Just to clarify something:
In NMR, is chemical shift proportional or inversely proportional to frequency of the radio waves?
I thought that shielded atoms experience the magnetic field of the NMR machine to a lesser extent, and hence require less energy for resonance to be induced (ie. chemical shift is proportional to frequency), but TSFX says the opposite, that is shielded atoms require more energy for resonance to be induced. Does the discrepancy arises due to there apparently being 2 kinds of NMR machines (ones where the magnetic field varies and ones where the radio wave frequency varies)?
Please correct me if any of these information is wrong. And I believe VCAA does not require too much details about these:
Radio waves with sufficient energy can break the magnetic field, causing the nuclei to “flip” and create a peak.
Normally in NMR, they might vary the magnetic field so that if you have a high magnetic field, radio waves strength will not be strong enough to cause flipping. And if you have a low magnetic field, radio waves will be strong enough to cause flipping.
However, since different nuclei experiences different chemical environment, different magnetic field is needed in order for that nuclei to flip. Electrons play a vital role in the chemical environment because they shield the nucleus from the magnetic field, forcing them to experience less energy, hence, larger magnetic field strength is needed.
In short, a highly “exposed” nucleus requires lower magnetic field to cause flipping (especially those that close to a high electronegativity atom because these atoms will draw the electrons from the nucleus away). You can called these being “deshielded”.
Now, in the NMR spectrum, on the LHS, we call that the “downfield” region and on the RHS, we call that the “upfield” region. Downfield is the region of nuclei that experiences the MOST magnetic field and upfield is the region of nuclei that experiences the LEAST magnetic field. Since chemical shift is measure in ppm, we can say that when a peak is 7ppm, it means the magnetic field required to cause flipping of the nucleus is 7 millionths less than that of the TMS. Hence, higher chemical shift (the more downfield it is), the more exposed a nucleus.