NMR spectroscopy- Basic principle, Nuclear spin, Absorption process for a proton


Nuclear Magnetic Resonance is NMR, Which is a spectroscopic method that is very important for an organic chemist. NMR Techniques study is mostly common atoms/Nuclei are one is Hydrogen and another carbon nuclei. NMR study gives information about how many numbers of magnetically distinct atoms are present in the molecule. A 1H NMR spectroscopy or 13C NMR spectroscopy is mostly studied in the organic chemistry portion. Mainly a combination of NMR and IR spectroscopy data gives the complete structure determination of unknown molecules.


1. Basic principle of NMR spectroscopy
2. Nuclear spin state of NMR studies
3. What is Nuclear spin quantum number (I)
4. Nuclear magnetic resonance spectroscopy (NMR) activity and inactivity.
5. Which isotope nuclei are NMR active and inactive?
6. Basic principles of Nuclear Magnetic Moments
7. Hydrogen NMR studies (1H NMR)
8. Absorption of Energy in NMR 

Basic concepts of Nuclear Magnetic Resonance(NMR) Spectroscopy:

NMR spectroscopy mainly concepts of Nuclear spin Resonance, Which is dependent upon the nuclear spin (I). In the case of nuclear spin I=0; that means they have non-magnetic atoms that occur in a particular molecule or atom, corresponding NMR inactive. Nuclei with the atomic number even and mass number also even will have a nuclear spin Quantum number equal to zero( I=0) and will be NMR inactive, they are example are 12 C, 16O, 4He nuclei  I=0. And the other hands nuclear spin (I) is not equal to zero, which means nuclei will be magnetic I=1/2, 1, 0, 3/2, 5/2, etc, corresponding to NMR spectroscopy will be NMR active. Nuclei with the atomic number even and mass number odd, atomic number odd and a mass number even, atomic number odd and mass number odd, which are cases nuclear spin quantum number (I) not equal to zero and will be NMR active. These examples are 1H, 2H, 17O, etc. 

For each nucleus, the spin quantum number I is a physical constant and there are 2I+1 allowed spin states with integral, which range from +I to -I. 
+I, (I+1), …..(-I+1), -I

The basic principle of NMR spectroscopy is that all magnetic nuclei have a different spin and there all are electronic charges. When applied any external magnetic on these nuclei an energy state should be transferred from the Ground-state (lower energy state) to the Excited energy state (higher energy state).

Nuclear Magnetic Moments in NMR studies:

In an applied external magnetic field, the nuclear spin state is not equivalent to energy because the nucleus is a change particle, Which is forwarded to the magnetic field itself. Nuclear magnetic Moments are generated by change and their spin.

A hydrogen nucleus has two spin-states one clockwise (+1/2) and another one anticlockwise (-1/2) spin. Nuclear magnetic Moments are in two opposite directions, +1/2 spin is aligned with the magnetic field (B 0), and -1/2 is opposite with magnetic field (B 0).

Hydrogen nuclei have two spin-states one aligned with the magnetic field and another opposed to the magnetic field. The nuclear spin state +1/2 is lower energy since it is aligned with the external magnetic field which the spin state is much more stable, on the other hand, the -1/2 spin state is higher energy due to it being opposed to an applied field. The opposed spin state concerning the external field is less stable due to the center magnet being repelled out of it is current orientation. When the external magnetic field is applied this work should occur.
Without a magnetic field, two spins degenerate.

What is the absorption process in NMR spectroscopy?

The Nuclear Magnetic Resonance process occurs when nuclear spin aligned with an external field is induced to absorb energy and correspond to change their spin orientation concerning the applied magnetic field.

In cases, the absorption energy will be a quantized process, which is the equal energy difference between the two energy states involved.

E(absorbtion)= (E-1/2 state – E+1/2 state ) =hv

The stronger the applied external magnetic field the greater the energy difference between the nuclear spin state.

∆E= f( B)
The magnitude of the energy level also depends on the particular nucleus. Each nucleus has a different magnetic moment to its magnetic momentum due to each nucleus having a different charge and mass. mainly this is called magnetogyric ratio, which is a constant for each nucleus.



When the magnetic field is applied, the nucleus is processed with its own axis and spins with angular frequency (w) which is called Larmor frequency.

Since the nucleus has a charge, the precession of nuclei generates an oscillating electric field of the angular frequency. When the radiofrequency waves of this frequency are supplied to the processing of the proton nuclei, energy can be absorbed. This process occurs the frequency of the oscillating electric field component of the incoming radiation just matches the frequency of the electric field generated by the precessing nuclei, two electrical fields are coupled to each other and their energy can be transferred from the incoming radiation to the nucleus, thus causing nuclear spin changes. This condition is called Nuclear Resonance.


We have finally concluded the nuclear spinning with processing corresponding to their angular momentum will be tilt rotation. Nucleus act as a tiny magnet that is rotating by an Applied magnetic field. Nuclear Magnetic Resonance (NMR)  basic reasonable spin is aligned or opposite to the applied magnetic field. They are two different energy states distributed in presence of an external magnetic field.

Next: What is Chemical Shift?

Frequently asked questions:

Which elements or isotopes are used in NMR spectroscopy?

Ans: Mostly Magnetic nuclei like Hydrogen (proton), 13C, etc are NMR active. These types of isotopes or nuclei are used in NMR spectroscopy. Mainly 13C NMR spectroscopy has a little bit rear comper to Hydrogen or proton NMR due to the natural abundance of the 13 C isotopes being very less than 1.1%, therefore, comparatively 12C isotopes are present in nature larger amount ( larger natural abundance 98.9%).

What is NMR spectroscopy used for?

Ans: In this organic chemistry NMR spectroscopy study for structure determination of the unknown compounds. And NMR spectroscopy has several uses for physical, chemical, and biological. Like for medical purposes use magnetic resonance imaging (MNI).

What are the types of NMR spectroscopy?

Ans: NMR spectroscopy technique mainly two types 1. Continuous-wave (CW) 2. Fourier transform (FT NMR).