# pulse NMR principle...

by schordinger
Tags: principle, pulse
 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[:D] [:D] [:D] [:D] [:(]
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[:D] [:D] [:D] [:D] [:(]
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.
 PF Patron Sci Advisor Emeritus P: 5,539 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.
P: 463

## pulse NMR 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.
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
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.
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