WTF NMR?

[Mao]

i read the textbook, my initial reaction was "ok..."
then i read the example question, and i was "WTF?"
then i tried A+ notes for explanations, and the result was "F@&#~~~~!"

so, here's my problem:

1:

so, naturally, looking to the data booklet
it was all good, and i did the problem, the answer came as Methyl Ethanoate
until when i checked the explanation in my textbook (Jacaranda)

The chemical shift D=.95 indicates that the CH3 group is next to a slightly electronegative group, such as a carboxyl group. This could mean a CH3COO— structure.
The peak at D=3.9 indicates that the CH3 group is next to a strongly electronegative group, i.e. the oxygen atom. This could mean a CH3—O— structure.

what? electronegativity what??? i sense somehow that's related to the resonance, but i have not figured out how it worked yet...

the next question is even wierder
2.

again, the same principle...
reading the data booklet, i was able to pin down the two peaks @ 4.1 and 2.0, so there was definitely a structure, which with the molecular formular, became of course
the book says so, but the explanation was crazy:

The triplet at D=1.2 indicates a simple alkane group, and the integration trace (3) shows that there are 3 hydrogen atoms present, therefore this is a CH3 group. The fact that it is a triplet, following the n + 1 rule, indicates that it is
next to a CH2 group.
The single peak at D=1.95 indicates a CH3 group; this chemical shift indicates that it is next to an electronegative group, possibly a carboxyl group. The fact that it is not split into a series of peaks implies that there are no hydrogen atoms on the next group.
The quartet at D=4.2 shows that this CH2 is next to a CH3 group and next to an oxygen atom.
Therefore the structure is ethyl ethanoate.

my methods and the results i drawn from the data were very different from this "QFT"

what the heck, am i missing something?? this is doing my head in...

MUCH APPRECIATED

[Collin Li]

I doubt you need to know how chemical shifts relate to electronegativity. I have little to no theoretical understanding of NMR spectroscopy.

The "integration trace (3)" refers to the fact that in the "low resolution" NMR print out (i.e: without splitting), the area under that peak showed there were 3 protons represented by this peak. Remember that the ratio of the area under the peaks correspond to the ratio of protons belonging to each peak.

[midas_touch]

Seems they have introduced NMR into the VCE chem syllablus this year. I remember we briefly went over this at uni last year.
Basically, the different peaks arise due to the different atoms that a particular Carbon atom is bonded to. For example, a Carbon atom which is directly bonded to three hydrogen atoms will register a different peak compared to a Carbon atom directly bonded to an Oxygen atom. The sub peaks in the second image represent the number of hydrogen atoms on a particular Carbon atom, with the number of peaks equal to n + 1, where n is the number of Hydrogen atoms bonded on a Carbon atom. However, you will only see this 'splitting' if the hydrogen atoms are on adjacent Carbon atoms since they interact with one another in such cases, resulting in the splitting of the peaks.

[J.Z-M]

I personally don't get NMR as well (well I don't get many things in chem, but NMR is one concept that literally shocks me)

[Mao]

...

i have some answers

to answer myself for the electronegativity part, (and btw both graphs are ¹H NMR), the further away the peak is from 0 (Si(CH₃)₄), the more resonance it has, and because of the electrons coupling and shielding (understanding of these not required in VCE syllabus), etc, the resonance tend to be higher when the hydrogen is near electronegative element(s) (but not specifically bonded to it, as that takes the readings off the scale due to "shielding", hence why OH bonds is hard to determine).

this, with the integration trace (the "(x)" shown above the peaks) allow you to determine how many hydrogens there are, and how close it is to electronegative element(s) (if any) and other CH₂ or CH₃ nearby when the high res result is given, this allows you to work out the molecular structure without having to consult the data-booklet, but you should still doublecheck to make sure the peaks are where they should be.

*types up craploads of notes on it* when the new notes system comes up, i'll put all that there, hopefully that'll make more sense than my rant here

[Collin Li]

NMR is simple. You don't need to understand how it works in much detail, you only need to be able to interpret the graph printouts. I studied NMR and IR in first year university, and they don't talk about the mechanics of it at all. I'm guessing that the theory delves into quantum theory that is too deep for VCE students.

It is hard to explain in words, but in order to make the task easier: proton NMR is the harder type - you have to deal with the normal graph printouts as well as the "high resolution" printouts that show splitting. Carbon-13 NMR is incredibly easy, it is just about counting the number of unique carbon atoms (which can be found out simply by drawing axes of symmetry).