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NMR Spectroscopy or Nuclear Magnetic Resonance spectroscopy uses the concept of the chemical shift to record the signal. One needs to understand what is NMR spectroscopy to get the concept of chemical shift.
The concept of NMR spectroscopy
Only certain nuclei show NMR phenomena. Nuclei that have an odd atomic number or atomic mass number show NMR phenomena. For each nucleus, the number I is a physical constant and there are 2I + 1 allowed spin states with integral differences ranging from +I to -I.
In the absence of a magnetic field, all the spin states of a given nucleus are of equivalent energy. But when the magnetic field is applied, the nucleus is a charged particle, and any moving charged particle generates a magnetic field of its own. In the applied magnetic field, any proton will have either an aligned or opposed spin with respect to the applied magnetic field. Nuclei that show NMR phenomena when placed in a magnetic field precess at a certain frequency which is called precessional frequency.
When a nucleus that shows NMR phenomena is placed in a magnetic field, it precesses at a certain precessional frequency which depends on the magnetic field experienced by the nucleus. When such nuclei are exposed to radiowaves, they can absorb radiofrequency which is equal to the precessional frequency of that nucleus.
What is Chemical shift
Definition and formula
Chemical shift is the difference between the precessing frequency of the sample and the internal standard divided by the operating frequency of the NMR instrument.
It is denoted by delta (δ).
Chemical shift (δ) in ppm = υ(sample) – υ(standard) in Hz / Operating frequency of NMR instrument in MHz
Explanation
In H1 NMR, not all the nuclei in a molecule have resonance at exactly the same frequency. This is due to the fact that protons in a molecule are surrounded by electrons and exist in a slightly different electronic environment from one another.
Due to the local diamagnetic current generated by the circulation of valence shell electrons, each proton in a molecule is shielded from the applied magnetic field. Shielded nucleus precess at a lower frequency.
It is very difficult to measure the exact frequencies of all the types of nuclei. So, we add an internal standard to the sample solution. One of the most commonly used internal standards in NMR spectroscopy is tetramethylsilane (TMS).
The protons in TMS are more shielded than most of the protons in other known compounds. So, the protons present in TMS precess at a much lower frequency. In the spectrum scale of NMR, we plot chemical shift on the x-axis and we start the scale from zero where the precessing frequency of TMS is adjusted to zero.
Now, the difference between precessing frequency of the sample and that of TMS is plotted on the x-axis as a chemical shift.
But since when we increase the strength of the magnetic field, the value of chemical shift will proportionally increase and is still not completely constant no matter what the magnetic field is.
Therefore, we divide the chemical shift by the operating frequency of the NMR instrument in MHz. So, now no matter what the magnetic field is, the chemical shift will remain constant.
Why is Chemical shift plotted on the x-axis and not radiofrequency?
In an instrument operating at 60 MHz with 1.41, Tesla applied a magnetic field, the magnetic field for each use will not remain at 1.41 Tesla. There will be fluctuations in the magnetic field applied. As a result of these fluctuations, the precessing frequency of the same proton will be slightly different when performed twice.
So, we add an internal standard assuming that the fluctuations experienced by the proton will also be experienced by the internal standard to the same extent.
So, even though the precessing frequency of the sample fluctuates, the difference between the precessing frequency of the sample and the standard will remain the same.
And we know that the chemical shift is the difference between the precessing frequency of the sample and the standard divided by the operating frequency of the instrument.
Therefore, to avoid fluctuations and obtain reproducibility of results, we plot the chemical shift on the x-axis and not the radiofrequency.
Factor affecting chemical shift
There are the following factors that affect the chemical shift in NMR spectroscopy:
- Electronegativity
- Anisotropic effect
- Van Der Waals deshielding
- Axial and equatorial effect
