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FT NMR WORKSHOP/===/ S.A.I.F./===/ NEHU/==/ Shillong

INTRODUCTORY LECTURE

S.ARAVAMUDHAN

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In a Scheme of an Atom

Electrons circulate in Orbits

NUCLEUS is stationery at the center

Chemical Molecular structure depends on the electronic structural changes due to bonding between atoms. Nucleus plays no role in determining the optimum geometry except that they get an assigned place as they occupy in the molecule.

Molecular spectroscopic studies involve studies assuming an equilibrium structure, but the molecular phenomenon responsible for spectroscopic absorptions require changes in the electron dispositions around the equilibrium geometries

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One of the aspect to be reckoned with is the fact that all the chemical consequences are because of the electrons present in the elemental atom or ion because of which these elements exhibit chemically binding characteristics.

It is known that the atoms and ions of such elements have their characteristic nucleus around which the electrons of the system revolve in orbits. For the chemical consequences there is not any significant role assigned to the nuclear characteristics unless it is a radio active element and the nuclear radiations can make it possible to be tracked by radio active tracer techniques.

The radio activity itself can be hazardous besides the toxic effects of such elements by chemical reactivities

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When all the extra nuclear electrons are in such continuous motion and participate in the bonding, if there can be a stimulation of nucleus which does not in any way affect the electronic dispositions, then would that be in any way useful for such studies of molecular structure?

In magnetic resonance, the nuclei are stimulated in such a way that the electronic dispositions are not influenced by the perturbation of nuclei, but the resulting stimulated response is indicative of the specifics about prevailing electron dispositions

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It is at this juncture it is worth trying to inquire the possibilities of using the Nuclear Magnetic Resonance [N.M.R] spectroscopy to follow the characteristics due to the presence of such nuclei invariably with the electron system to be identified as an element. An elementary description of Nuclear Magnetic Resonance [NMR] phenomena is given in the following paragraph.

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The central nucleus of some of the elements posses intrinsic spin

and hence also can be associated with a magnetic moment

SPIN

Magnetic moment

These tiny nuclear magnetic moments are similar to the bar magnets which are influenced by Externally applied magnetic fields

Applied magnetic field

Only Discrete orientations of the spin are possible due to the quantization criteria at atomic regimes

or

or

or

Electromagnetic radiation with

frequency ν can cause transition between these levels and this is the resonance phenomenon

2π ν = γ HSimilar effects are possible with electrons also but only in PARAMGNETIC IONS or Molecules when there are unpaired electrons present. Here the reference is only to Diamagnetic Samples (compounds) to introduce NMR exclusive of any other effects.

The value of γ differs from one nucleus to the other.

This unique value of ‘γ’ for each element’s nucleus different from every other element is what makes multi nuclear NMR possible

SPIN

Magnetic moment

Discrete orientations RESULTS in discrete energy levels

Illustration is a case of SPIN=3/2 results in 4 equally spaced energy levels

Spin Quantum number value= 3/2

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This NMR phenomenon is due to the fact that nuclei, placed in a strong external magnetic field, can resonate with externally applied electro magnetic radiations in the radio frequency range of the electro magnetic spectrum. Such of those nuclei which have nuclear magnetic moments are the candidates which can be detected by this resonance phenomenon. The frequency of the electro magnetic radiation at which the resonance can occur is governed by a specific equation which relates the frequency to the strength of the external magnetic field.

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Magnetic Resonance Phenomenon is a manifestation due to the presence of INTRINSIC SPIN angular momentum and the associated Magnetic Moment characteristically in electrons and Nuclei

When the experimental conditions are set for the NUCLEI to resonate, then it is the Nuclear Magnetic Resonance.

A definition:

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NMR Nobel Prize Laureates

 

A brief historical account of the Nobel Prize Laureates clearly shows the track of the discovery, development, and applications of NMR spectroscopy.•Otto Stern, USA: Nobel Prize in Physics 1943, "for his contribution to the •development of molecular ray method and his discovery of the magnetic moment of the proton"

•Isidor I. Rabi, USA: Nobel Prize in Physics 1944, "for his resonance method for• recording the magnetic properties of atomic nuclei"

•Felix Bloch, USA and Edward M. Purcell, USA: Nobel Prize in Physics 1952, "for their •discovery of new methods for nuclear magnetic precision measurements •and discoveries in connection therewith"

•Richard R. Ernst, Switzerland: Nobel Prize in Chemistry 1991, "for his contributions to the• development of the methodology of high resolution nuclear magnetic resonance (NMR) spectroscopy

•Kurt Wüthrich, Switzerland: Nobel Prize in Chemistry 2002, "for his development of nuclear magnetic resonance spectroscopy for determining the three-dimensional structure of •biological macromolecules in solution"

•Paul C. Lauterbur, USA and Peter Mansfield, United Kingdom: •Nobel Prize in Physiology or Medicine 2003, "for their discoveries concerning magnetic resonance imaging"

http://www2.chemistry.msu.edu/facilities/nmr/900Mhz/MCSB_NMR_nobel.html

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Spin and Magnetic moment:

Nucleus rotates about an axis within itself, which is referred to as the spinning of nuclei.

The spinning object acquires an angular momentum

The NUCLEI thus possess angular momentum; and the angular momentum in atomic system are quantized. Due to this quantization, the angular momentum component in any chosen direction can take only specified discrete values.

Spin Angular momentum

Nucleus has electrical Charge. Thus a rotating charge has associated magnetic moment.

For protons: Spin quantum number=1/2

The angular momentum and magnetic moment are in the same direction because of the positive charge of the nucleus

Illustration:

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+1/2

-1/2The ensemble of spins, have equally distributed population between the two levels for the spin ½ protons

+1/2

-1/2

No external magnetic field. The energy levels are degenerate

Magnetic field

+1/2

-1/2No radiations

are present

Not stimulated transitions: but spontaneous relaxation transitions

Degeneracy removed/Energy levels split

On the application of field…..

Splitting is instantaneous & population redistribution requires more time called the relaxation time

Thermal equilibrium Boltzmann distribution

random

No net magnetization

Net magnetization‘h ν = g β H’ is the magnetic resonance condition

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Transmitter ON time

Probe & sample

Crossed Diodes

ReceiverReceiver OFF time

Receiver Silent or dead time

DATA acquisition starts at this time

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After the RF pulse, the FID is the impulse response from the sample spin system. The pulsing and FID can be repeated and added to acquire the averaged signal for better signal to noise ratio The RF frequency of the pulse is ν as given by the equation ‘h ν = g

β H’ where H is the applied homogeneous magnetic field

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Randomization in XY plane

I h NET Magnetization

Transverse T2

Relaxation

Relaxation Longitudinal and transverse

Longitudinal T1 Relaxation

π/2 pulse

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N

S

N

S

N

Set of equally spaced parallel lines indicates

homogeneous field

No. of lines within a unit area of cross section [in plane perpendicular]

Is the field strength/ Intensity of magnetic

field

Area of cross section

Inhomogeneous Fields

Presence of a Magnetic field is pictorially depicted by a vector-line pointing along the direction in which an isolated North pole would move

After this illustration on magnetic Field, in the next slide, Magnetic Moments would be considered

It is important to realize the difference between two poles forming magnet pole faces and two poles making up the dipole with a dipole moment

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H0

H0 - ΔH

H0 - ΔH

180°pulse

180°pulse

90°Pulse

90°Pulse

Thermal equilibrium Boltzmann population distribution

H0

Net Magnetization only along H0 and no magnetization in plane perpendicular (zero component)

Magnetization has non-zero component in XY-plane…

Tilted to XY plane

Homogeneous Field

Inhomogeneous Field

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This phenomenon proved itself to be capable of revealing the nature of nuclear environments in molecules (chemical compounds) because of the changes in the electronic structures due to the bonding criteria for the atoms forming the molecules. These are essentially the variations in the resonance frequencies due to electron circulations within molecules. And these variations called ‘Chemical Shifts’ are in the order of parts per million of the applied field/frequency. Thus if proton nuclei has a characteristic resonance frequency of 300MHz corresponding to a applied magnetic field of 7.05 Tesla, then the total range for the variation of the proton resonance frequency due to differences in molecular electron circulations (the Chemical shift range) is 10ppm. This corresponds to a total variation of 3 KHz in 300 MHz (since 1ppm=300Hz). This implies a stringent stability criterion for the Magnetic field and RF frequency sources and the required ratio must be also maintained to the same accuracy to obtain reliable readout parameters from the spectrum obtained from spectrometers. This is the requirement of field-frequency lock in NMR spectrometers. With that good stability ensured, the magnetic field must be shimmed to get high homogeneity of the field in the sample region. By such techniques a reproducibility of the chemical shift to the accuracy of 0.0001ppm seems possible with the current generation of spectrometers.

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1cc of water contains proton spins of the order of 10 exponentiated to 22 spins and the actual sample of water in the detectable region of nmr-probe would contain about 10 exponentiated to 21 spins corresponding to 100 microliter of water solvent. A typical spectrometer of the 300MHz frequency can detect conveniently a spin count of 10 exponentiated to 18 which amounts to volumes in a few ‘microliter’ ranges. But the present generation of Spectrometers at as much high field as corresponding to 900MHz can be sensitive enough detect 10 exponentiated to 11 spins which in terms of sample volume in the ‘pico liter’ range. All this is due to the advances in instrumentation on the rf detection side during the continuous wave mode of NMR detection and subsequently and the improvements in tuning of sample coils simultaneously used for the transmitter and receiver purposes with High Power [up to 3KW peak power for solid samples] pulsing detecting the response possibly in the range of 10 microvolts induced RF in the coil due to NMR induction in pulsed NMR detection. Up to 100MHz proton resonance frequency, Electromagnets (23 KG) can be used but for fields higher than this value Supercon Magnet Systems (with superconducting current carrying elements) are necessary. The possibility of realizing superconducting magnet systems has brought about a total revolution in what was possible by NMR Spectroscopic Technique.

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Updated 5/10/09

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