun at 30 - 60 rps. The spectrum is recorded, and then followed via integration of the peaks.

863_Schematic diagram of an NMR Spectrometer.jpg

Fig: Schematic diagram of an NMR Spectrometer

Application of NMR

The major utilize of this method is in the determination of the structure of organic compounds depend on the absorptions of the hydrogen nuclei. The method is as well vital in the learn of reaction rates and mechanisms.

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