L3 Coupling second order effects

What is roofing

If two multiplets get too close to each other they start to become skewed and appear as though they are starting to form one multiplet. The 1:1 ratio of intensities in a doublet may not be seen as such. The degree of roofing is affected by the frequency of the spectrometer.

Do you need to predict second order effects

No

Second order effects do not always appear even if they theoretically could.

What is magnetic inequivalence and how can it impact the spectrum

A nucleus can couple to chemically equivalent nuclei in different ways. H_A both see the H_B atoms in different ways (⁴J vs ³J) so couple to both separately, producing a doublet of doublets.

The spectrum appears as a messy doublet of doublets with 2 small additional peaks either side.

How do we differentiate magnetically inequivalent nuclei

H_A and H_A’

Are these magnetically equivalent

Yes. Coupling to each individual F happens in exactly the same way

Are these magnetically equivalent

No. Coupling to the same fluorine is cis for one H and trans for the other

Do magnetically inequivalent nuclei appear at the same chemical shift

Yes

How do enantiomers affect the chemical shift of H

They don’t - both H are equivalent and have the same chemical shift

What about diastereomers

They appear at different chemical shifts as they are cis to different groups.

Hence diastereomers can be distinguished using NMR (not enantiomers though)

What are diastereotopic H

If you replace one of the H with X, the molecule becomes a diastereomer.

They are chemically inequivalent and appear at different chemical shifts.

Can diasterotopic H couple to each other

Yes, they are not chemically equivalent.