NMR Pulse Principles: Exciting Protons & Lifetime Boarding

In summary: 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 :frown:In summary, NMR uses radio waves to change the energy levels of the electrons in orbit around the nucleus of the atom. The electrons absorb the radio waves and release the energy, providing an exact spectrum of the atom. The precise frequency of the pulse is selected to match the energy difference between the spin-up and spin-down states, and the photons in the pulse must have the same energy. This is known as the principle of FT NMR, which allows for high
  • #1
schordinger
23
0
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 :frown:
 
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  • #2
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 :frown:

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.
 
  • #3


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.
 
  • #4
"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.
 
  • #5


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
 
  • #6


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.
 
  • #7


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 going to see with RF waves!
 
  • #8
My Q.s. should be...

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... :smile: :smile: :smile:
 
  • #9
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.)
 

1. What is NMR pulse and how does it work?

NMR pulse is a technique used in nuclear magnetic resonance (NMR) spectroscopy to excite protons in a sample. This is accomplished by applying a short, intense pulse of radiofrequency energy to the sample. This pulse causes the protons to align with the magnetic field, and when the pulse is turned off, the protons return to their original state, emitting a signal that can be used to analyze the sample.

2. What is the principle behind NMR pulse?

The principle behind NMR pulse is based on the fact that protons have a property called spin, which causes them to behave like tiny magnets. When placed in a strong magnetic field, the protons align themselves either with or against the field. By applying a radiofrequency pulse at the right frequency, the protons can be flipped to align in the opposite direction. This flip is what produces the NMR signal.

3. What is the importance of NMR pulse in NMR spectroscopy?

NMR pulse is crucial in NMR spectroscopy because it allows scientists to selectively excite specific protons in a sample. This allows for the analysis of different molecules in a complex mixture. Additionally, the strength and duration of the pulse can be controlled to manipulate the NMR signal and provide information about the structure and dynamics of the molecules in the sample.

4. How does NMR pulse affect the lifetime of protons?

NMR pulse does not directly affect the lifetime of protons. However, it does cause the protons to flip their alignment, which changes the energy levels of the protons. This, in turn, affects the rate at which the protons return to their original state and emit the NMR signal, which is known as the proton's relaxation time. The duration and strength of the pulse can be adjusted to manipulate the relaxation times and provide information about the sample.

5. Can NMR pulse be used for any type of sample?

Yes, NMR pulse can be used for a variety of samples, including liquids, solids, and gases. However, the sample must contain nuclei with spin, such as protons or carbon-13 nuclei. Additionally, the sample must be placed in a strong magnetic field to observe an NMR signal. Overall, NMR pulse is a versatile technique that is widely used in many fields of science, including chemistry, biochemistry, and material science.

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