Spectroscopy how does it work?


by Chemist20
Tags: absorption, nmr, spectrum
Chemist20
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Jan21-12, 04:09 AM
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Okay, I'm havin trouble understanding the difference between NMR and spectrums.

As far as I know: in NMR you excite a nuclei from one state to another. The frequency of the energy absorbed is what we measure.

How is that related to normal spectrums and colour? For example, absorption of radiation corresponding to the visible region causes transitions of electrons between energy leves within the molecule. (difference in energy given by Planck's equation). Hence you get a spectrum


But how is this different to what we do in NMR?
Is itbecause in NMR its at the nuclei level and the other at a molecular level?


Thank you!
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Drakkith
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Jan21-12, 04:17 AM
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I believe that in NMR the nuclei are aligned with the applied magnetic field. The radio frequency used to excite them causes a shift in the direction of their spin in relation to the magnetic field. When they shift back I *think* they emit the same radio frequency which is then detected.

In regards to the spectrum of energy released by transitions of electrons, you seem to understand that.
Chemist20
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Jan21-12, 05:25 AM
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Quote Quote by Drakkith View Post
I believe that in NMR the nuclei are aligned with the applied magnetic field. The radio frequency used to excite them causes a shift in the direction of their spin in relation to the magnetic field. When they shift back I *think* they emit the same radio frequency which is then detected.

In regards to the spectrum of energy released by transitions of electrons, you seem to understand that.
Yes I think that too. The thing is, in NMR they talk about "spectrums" and Im getting confused because I'm not sure if these spectrums are somehow related to absorption spectrums defined by plank's equation. Im not sure if im explaining myself too well....

Drakkith
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Jan21-12, 05:34 AM
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Spectroscopy how does it work?


A spectrum is just a breakdown of the radiation you are measuring. If I look at the spectrum of the Sun, all I'm looking at is the breakdown of what wavelengths of light are coming in and how much of each. The spectrum of a particular atom from an NMR machine will have a certain frequency and certain relaxation time and other properties.

See here: http://en.wikipedia.org/wiki/Spectrum
Chemist20
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Jan21-12, 05:58 AM
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Quote Quote by Drakkith View Post
A spectrum is just a breakdown of the radiation you are measuring. If I look at the spectrum of the Sun, all I'm looking at is the breakdown of what wavelengths of light are coming in and how much of each. The spectrum of a particular atom from an NMR machine will have a certain frequency and certain relaxation time and other properties.

See here: http://en.wikipedia.org/wiki/Spectrum
Okay, so then there can be a spectrum of nuclei, or electrons right? thanks!!
jtbell
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Jan21-12, 06:46 AM
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Quote Quote by Chemist20 View Post
Im getting confused because I'm not sure if these spectrums are somehow related to absorption spectrums defined by plank's equation. Im not sure if im explaining myself too well....
If by "Planck's equation" you mean E = hf, it's a general relation that applies to photons produced in any process, whether transitions of electrons in atomic orbitals, or transitions between different states of nuclei, or...

Whenever you have discrete energy levels (in atomic electrons, or nuclei, or molecular vibrations, or whatever), you're going to get discrete spectra of photons produced from transitions between those energy energy levels.
Chemist20
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Jan21-12, 07:11 AM
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Quote Quote by jtbell View Post
If by "Planck's equation" you mean E = hf, it's a general relation that applies to photons produced in any process, whether transitions of electrons in atomic orbitals, or transitions between different states of nuclei, or...

Whenever you have discrete energy levels (in atomic electrons, or nuclei, or molecular vibrations, or whatever), you're going to get discrete spectra of photons produced from transitions between those energy energy levels.
Okay! Thanks!!!
sophiecentaur
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Jan21-12, 09:11 AM
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Quote Quote by Chemist20 View Post
Okay, so then there can be a spectrum of nuclei, or electrons right? thanks!!
ummm Not sure about that. Although the word 'spectrum' is used in everyday life to mean 'range of', I think that spectrum is used in Science to relate to frequency (or possibly wavelength) rather than particles.
However, you can get situations in which particles interact with EM waves with certain frequencies so a spectrum of frequencies can be associated with a range of particles. There is, of course, a correspondence between mass and EM energy (hence frequency).
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Jan21-12, 09:23 AM
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Quote Quote by sophiecentaur View Post
ummm Not sure about that. Although the word 'spectrum' is used in everyday life to mean 'range of', I think that spectrum is used in Science to relate to frequency (or possibly wavelength) rather than particles.
However, you can get situations in which particles interact with EM waves with certain frequencies so a spectrum of frequencies can be associated with a range of particles. There is, of course, a correspondence between mass and EM energy (hence frequency).
yes but, the thing is that for example in NMR you get a spectrum right, but this spectrum is a different "kind" to the one you get if you talk about absorption spectrum and colours right? The first one is due to the interaction of the nuclei with energy, and the second one due to the interaction of electrons with energy, if im not mistaken.. :)
sophiecentaur
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Jan21-12, 10:21 AM
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There are two aspects to NMR equipment. The first is Imaging. The protons resonate at a frequency that depends upon the magnetic field around them. Using a powerful magnetic field and a smaller 'gradient' field, the equipment can identify particular regions in the body ('like pixels') at which the (RF frequency, usually in the UHF band - say 900MHz) resonance occurs and the depth of the resonance tells you the density of protons in that region. Using gradient fields in this way, allows you to examine just one particular region in the body at a time and build up a 3D scan, eventually.

The next bit is Spectroscopy and this works because the molecule in which the proton happens to exist will modify the way the proton absorbs the RF is absorbed. This is something to do with the rate of build up of the resonance and the fine detail (frequency offset) of the resonance - which is characteristic of each chemical. Afaik, the term 'spectroscopy' here refers to this variation in frequency of the resonances.

So, between them, the processes 1. identify a position in the tissue and the density of protons there, and, 2. identify the actual molecules present at that point.
Pretty damned smart?


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