Chemistry question on NMR Spectroscopy?

[Jarrod K]

What exactly does "proton shielding", chemical shifts and exterior magnetic field in respect to NMR mean? What are the trends in the chemical shift? Is it the higher the shift, the lower the proton shielding, thus it is easier for them to "flip", or is it the other way around? And what does "resonance" mean?Thanks for your inspiring explanations, which I understood fairly well.

[Aaron H]

Proton "shielding" is when the positive charge of the proton is "shielded" (less prominent) by the electron cloud. When you have a molecule that includes electronegative atoms, such as Oxygen or halides, the electrons will spend more time closer to the electronegative atoms and less near their respective protons. So the "H" in "OH" will be less shielded than the H's in CH4. Deshielding moves the signal a bit to the left -- so when you're consulting your reference chart -- if it says "3.2", you won't want to rule out signals that show up a little above that (3.5-4.0).Resonance is the property of certain molecular structures that involve the sharing of a double bond. The archetypal example is a Benzene Ring (or an "Aromatic Ring") which is a C6H6 cyclohexane molecule with conjugated (alternating) double bonding. When the electrons in a double bond could just as easily be in an identical location elsewhere in the molecule, they'll jump around. For example:H3C-O=C-O-CH3In this diester, it is symmetrical and although the double-bond is currently on the left side, the electrons will alternate between that arrangement and this one.H3C-O-C=O-CH3

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