NMR - Introduction

 

     Introduction To Nuclear Magnetic Resonance (NMR)

Different Components of NMR

Nuclear magnetic resonance spectroscopy is most sophisticated instrument to for structural and dynamical study of chemical or biological samples. It can probe the internal structure, dynamics, composition, chemical environment of a large number of samples including small atoms to very large polymeric molecules.

A 700 MHz Bruker NMR

Nuclear magnetic resonance builds on the physics concepts of resonance and nuclear spin (angular momentum of elementary particles of an atom). The basic theory behind NMR spectroscopy is based on probing the interaction between the spin magnetic moment of nuclei and the r.f. field applied in the presence of a strong field. Protons, electrons and other elementary particles have an intrinsic spin which does not have any classical analogy. When atomic nuclei, or other charged particle also have an intrinsic magnetic moment developed due to their spin or orbital motion. A nucleus or an elementary particle having intrinsic magnetic moment precess when they are placed in a magnetic field. A nucleus in a magnetic field precesses at a frequency, called the Larmor frequency, which is proportional to the magnetic field. When an atom is placed in a constant magnetic field, its nucleus precesses at the Larmor frequency. If in addition to the constant magnetic field, a weaker perpendicular magnetic field that varies at the same frequency as the Larmor frequency for the nucleus is applied, then the nucleus resonates. The phenomenon is called Nuclear Magnetic Resonance (NMR).

            

NMR or the Nuclear Magnetic Resonance spectroscopy is basically a absorption spectroscopy. In this spectroscopic technique a resonant r.f pulse is applied to the nuclei precessing in a strong static magnetic field. This perturbing r.f field is absorbed and reemitted during the relaxation process. The resonance frequency depends on the strength of the magnetic field and the magnetic properties of the isotope.

NMR spectroscopic study can be done for the isotopes having non zero spin. All the isotopes containing unpaired neutron or proton possess a net spin magnetic moment. All the other isotopes which have both proton and neutron in even numbers, do not have any net spin magnetic moment and as a result can not be studied by NMR spectroscopic method. ¹H and ¹³C are two most commonly studied isotopes by NMR technique. Other isotopes which have been studied by using NMR technique are – ²H, ⁶Li, ¹⁰B, ¹¹B, ¹⁴N, ¹⁵N,¹⁷O, ¹⁹F, ²³Na, ²⁹Si,³¹P, ³⁵Cl, ¹¹³Cd, ¹²⁹Xe, ¹⁹⁵Pt etc.

NMR SPECTRUM OF AN ORGANIC COMPOUND

            

By probing the NMR signal, we can get a large no of informations about three-dimensional molecular structure, molecular motion and chemistry in solution and ordered states; the physic-chemical properties and structure of solid materials; the composition profile of complex chemical and biochemical mixtures; and the time evolution of biochemical and chemical processes.