[veracity]

Hi all! When writing my ochem lab report, I became confused by the presence of the AA'BB' coupling of p-iodonitrobenzene. From what I know regarding the effects on chemical shifts of I and NO2, I would have predicted AA'XX' coupling.

I decided to attempt to decipher this by looking up the literature spectra of both iodobenzene and nitrobenzene. I can now understand the similar environments in p-iodonitrobenzene, however the WHY behind the chemical shifts of the iodobenzene greatly confuses me. I have attached pictures detailing my thought process.




Thought process:
- Iodide is not particularly electronegative and would not have a huge deshielding inductive effect
- Iodide should display the heavy atom effect due to its huge, shielding electron clouds (just how far could that shield? Just the carbon to which it is directly attached (which of course would not be on 1H NMR), or further?)
- Resonance should place added shielding electron density ortho and para

What I observe:
Ortho most deshielded (7.667 ppm), para second most deshielded (7.30 ppm), and meta most shielded (7.05 ppm) -- What??

General question:
- What effect does iodide have on a benzene ring with regards to shielding and deshielding? All the little components on each position, not just the net effect

Side question:
- Relative to one another, the nitrobenzene shifts are as I had expected, however why is the ortho position super deshielded (8.193 ppm) relative to the para position (7.650 ppm)? I understand that the N in NO2 has a fairly strong inductive effect, but would that reach past the carbon to which it is directly attached to the ortho carbon with *that strong* of an effect? Or is there another reason for the deshielding difference between ortho and para? What is deshielding the meta position from the lit -H of normal benzene (7.339 ppm)... inductive through the ring?

Where did my thought process go awry?

Any insights are super duper appreciated!