Hi

Please, find in attach a paper on HSQC as well as some links.

https://www.google.com.eg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=12&cad=rja&uact=8&ved=0ahUKEwi1wrH8zKrMAhWF7hoKHcG3CLAQFghZMAs&url=http%3A%2F%2Facdlabs.typepad.com%2Felucidation%2F2009%2F12%2Fhow-to-interpret-an-hsqctocsy-experiment.html&usg=AFQjCNEKHgBFd3n1E3dtdmItVnCSDonZ8A

https://www.google.com.eg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwi6ypGCzqrMAhVBiRoKHdliCN8QFgggMAE&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FHeteronuclear_single_quantum_coherence_spectroscopy&usg=AFQjCNGM0c9ZtaTeerVixj0JuSuu1UXocQ&bvm=bv.120551593,d.d2s

https://www.google.com.eg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwi6ypGCzqrMAhVBiRoKHdliCN8QFggsMAI&url=https%3A%2F%2Fwww.chem.fsu.edu%2F~shatruk%2Fdocs%2Ftutorials%2FTutorial%2520on%25202D-NMR.pptx&usg=AFQjCNGre5LApVuoh_CH8lPyuqQhjGfd-A&bvm=bv.120551593,d.d2s

Hello, thank you for your response. But my issue does not lie in the interpretation of the HSQC spectrum, rather I understood the HSQC spectrum but stumbled upon this situation where I need more explanation. Let me elaborate on my question slightly further. Try to picture this. Note that the signals are only in the aromatic region.

Aromatic Ha in 1H NMR spectrum is downfield.

Aromatic Ca in 13C NMR spectrum is upfield.

Aromatic Hb in 1H NMR spectrum is upfield.

Aromatic Cb in 13C NMR spectrum is downfield.

The assignment is confirmed by HSQC.

So my question is, how does Ha appear downfield, but Ca appear upfield? And vice versa. Is this a common occurrence? If not, what's the reason behind the difference in the deshielding effect on carbons and protons?

Hello David.

To clarify, I have mentioned that the signals in discussion are in the aromatic region. So, it's not that the aromatic signal is in an upfield chemical shift. It is very much in the aromatic region. I'm sorry if I haven't been clearer at phrasing my words.

I've decided to attach a HSQC spectra, expanded on the aromatic region (observe ppm values). Do notice that the chemical shifts are in the opposite direction in 1H and 13C, as labelled. If you could have a look, maybe offer me some advice or clarity, I would appreciate that very much. Thank you in advance.

Miyeko:

You need to consider that total electron density in a bond is a constant.  So if there is more electron density near H the opposite must be true for C.

Thank you Donald!

I realise now that I have this misconception where a deshielded proton will always correspond to a deshielded carbon signal. While I did assign the protons based on the electron density, but why didn't I for carbon? I'll ponder upon this and hopefully conclude this matter once and for all. Thank you once again.

There is not a absolute correlation between proton and carbon chemical shifts. Substiutent effects on carbon and proton can be different enough to induce a stronger down- or up- field shift on one versus the other. Almost every aromatic compound has signals where you can find the behavior you describe.   After all the proton is one bond further away from a substituent that affects the shift.

Dr. Myeko Lotus, could you show your structural proposition?  

Clemens you are correct in saying that there is not an absolute correlation between C and H chemical shifts. 

The question I was answering is the narrow case presented by Mieko.  Looked at locally, after other effects are taken into account, what I said is true. For an isolated C-H bond, if the center of gravity of electron density is shifted toward H, the opposite must be true for C.

Thank you everyone for your response. I highly appreciate them.

Clemens and Donald, could you kindly share any reading materials that specifically discusses this behaviour? Especially since it's mentioned that almost every aromatic compounds display this kind of behaviour. Perhaps some's reported them, but somehow I'm missing it, so I would like to go through them.

Eduardo, I've included an image of the structure of my compound for your reference. -OR is an alkoxy chain.