A COMPLETE REVIEW ON NUCLEAR MAGNETIC RESONANCE (NMR)

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ABOUT AUTHORS:
*Md. JahaSultana, P. Vijaya Sri, G.Alekhya, S.VenkateswaraRao, Chaitanya PrasadMeher
Department Of Pharmaceutical Analysis
Vijaya Institute of Pharmaceutical Sciences For Women
Enikepadu, Vijayawada-521108, Andhra Pradesh, India
sohnivya786@gmail.com

ABSTRACT:
Over a past decades, nuclear magnetic resonance (NMR) has progressed rapidly in improvement of experimental method and development of novel approaches. NMR is a research technique that exploits the magnetic properties of certain atomic nuclei. Of all the spectroscopy methods, NMR is the only one of which a complete analysis and interpretation of the entire spectrum is normally excepted. Although larger amounts of samples are needed when compared with mass spectroscopy, NMR is non-destructive and with modern instruments good data may be obtained from samples weighing less than a milligram. In this review a brief introduction to the NMR is given which includes instrumentation, parameters influencing NMR and applications.

REFERENCE ID: PHARMATUTOR-ART-2076

INTRODUCTION(1, 2, 6):
Over the past fifty years Nuclear Magnetic Resonance spectroscopy, commonly referred to as NMR, has become the dominant method of analysis of organic compounds, because in many cases it provides a way to determine an entire structure using one set of analytical tests. It is also increasingly used in inorganic chemistry and biochemistry, where it also provides a lot of valuable structural information. NMR is a property of the nucleus of an atom, concerned with what is known as nuclear spin (I). This is equivalent to the nucleus acting like a tiny bar magnet. Isotopes can have a variety of values for I (including zero). In this NMR spectroscopy only nucleus containing odd mass number (A) or odd atomic number (Z) are most useful which includes hydrogen 1 (1H), carbon 13 (13C), fluorine 19 (19F) and phosphorus 31 (31P). Whereas nucleus containing even mass number (A) or even atomic number (Z) are not used , This includes 10B, 14N etc.

Resonance frequency of a particle, substance is the key feature of NMR and is directly proportional to the strength of the applied magnetic field.The effectiveness of NMR can also be improved by using following techniques

  • Hyperpolarization
  • Two-dimensional
  • Three-dimensional
  • Higher-dimensional multi frequency techniques

Although large amounts of sample are needed when compared with mass spectroscopy, NMR isnon-destructive and with modern instruments good data may be obtained from samples weighing less than a milligram.The 1H nucleus is most commonly studied by using NMR spectroscopy because of its high natural abundance (99.98%) and the fact that it is invariably present in the majority of organic compounds. PMR provides information about the number of different types of protons and also regarding the nature of the immediate environment of each of them. Despite of NMR, Carbon- 13 is also an important nucleus because carbon forms the backbone of all organic compounds and valuable structural information can be derived by 13C NMR spectroscopy.

DEFINITIONS(2, 3, 6):
Spectroscopyis a method used to study the light as a function of length of the wave that has been emitted, reflected through solid, liquid or gas. Nuclear Magnetic Resonance (NMR) is a spectroscopy technique which is based on the transition of Electromagnetic radiations in a radio frequency region 4 to 900 MHz by nuclei of atoms in the presence of magnetic field. In this radio frequency radiations are used to induce transitions between different nuclear spin states of samples in a magnetic field. When proton (hydrogen) is studied then it is called as Proton Magnetic Resonance (PMR). When other nuclei like 13C, 9F, 35Cl etc. are studied then it is called as NMR.

HISTORY(2, 3, 5, 6):
NMR was first described and measured in molecular beams by Isidor Rabi in 1938 by extending the Stern-Gerlach experiment and in 1944; Rabi was awarded the Nobel Prize in physics for this work. In 1946, Felix Bloch and Edward Mills Purcell expanded the technique for use on liquids and solids, for which shared the Nobel Prize in physics 1952. Purcell had worked on the development of radar during World War II. His work during that project on the production and detection of radio frequency power and on the absorption of such radio frequency power by matter laid the foundation for Rabi’s discovery of NMR.  In 1991 Richard Ernst awarded the Nobel Prize for his contributions to high resolution NMR and in 2002 Wuthrichawarded the Nobel Prize for his development of three dimensional structure of biological macromolecules in solution. In 2003 Lauterbur and Mansfield awarded the Nobel prize for their discoveries regarding magnetic resonance imaging (MRI). In the past decades, NMR spectroscopy has achieved a great development in multinuclear studies, very high field operation comes to being, applications of new pulse series appear, new technology of multi-dimensional spectra and high resolution work on solids.

PRINCIPLE(2, 3, 6):
Nuclei that exhibit the NMR phenomenon are those whichhave the spin quantum number I greater than 0 (I>0). A nucleus with an odd mass or an odd atomic number possess a nuclear spin, due to spinning a magnetic field is generated along the axis. The spin quantum number I of the nuclei as follows:

Mass number (A)

Atomic number (Z)

Spin quantum number (I)

odd

odd or even

1/2, 3/2, 5/2…

even

even

0

even

odd

1,2,3, 4…..

Table-1: The spin quantum numberI is associated with the mass number (A) and atomic number (Z) of the nuclei

Without externally applied magnetic field, the nuclear spins are random in all directions. But when externally magnetic field is applied; the nucleus align themselves by creating magnetic momentum.


Fig-1: Orientation of spinning nuclei in absence and presence of external magnetic field

Hence, nucleus spins on their own axis when placed in an external magnetic field resulting in a circular motion creating aprecessional orbit, with a frequency called precessional frequency. When energy in the form of radio frequency is applied and is equal to precessional frequency, then the transition of protons from lower energy (α state) to higher energy (β state) take places and NMR signals are recorded.


Fig-2: Energy level transitions of protons

When application of radio frequency energy is stopped nucleus returns to ground state. Increasing in strength of magnetic field does not cause transition from lower energy (α state) to higher energy (β state). But it merely increases precessional frequency.

RELAXATION PROCESS(2, 3):
It is the process of transition from excited state to ground state where absorbed radio frequency energy can be lost by two ways:

1.Radiation Emission: lost with emission of radio frequency radiations itself.

2.Radiation Transition: (without radiation) It occurs in two ways:

a) Spin-lattice/ longitudinal relaxation process: where the energy is lost by means of translational/ vibrational/ radiational energy.

b) Spin-spin/ Transverse relaxation process: where the energy is lost to neighboring nuclei.

In a homogenous magnetic field, an atomic nucleus will have (2I+1) orientations. The frequency of radiation needed to flip the proton to higher energy state is


Where, v = frequency,H0= Strength of magnetic field (guass), γ = nuclear constant (26750 for protons)

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