NMR Analysis Using NMR Simulator

Theoretical Background / Context

• Nuclei with odd atomic (number of protons) or mass number (total number of protons and neutrons) or both have quantized spin angular momentum and a magnetic moment. 
• Interaction of the magnetic moment of an atomic nucleus (µ) with an external magnetic field results in the absorption of energy and changes their spin orientation with respect to the external field. 
• NMR gives the chemical properties of molecules by studying the magnetic properties of the atomic nuclei which are measured by resonant absorption by nuclei.
• The electrons around the nucleus shield the nuclei from an effective magnetic field and require the energy of lower frequency to cause resonance. 
• Therefore different nuclei in the same compound come into resonance at different frequencies. 
• In general, hydrogens bound to carbons attached to electron withdrawing groups tend to resonate at higher frequencies. 
• The position where a particular hydrogen atom resonates relative to TMS is called its chemical shift, which can be predicted using NMR predictor tools.

Chemical Shift: 

• Nuclei in a different chemical environment experience different magnetic fields. 
• The electron cloud around the nuclei induces electronic angular momentum which gives rise to an additional magnetic field ß, through the usage of NMR simulator.

Solvent and Internal Standard:

Tetramethylsilane is used as an internal standard during the NMR measurements for the following reasons:
1. It is inert, non-toxic, and low boiling point, and can be recovered by distillation
2. A very small amount is required for measurement
3. It has 12 equivalent protons and thus gives a single strong peak
4. The signal appears outside (more upfield) away from most of the protons signals

• When using the modern NMR instruments, no internal standard is added to the sample to be measured.
•  The difference between the solvent signal and the TMS is known. 
• The modern instruments detect the solvent signal and lock them, which now can serve as the internal standard.

Principle Work of NMR Simulator

• Different pesticides will be analyzed for their chemical structure using a proton NMR. 
• Each sample is dissolved in a deuterated solvent and purified before NMR analysis. 
• For a non-polar to moderately polar compounds deuterated chloroform is commonly used. 
• For polar compounds, deuterated Dimethyl sulfoxide (DMSO) is commonly used. 
• An NMR spectrum gives various information of the compound to help in the structural elucidation of the organic molecule, e.g.: 

1) The number of signals that gives the  number of different sets of protons present in the sample measured.

2) The chemical shift that gives the environment of the protons.

3) Multiplicity that gives information on the number of protons on adjacent atoms, and integration of the peaks (peak area) that gives the number of protons in each environment using NMR simulator.