Theory of NMR!?!?

[Dark Horse]

Hey guys, I understood the theory behind NMR when I learnt it at school, however, I went to a tsfx lecture today, and I do not understand the theory behind the technique. I know do not understand why a nuclei in a more electronegative bond (eg. O-H) is shifted more downfield than ( C-H). I mean, before I learnt that nuclei that were shielded to a greater degree (eg. C-H) absorbed less energy, and hence had a lower chemical shift than a nuclei that are less shielded (O-H). However, at tsfx, the lecturer kept talking about magnetic field energy, and how this is actually increased. Apparently relatively shielded nuclei absorb at a higher magnetic field energy (like tetramethylsilane) and everything is compared to TMS. Could someone please explain what I should know on this concept for VCE chem exam?

eg. Why is O-H more downfield than C-H? Answering this and addressing my concerns would be awesome, cos I'm kinda in panic mode =P   

if you dont know what im on about just tell me

[vibs23]

Depends on the molecule...

[vibs23]

It's kind of hard for me to follow your question. but dont worry, they wont ask for the exact scientific reasoning behind NMR spectroscopy because not everyone would have access to such technology, just questions where you have to interpret the data from the table.

[chem-nerd]

most of the theory behind this is beyond the scope of VCE.

nuclei that are strongly shielded by electrons need a higher external magnetic field to bring them into resonance hence are closer to TMS and vice versa.

protons on carbons next to electronegative atoms (halogens, O, N) are deshielded because of the electron-withdrawing ability of these atoms.

[Dark Horse]

Yeah lol, it wasnt very clear was it  well, i went to tsfx today, and got confused, because the lecturer kept going on about increasing the external magnetic field, which I had never heard before. So basically, I was wondering how chemical shift values are obtained?

"You have the proton NMR spectrum of ethanol. Why does the H in O-H have a higher chemical shift than the H in the CH3?" If that was a VCE exam question, how would you answer it?   

[lacoste]

What lecture was it by the way?

I thought TSFX dont do any easter programs

[Dark Horse]

lol, i dont know, it was part of the masterclass The teacher was Irena (thats what i think her name was) she was really good, its just that I reckon the NMR theory she taught was hardert than it needed to be.

[Gloamglozer]

lol, i dont know, it was part of the masterclass The teacher was Irena (thats what i think her name was) she was really good, its just that I reckon the NMR theory she taught was hardert than it needed to be.

Yeah, it was part of the Intense Workshop (ie. Master Classes, weekly classes, etc) and I reckon Irena did a pretty good job doing it although half the time, I was pretty confused.

[lacoste]

probably trying to scare you, thats what tsfx does best, throw in hard qsts relating to irrelvent theory.
 

[lacoste]

lol, i dont know, it was part of the masterclass The teacher was Irena (thats what i think her name was) she was really good, its just that I reckon the NMR theory she taught was hardert than it needed to be.

Yeah, it was part of the Intense Workshop (ie. Master Classes, weekly classes, etc) and I reckon Irena did a pretty good job doing it although half the time, I was pretty confused.

Does Byron teach there?

[shinny]

"You have the proton NMR spectrum of ethanol. Why does the H in O-H have a higher chemical shift than the H in the CH3?"

The hydrogen atom in a hydroxyl group has a higher chemical shift than a hydrogen atom in a methyl group because the oxygen atom in the hydroxyl group is highly electronegative. This causes the electron on the hydrogen atom in the hydroxyl group to orbit closer towards the oxygen atom, and hence, orbit further away from the hydrogen nucleus. This deshields the hydrogen nucleus as less radio waves will be absorbed by this electron, and consequently, a different amount of energy, or rather, a different radio frequency is now required to 'flip' the direction of the hydrogen nuclei's spin.

However, I really don't think you'll be asked this. It's been stated many times that the emphasis is on interpretation of data, not the principles behind it. Check last year's exam for the kind of question you should be expecting.

[Dark Horse]

Thank you shinny, that was awesome, I get it now! (lol, I would give you karma but I dont have enough posts)

btw, byron doesnt teach there anymore, I put him as my first pref but he only does the summer school now

[shinny]

I gave a bit more detail there than necessary in an exam. From what I know, just saying deshield without stating what that means or how it occurs is enough for VCE standards (if you ever even need to state this to begin with).

[nicholasysc]

Irena is awesome but she only teaches the Friday Chem class + Summer School etc. :S
What day + time do you (those who attends Chem Master Class) go to? n which teacher do you have?

[Dark Horse]

Does Irena teach on Friday nights as in 4-6 or 7-9? I go on Friday 7-9, have Graeme Morgan, he goes slow but makes sure you understand the concepts.