theory and principles of ft nmr

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THEORY AND PRINCIPLES OF FT-NMR

CHAITHRA B

I M.PHARM

DEPT. OF PHARMACOLOGY

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NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY NMR is a physical phenomenon in which nuclei in a magnetic field absorbs and

re-emit electromagnetic radiation. This phenomenon occurs when the nuclei of certain atoms are immersed in a

static magnetic field and exposed to the second oscillating magnetic field. It is an analytical chemistry technique used in quality control and research for

determining the content and purity of a sample as well as its molecular structure.

It is used in various fields like scientific research, various industries, medical field etc.,

It is now the most versatile spectroscopy technique that is used in regular analysis of Bio macromolecules.

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FOURIER TRANSFORM NMR SPECTROSCOPY In FT-NMR instrument, small energy change takes place

in the magnitude, present in NMR and hence the sensitivity of this instrument is very less.

The sensitivity in FT-NMR can be increased by adding the square root of recorded spectra's together.

Simultaneous irradiation of frequency occurs in a spectrum having a Radio frequency pulse and then the nuclei returns back to thermal equilibrium on its normal state.

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FOURIER TRANSFORM NMR SPECTROSCOPY The Fourier Transformation is the basic mathematical

calculation necessary to convert the data in time domain(interferogram) to frequency domain(NMR Spectrum).

i.e, time domain Intensity v/s Time.

Frequency domain Intensity v/s Frequency. It was developed by JEAN BAPTISE JOSEPH FOURIER.

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THEORY OF FT-NMR When magnetic nuclei are placed in a magnetic field and

irradiated with a pulse of radio frequency close to their resonant frequency, the nuclei absorb some of the energy and precess like little tops at their resonant frequencies.

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THEORY OF FT-NMR (CONTINUED..)

This precession of many nuclei at slightly different frequencies produces a complex signal that decays as the nuclei loses the energy they had gained from the pulse. This signal is called as free induction decay(FID) or transient, it contains all the information needed to calculate a spectrum.

The free induction decay can be recorded by a radio receiver and a computer in 1-2 seconds and many FIDs can be averaged in few minutes. A computer converts the averaged transients into a spectrum.

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THEORY OF FT-NMR (CONTINUED..)

A Fourier transform is the mathematical technique used to compute the spectrum from the free induction decay. This technique of using pulses and collecting transients is called Fourier transform spectroscopy.

A Fourier transform spectrometer is usually more expensive than a continuous wave spectrometer, since it must have fairly sophisticated electronics capable of generating precise pulses and accurately receiving the complicated transients

A good 13C NMR instrument usually has the capability to do 1H NMR spectra as well. When used with proton spectroscopy, the Fourier transform technique produces good spectra with very small amounts( less than milligram) of sample.

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CHEMICAL SHIFT:

• Resonance frequencies of the same isotopes in different molecular surroundings differ by several ppm (parts per million). For resonance frequencies in the 100 MHz range these differences can be up to a few 1000 Hz. After creating a Mx,y coherence, each spin rotates with its own specific resonance frequency w, slightly different from the B1 transmitter (and receiver) frequency w0. In the rotating coordinate system, this corresponds to a rotation with an offset frequency W = w - w0.

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SENSITIVITY:

The signal induced in the reciever coil depends

• On the size of polarization Mz to be converted into Mxy coherence by a 900 pulse.

• On the signal induced in the receiver coil at detector, depending on the magnetic moment of the nucleus detected and its precession frequency

• unfortunately the noise also grows with the frequency.

MICHELSON INTERFEROMETER

A Michelson interferometer is used to observe interference.

It does this through a setup involving a light source, a light detector, a beam splitter, and mirrors.

By splitting the beam of light and introducing differences in path length for the resulting beams, interference can be induced.


The following slides will explain this concept in greater detail.

Legend

Light wave (original)

Light wave (split)

Light wave (recombined)

Mirror

Light source

Beam splitter

Light detector


First, the Michelson interferometer emits a beam of light of a fixed wavelength from the

source.

This beam travels through the beam

splitter, resulting in 2 waves (still same

wavelength) being sent to different mirrors.


The mirrors each reflect their respective beam

back toward the splitter. In this case, the distance between each mirror from the splitter

is the same.

MICHELSON INTERFEROMETERWhen the beams reach

the splitter, they are both in the same spot and

aimed in the same direction. Because they occupy the same space, interference must occur.

In this case, it is constructive because the

mirrors are the same distance away, thus the

number of wavelengths is the same. Note the

resulting amplitude is now 2A. This should result in a

bright light being observed on the detector.


Now let’s modify the experimental settings by moving the right mirror to the right by λ/4

(one quarter of the beam’s wavelength)


Just as before, a light of a fixed wavelength is emitted, is split into two, and each beam

travels to its respective mirror.

This time, however, the right beam’s mirror is

slightly further away, a length of λ/4.

MICHELSON INTERFEROMETERBecause the right mirror has been shifted, a phase

difference has been introduced between

the waves corresponding to the two mirrors. Because the distance moved is λ/4, and that distance

is travelled twice (oncoming and

reflected beam) the phase is now λ/2 or π.


Once these 2 waves combine at the same spot as before, their

phase difference results in complete destructive

interference.

As a result, it is expected that no light will be observed at the

detector.

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CONCLUSION:• importance of detecting the nucleus with the highest γ

(i.e., 1H), important in heteronuclear H,X correlation experiments: "inverse detection"

• double sample concentration gives double sensitivity, but to get the same result from longer measuring time, one needs four times the number of scans

• sensitivity should increase at lower temperatures (larger polarisation), but lowering the temperature usually also reduces T2 , leading to a loss of sensitivity due to larger line widths.

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REFERENCE:

• L G Wade, Organic Chemistry, 6th edition.• FT NMR article from ©Gerd Gemmecker, 1999.• Skoog, Holler and crouch, Instrumental analysis,2007.• Internet sources

theory and principles of ft nmr

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