What exactly is the chemical shift in NMR?
In a compound, how would I know where to put each peak (whether C-13/H1 NMR) along the horizontal axis with the chemical shift(ppm) on it?
I heard it was something to do with electronegativity or polarity of the different chemical environments of the compound..
help would be appreciated!
Thanks!
Mmm well you have to have a pretty good understanding of the background information of NMR. However, I have never come across a questions which asks to literally place a peak onto an NMR spectrum! The questions you will normally recieve would have something to do with identifying what the molecule is.
You are right when you say it has something to do with electronegativity. BUT YOU DO NOT NEED TO KNOW THIS. THIS IS ONLY FOR YOUR UNDERSTANDING. From what I gather, we won't be asked to describe this background info at all.
The reason for the different chemical shifts depends on the 'shielding' of a nucleus and how 'exposed' it is. The energy from the radio waves cause the 'spinning' of electrons around each nucleus from the applied magnetic field. THe amount of nuclear shielding depends on other atoms surrounding the nucleus.
The chemical shift is basically the difference in energy needed to change the spin state compared to TMS.
Sooo back to electronegativity.. the more electronegative an atom is, the greater pull it will have from the other atoms .. decreasing the shielding.. leaving it more exposed and .:. allowing it to have a greater chemical shift
Like the hydrogen in -CH3- will absorb at a different rate than the hydrogen in -CH2- because of the different electronegiativity and thus the 'shielding'
CH2 CH3 TMS
That is normally the order ^^^^ (LMAO worst NMR ever). As you can see, the CH2 requires more energy to 'spin' because it is more 'exposed' as the electronegativity is greater
We add TMS as the reference sample because all the protons are in the same chemical enviro and will need the smallest chemical shift becuase it is the least 'exposed' ... because TMS is surrounded by CH3
Would love it if someone could please read over that and check my understanding. Thanks