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Topic: NMR Spectroscopy  (Read 10490 times)

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jena

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NMR Spectroscopy
« on: July 21, 2005, 04:57:17 PM »
Hi,

How does NMR spectroscopy work?

Thank You :D

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Re:NMR Spectroscopy
« Reply #1 on: July 21, 2005, 05:43:27 PM »
I'm sure AWK will have a bunch of links to more complete explanations, but the basic idea is that you subject a molecule to a pulse of magnetic energy which excites the nuclei to a higher energy spin state.  As the nuclei relax to the lower energy spin state, the spectrometer records the resultant energy emissions.  These specific energy of these emissions depends on the electronic environment of the nucleus that was excited, that is why you see a difference in the NMR spectrum for electronically different nuclei.

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Re:NMR Spectroscopy
« Reply #2 on: July 21, 2005, 06:51:33 PM »
the spectrometer records the resultant energy emissions.

I always thought it is absorption that is measured, not emissions?

Regardless of the fact that it may seem as a toally different method - it is only a technical detail :)
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Re:NMR Spectroscopy
« Reply #3 on: July 21, 2005, 07:11:50 PM »
I just checked in a textbook and it says that the emissions are what are detected.

Absorbtions are detected in other forms of spectroscopy like IR though.

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Re:NMR Spectroscopy
« Reply #4 on: July 21, 2005, 07:56:13 PM »
OK, memory is not a thing that always work 100% correctly ;)

Technically speaking it shouldn't matter in case of NMR, unless I am missing something. If you put a sample on the beam way (is it in English?) for some wavelengths beam will be absorbed - but at the same time absorbed energy is emitted in other directions (dispersed) so whether you measure emission (perpendicular to beam) or absorption (of the beam) doesn't matter. What matters is the fact that at this particular wavelength your sample interacts with the beam. Whether you measure emission or absorption is rather an engineering selection than chemical one.

Situation is not always as easy. For example if the excited state is stable (slow relaxation) one can see only some absorption at the beginning of the measurement (it makes NMR in case of solids pretty difficult). In Raman spectroscopy frequency of emission is important, not the absorption.

Oh man, what am I writing about? 2 a.m., time to sleep not to shine with some dull, second-hand knowledge ;D
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Re:NMR Spectroscopy
« Reply #5 on: July 22, 2005, 12:25:52 AM »
I'm sure AWK will have a bunch of links to more complete explanations, but the basic idea is that you subject a molecule to a pulse of magnetic energy which excites the nuclei to a higher energy spin state.  As the nuclei relax to the lower energy spin state, the spectrometer records the resultant energy emissions.  These specific energy of these emissions depends on the electronic environment of the nucleus that was excited, that is why you see a difference in the NMR spectrum for electronically different nuclei.

Actually, you subject the molecule to a pulse of radio energy while applying a strong magnetic field.  The magnetic field will create a difference in potential between nuclei whose spin is aligned with the magnetic field and nuclei whose spin is aligned against the magnetic field.  The radio wave pulse will excite all of the molecules to the higher energy state (spin against the applied magnetic field).  Then the NMR machine detects the radio waves emitted when the nuclei return to the ground state.  (Radio waves are used because they correspond to the energy difference between nuclear spin states.)

Different nuclei will emit radio waves of different frequencies because the size of the energy difference between spin states depend on the strenght of the magnetic field felt by the nuclei.  Although the applied magnetic field is uniform throughout the sample, the magnetic field felt by the individual nuclei is affected by shielding from electrons surrounding it (as well as magnetic fields induced by neighboring electrons).

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Re:NMR Spectroscopy
« Reply #6 on: July 22, 2005, 02:43:14 PM »
I see, so the magnetic field is there to create an energy difference between the two spin states, then the radio wave actually does the excitation.

Yggdrasil, do you know why the spectrometer detect the emission from relaxation rather than the absorption of the inital pulse?

Could it be that the initial pulse is a broad pulse over the whole range of energies needed to excite the different nuclei so that all of them are excited?  If you measured absorption, wouldn't the pulse have to scan across the possible energies?

As Borek said, this sounds like an engineering problem to me, and I never took an engineering class!

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Re:NMR Spectroscopy
« Reply #7 on: July 22, 2005, 06:28:12 PM »
This question I'm not so sure about.  Looking over my notes on NMR, I can offer this as a guess:

NMR spectrometers use a broad pulse of radio energy to excite all magnetically active nuclei simultaneously, then record the emissions on an intensity-v-time graph.  The computers in the spectrometer then subject this data to a Fourier transform which converts the intensity-v-time graph to an intensity-v-frequency graph based on the decay of the signals.  This is probably easier than producing or recording radio waves of a specific frequency because radio waves have such long wavelenghts and would require uneccessarily large equipment to produce and/or record radio waves of a specific frequency with the neccessary degree of accuracy.  In addition, using the Fourier transform method probably makes the spectrometers cheaper to produce since they require less sophisticated radio wave generators and detectors.

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Re:NMR Spectroscopy
« Reply #8 on: July 22, 2005, 06:45:04 PM »
Broad pulses and FFT are fancy solution used in contemporary NMR, historically speaking both field sweeping and frequencey sweeping were used (although I have a gut feeling that I have more often heard about constant frequency).
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Re:NMR Spectroscopy
« Reply #9 on: August 09, 2005, 10:54:22 PM »
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

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Re:NMR Spectroscopy
« Reply #10 on: August 10, 2005, 10:47:05 AM »
For proton (H) NMR, the nuclear spin quantum no. (I) of protons is 1/2.

The no. of nuclear spin states of protons = 2I + 1 = 2

The nuclear spin states exist when protons are under magnetic field (Bo).

Spinning protons just like spinning charged matters that can generate magnetic moments which look like magnets.

The magnetic moment of each proton align either parallel to the external Bo (ground st.) or anti-parallel to the external Bo (excited st.).
(If no external Bo applied to protons, it will gives random alignments of magnetic moment)

By Boltzmann distribution,
N(upper) / N(lower) = exp -(delta E / kT)

In room temp., the no. of protons in ground state is slightly more than that in excited state.

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Re:NMR Spectroscopy
« Reply #11 on: August 10, 2005, 11:30:38 AM »
However, protons are not only spinning, protons are also precessing at a frequency, called Larmor frequency.

You can find that the NMR spectrometers have their field strengths (in Tesla).
e.g. 7 Tesla will give 300 MHz of precession frequency of protons
Larmor frequency = gyromagnetic ratio (in MHz/Tesla) x Bo (in Tesla)

As the no. of protons in ground state is higher (assume the external Bo is pointing upward, z-axis), the net magnetization is pointing upward also.
Then, applying a 90 degree pulse (for example) using radio frequency.
(This pulse can generate a continous radio frequency in particular range, but I am not familiar with this principle.)

This pulse will exite the gd. state protons to ex. state. Not all protons are being exited for 90 degree pulse, it will only make the population between 2 states are the same.
The net magnetization is now on xy plane, and the receiver (on xy plane) convert the signals to peaks by FT.
(I am not quite familiar with this part, just briefly describe it)

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