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Pulse NMR principle...

by schordinger
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schordinger
#1
May3-03, 12:58 AM
P: 23
I would like to ask why a radio-wave pulse with defined frequency was used in NMR, why it can excited different proton in sample measured.
Is it related to so-called "lifetime boarding" ??

Thx
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wuliheron
#2
May3-03, 11:40 AM
P: 1,967
Originally posted by schordinger
I would like to ask why a radio-wave pulse with defined frequency was used in NMR, why it can excited different proton in sample measured.
Is it related to so-called "lifetime boarding" ??

Thx
I have no idea what this "lifetime boarding" is, but NMR uses radio waves to change the energy levels of the electrons in orbit around the nucleus of the atom. The electrons first absorb the radio radio and contract their orbit, then expand their orbit again releasing the energy they absorbed. In so doing, the frequency they emit provides an exact spectrum of the atom. In other words, the radio waves used in MNR do not excite the protons, but the electrons. It is the assortment of electron orbits around the nucleus that tell the machine precisely what kind of atom it is.
Tom Mattson
#3
May3-03, 12:24 PM
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NMR Pulses do not excite anything; they flip the spins of the nuclei (not the electrons).

I don't know what "lifetime boarding" is, but the precise frequency of the pulse is selected in accordance with the difference in energy between the spin-up state and the spin-down state in a magnetic field B0.

That energy difference is:

ΔE=hγB0/2π

edit: γ is the gyromagnetic ratio.

and the photons in the pulse have to be of the same energy. Since E=hf, the frequency is determined.

Mike H
#4
May3-03, 12:37 PM
P: 485
Pulse NMR principle...

"Lifetime broadening" is a spectroscopic consequence of the Heisenberg indeterminacy principle.

Magnetic resonance on electrons is called EPR (electron paramagnetic resonance) or ESR (electron spin resonance), and you use microwave frequency radiation to obtain transitions between spin states. There is also a phenomenon known as ENDOR (electron-nuclear double resonance). You observe a nuclear spin transition through its effect on the EPR/ESR signal, as there exists a coupling between an NMR-active nucleus and an EPR/ESR-active electron spin.
eNtRopY
#5
May3-03, 03:25 PM
P: n/a
Originally posted by wuliheron
I have no idea what this "lifetime boarding" is, but NMR uses radio waves to change the energy levels of the electrons in orbit around the nucleus of the atom. The electrons first absorb the radio radio and contract their orbit, then expand their orbit again releasing the energy they absorbed. In so doing, the frequency they emit provides an exact spectrum of the atom. In other words, the radio waves used in MNR do not excite the protons, but the electrons. It is the assortment of electron orbits around the nucleus that tell the machine precisely what kind of atom it is.
Dude... I think you're talking about ESR.

eNtRopY
Chemicalsuperfreak
#6
May3-03, 03:37 PM
P: 324
Originally posted by Tom
NMR Pulses do not excite anything; they flip the spins of the nuclei (not the electrons).

I don't know what "lifetime boarding" is, but the precise frequency of the pulse is selected in accordance with the difference in energy between the spin-up state and the spin-down state in a magnetic field B0.

That energy difference is:

ΔE=hγB0/2π

edit: γ is the gyromagnetic ratio.

and the photons in the pulse have to be of the same energy. Since E=hf, the frequency is determined.
Oh, but they do indeed excite the protons. Protons that are spin opposed to the magnetic field are more excited than protons that are spin aligned.

Back to the original question. If I'm following right, you're asking how NMR differentiates between two protons?

In molecules, different protons experience different environments. Protons that have a lot of electron density around them are shielded from the external magnetic field and thus will have a different chemical shift than the protons that are deshield, that is the electron density is being stripped away.

So, for example, a hydrogen bonded to a carbon will come in at 1 ppm, where as a proton bonded to a more electronegative oxygen will come in around 2.5 ppm.
Tom Mattson
#7
May3-03, 03:39 PM
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Originally posted by Chemicalsuperfreak
Oh, but they do indeed excite the protons.
Sorry, I meant that it doesn't excite the proton to one of its resonances. The lowest lying resonance is the Δ(1232), which you ain't gonna see with RF waves!
schordinger
#8
May4-03, 07:27 PM
P: 23
Thx for all you reply....

I think my question was poorly asked...
Actually, my q.s. should be
"what is the principle of FT-NMR" and "why Pulse excitation source is used"

Thx.....
Mike H
#9
May4-03, 11:47 PM
P: 485
What is the principle of FT NMR? Using a pulsed source and the principles of Fourier analysis one can obtain high resolution NMR data in less time and increase the complexity (and therefore the total extractable information about the spin system and relaxation/dynamical processes) of the experiments. If you desperately need details, I'd suggest Ernst, et al (1987) Principles of Nuclear Magnetic Resonance in One and Two Dimensions Oxford Science Publications.

Pulsed methods also take a heck of a lot less time, otherwise you could spend more time than you'd like doing CW experiments. You, naturally, will have issues with signal/noise and averaging with CW experiments. Case in point, back when I was still doing EPR on a semiregular basis, I used to spend ages collecting data. (Of course, now that I'm in NMR land, I'll still be spending ages doing spectrum collection, but am working with proteins, so is not totally unwarranted.)


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