NMR , Carbon And Proton Spec Flashcards
What is NMR spec and how does it work
What is it based on
Nuclear magnetic resonance
- infrared spec was based on the electrons in covalent bonds absorbing IR radiation, well this time NMR spec is based on the NUCLEONS in atoms absorbing radio wave radiation.
- based on idea that different atoms will need different frequency of radio waves based on characteristics they have to absorb this, and thus we can detect this, and work out what is in a molecule
How does it work with the nucleons
Why odd
So how can nmr be used to differentiate what’s in molecules
Basically like electrons, nucleons also have SPIN property.
- if they have an even amount if nucleons , then no matter what the same number will lie with the MAGENTIC field and against jt. Here, none will absorb Radio waves
- however if there is an ODD AMOUNT OF NUCLEONS, then the molecule will absorb Radio waves to flip the spin
Now again based on what the atom is bonded too/ envionrment will it require different FREQUENCIES of radio wave
And we can detect this and match to a database, and use it to work out the molecules
So how can nmr be used to differentiate what’s in molecules
Based on what atoms bonded to atom with odd nucleons + the different environments, causes a SHIFT in frequency that is needed to be absorbed For resonance to take place
Based on this shift we can detect, and compare it to a book of data and a sta card to work out the molecule
Again what actually happened with the odd nuelcoens ; when is the radio wave absorbed?
It is absorbed when the spin state of the nucleons rapidly flips . At this point, it is called resonance , and so for resonance to occur a certain frequency of radio wave must be given based on the 1) magnetic field) 2) atoms it’s bonded to 3) chemical enrbioment it’s in
So why is NMR spec useful for organic chemist?
CARBIN 13 and Hydrogen exists naturally, which are odd nucleons
Everyhting had hydrogen , which is badicsllt s proton, so that’s why useful
Not everything has isotope C13, but in like few hundred molecules yiu,, have a few with carbon 13 isotopes, thus NMR spec can definitely be used
As organic chmiestu based on carbon and hydrogen this proves very useful
How are values for frequency absorption of things seen in NMR? what is present in all samples as a result
where will there always be a peak in NMR spec and why?
Here we compare all “shifts in frequency” frequency shifts REALTIVE TO A MOLECULE CALLED TMS. TMS is in every sample of solution presented and the frequency is set to 0, with everything relative to it
Thus on every spectra there will Always be a peak at 0 because of presence of TMS
Why is TMS used in general
3 reasons AND WHY (first reason more IMPORTSNT )
-It is a symmetrical molecule, and so all the carbins exist in one environment and same for all the hydrogens. Thus whatever spec you run, it will only every absorb one frequency, making it a good thing to compare to and MAKE RELATIVE
- also unreactive (good)
- and volatile (also good because we can remove easily)
Again quickly how will all specs look like no matter what
Will always be a peak at 0 PPM (the unit for frequency) at the right, and then different frequencies to the lift measured in ppm
How is a sample prepared for NMR (4)
1) dissolved in a solvent
2) then TMS is added
3) placed in narrow tube and spun to even out imperfections in mansgtic field )
4) finally a RANGE of radio wave frequencies are given to the sample, to see which ones are absorbed
Why is NMR useful then in general to identify the molecule
Because you just ran a test that doesn’t require conventional decolorisation or long tests, the origanl molecule is in tact and still easily recovered
What is deuterium in context of NMR spec and so why are solvents used to dissolve the sample to be tested normally deutarated molecules?
Deuterium is the 2 hydrogen so has even nucleons . Anything with even nucleons won’t produce a peak on NMR spec.
Therefore Deutrareted substances are typically used as solvents for NMR spec, as they produce either no peak or very small that is filtered out before the graph is shown
Again this because even nucleons
How to identify chemical environments
Look at each carbon and first differentiate what they are bonded to. If they are bonded to different things they have different environments
If the molecule is symmetrical, it s possible thst carbons will exist in the same environment . Thus look at the PROXIMITY it is from double bonds and electronegative FON atoms. The further away it is, also means a new environment
Make sure for aromatic compounds to draw all lines if symmetry
What does the number of carbon environments tell you about how thr spectra will look
If there is 4 CARBIN environments there will be 4 PEAKS IN THR SPECTRA, ONE FOR EACH ENVIRONMENT
What if there are two peaks in the same range on the spectra - what does this mean?
Looking at a spectra, you can see where the peak is range ti give you clue about what is in the molecule
However if there are two peaks in the Same range, this should tell you that THEY ARE IN DIFFERENT ENVIORNMENTS
the thing is thr carbon closer to the more elector active environment will be shifted more LEFT than the one further away, this should tell you why there are few peaks . Don’t get caught out
