Chemical shift NMR - population density, saturation, TMS standard preference

In NMR, their peak intensity depends upon the population density of nuclear spin state. Chemical shift depend upon shielding and deshielding effect.


Nuclear Magnetic resonance or NMR spectroscopy is very very important for the identification of molecular structure in organic chemistry. Scientists are determined the unknown molecular structure using the proton NMR and carbon NMR, using There are several types of methodology like pulse determination of molecular structure.

Chemical shift NMR
Chemical Shift value calculation, TMS reference banner: Created by Chemarticle.

What is the Basic Concept of Nuclear Magnetic Resonance?

NMR basic concept or basic principle in presence of applied field and absent of applied field, what happening the nuclear spin state distribution and their population distribution in the discussion.

In nuclear Magnetic resonance spectroscopy chemical environment of the molecular structure, how determines the chemical shift value using the shielding and deshielding effect, the local diamagnetic current on the nuclei.

We are already discussed what is the nuclear spin state, nuclear magnetic moment, absorption energy, and mechanism of absorption in NMR.

The Nuclear Magnetic resonance process occurs when the Larmor frequency is equal to the spectrometer frequency.

Table of contents

Popularity density of nuclear spin state

According to Boltzmann distribution low, applied magnetic field two types of nuclear distribution one is upper level and another is lower level. Two nuclear energy states are energetically unequal, corresponding to the number of nuclear are unequally distributed. The number of nuclear spins is large in the upper nuclear spin state compared to the lower nuclear spin state. Or almost the same.

Excess Population in the Nuclei

The difference between the number of nuclei population density in these two spin states is called Population difference or excess population. This is very much lower compared to two different spin state population densities.

Population Density with proper Example:

Let us, 60 megahertz instrument applied magnetic field nuclear are distributed in two different energy states they are one is higher energy and one is lower energy. The number of nuclei in the higher nuclear spin state is 1 million 9 and in the lower nuclear spin state is 1 million.

Population difference or excess population
The excess population of nuclei in a lower energy state.

That means Population difference or excess population is 9 in the total 2 million nuclei.

The excess nuclei are allowed for or observe nuclear resonance. When 60 megahertz instrumental applied field radiation is applied the transition occurs upward and also be stimulated transition downward. The population of the nuclear spin state and lower nuclear spin state become exactly equal when we have to observe that signal is not formed.  This is called saturation.

  • The saturation occurs in NMR When the NMR radio frequency signal is too high.

What is the Boltzmann distribution equation in the Popularity density of the Nuclear spin state?

The ratio of the number of nuclei in the upper nuclear spin state and lower nuclear spin state, it's called the Boltzmann ratio of nuclear spin. This is the dependent energy difference between two energy states (∆E). Since, ∆E=hv, where v is frequently of NMR instruments.

Popularity density of nuclear spin state

Therefore we can see that in case NMR instrument frequency to high then the energy difference between the two states becomes high. Clearly, instrument frequency and the energy difference between the two-state are proportional. Then NMR signal intensity of the peak becomes high.

Shielding and Deshieding effect

Nuclear Magnetic resonance has effectiveness because every proton has a different resonance frequency in a molecule. This Variation occurs in electronic clouds surrounded by nuclear which is a slightly different electronic environment from one to another.

What is Shielding Effect?

The variable fact is that electron density in valence shell electrons they have surrounded nuclei or Proton are shielded by electronic cloud. An applied or external magnetic field shielded for the electronic cloud is generated by the electronic magnetic current that is circulating in the proton. Circulation is called local magnetic current which is generated by the magnetic field, their direction is opposite to the applied or external magnetic field. This is called the shielding effect.

Shielding and Deshieding effect in NMR
  Diamagnetic  anisotopic -Shielding effect in NMR spectroscopy

What is the De-Shielding Effect?

Increase electron density of the nuclei is Less due to any reason like electronegative group effect or any electron negative group are attached to nuclei which cases local magnetic current or induced magnetic field is less. That means the electronic magnetic field is less affected by the opposite direction applied or the external magnetic field. This effect is called the Deshielding effect.


Any case-induced magnetic field is aligned with the applied magnetic field when the magnetic field will be increased in cases Deshielding has occurred and de-shields the proton.

What is the chemical shift?

Definition of the chemical shift:

Using the reference substance difference between TMS required radiofrequency or Larmor frequency and sample like Methyl Bromide radio frequency in hertz are called chemical shift.

The chemical shift value in hertz is divided by the instrument frequency value in megahertz is called the chemical shift in ppm.

Chemical shift definition -ppm unit

What is the unit of the chemical shift value?

The unit of shift value in NMR is Hertz. The shift value is divided by instrument frequency in  Megahertz, chemical shift unit becomes ppm

What is the standard reference for NMR Spectroscopy

What is the standard reference for chemical shift

TMS is the international standard reference for the chemical shift of NMR spectroscopy, it is a very useful standard solvent TMS. It is full name Tetra methyl Silane. 

Why is TMS used as a standard reference

  • TMS low boiling point liquid
  • It is a volatile compound, therefore, we can readily remove it from a sample after using it.
  • TMS has four hand methyl groups that are the same, therefore there has a present equivalent proton, and all methyl protons are chemical equivalent. A total of 3 into 4 is equal to 12 Protons are present they have observed the same position. 
  • Therefore their signal intensity will be high compared to another solvent.
  • TMS all protons are shielded causes it is resonance position is high field almost other hydrogen resonance in organic molecules solvent.


Finally, we will have concluded that the enema study fore structure determined very very useful to chemical shift and their population it depends on the peak intensity of the signal.

Frequently asked question

Why ppm is a chemical shift?

Ans: The problem is that different spectrometry instruments have different types of signals formed. That means one molecule gives a different signal in different NMR spectroscopy instruments in hertz. For problem-solving one molecule give the identical 1 signal, the chemical shift in hertz divided by instrument frequency in megahertz that is equal to ppm.

The chemical shift value in ppm gives the one signal for one molecule that is used or internationally or uniquely.