Nuclear magnetic resonance (NMR) is frequently used by chemists to ellucidate structural information of molecules. How magnetic energy is used can be understood if we look at the principle of nuclear magnetic resonance.
As we had seen earlier in Basics of Magnetism – A charged particle in motion defines an electric current and any current gives rise to a magnetic field. Nuclei of atoms contain net positive charge and can spin (I).
Fig. 1 – shows a spinning nuclei generating a magnetic field indicated by N and S i.e. North and South poles of a magnet.
The motion of the nuclei is restricted to the spin which generates an angular momentum (P). Due to this angular momentum of P, the nuclei will have a magnetic moment of μ which is proportional to the spin. Thus nuclei with I=1/2 has 2I+1 i.e. two spin states in the presence of a strong magnetic field – either +1/2 or -1/2.
Fig. 2 – in the presence of a strong magnetic field the nuclei with I=1/2 (like protons) can either spin in +1/2 spin energy state or -1/2 spin energy state.
The angular momentum μ is proportional to the gyromagnetic ratio of the nuclei γ which is characteristic of the nucleus.
i.e. μ = ± 1/2 γ h
where h is a constant.
Thus the angular momentum of each nucleus is also a characteristic of the nucleus. The energy of the nucleus E in the presence of a strong magnetic field H0 is given by
E = – μ . H0
In the presence of a strong external magnetic field nuclei oriented in the +1/2 spin energy state will have a net lower energy as the orientation of the magnetic field is in the same direction as the strong magnetic field. While the nuclei in the -1/2 energy state will be in a higher energy state as the magnetic field produced by the nucleus is now opposing the strong magnetic field. This is analogous to a boat in a river. If the boat is with the flow of the river then less energy needs to be put while if the boat is against the river then more energy needs to be applied thus it will have higher energy. In our case the boat being the nucleus and the river being the strong magnetic field.
Thus in the presence of a strong magnetic field the nuclei will orient themselves either with the strong magnetic field or against the magnetic field. However since the nuclei with +1/2 spin energy state will be at a lower energy state, the population of nuclei which will be at a lower energy state will be higher than the population at the higher spin energy state of -1/2. This creates a difference in energy states, and an addition of energy to the lower energy state would result in absorption of energy of the nuclei to reach the higher energy state.
The energy difference between the two energy states is given by:
ΔE = μ . H0 / I
In turn the difference in energy level can be defined as
ΔE = hν
where h is Planck’s constant and ν is the frequency of the radiation which should be applied to change the energy level from lower to higher energy level.
Therefore the equations above turns to
h ν = μ . H0/ I
Thus in a given magnetic field H0, an irradiation of frequency ν would produce a change in the lower spin energy state population of nuclei i.e. +1/2 to rise to a higher energy state of -1/2. This frequency at which the nucleus absorbs energy is characteristic of the nuclei as it depends on the magnetic moment of the nucleus μ. NMR uses this characteristic frequency of absorption of energy by the lower spin energy state nuclei to identify different nuclei.
In all spectroscopic techniques there is a difference in energy states, and the absorption of energy leads to a rise in the energy state, and this absorption of energy gives us information in the energy state. In most spectroscopic techniques the different energy states are intrinsic in the atom or molecule i.e. there is a naturally present ground state and excited state. However, in NMR, this difference in energy states is created extrinsically by applying a strong magnetic field H0. The sensitivity of the NMR is therefore depended on the applied strong magnetic field to create the energy difference e.g. stronger the magnetic field, higher the energy difference ΔE therefore more sensitive is the absorption.
Once this energy difference is created, a second radiation magnetic field has to be then applied in order to send nuclei from the lower to the higher energy state and the absorption of this second magnetic field or applied magnetic field is measured and is characteristic of the nuclei. The second applied magnetic field is to be perpendicular to the H0 magnetic field and has to be of lower energy than the strong H0 magnetic field.
The international units of magnetic flux is Tesla (T). These days magnets with 1 to 20 T are required for NMR spectrometers. It is common practice to measure strengths of the magnets in Hz units which is generally about 20-900MHz.