ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: vce01 on June 09, 2008, 06:26:40 pm
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okay this might sound like an incredibly noob question but i gotta ask...
with pretty much every sac/test/information resource ive come across, the peaks in the NMR do not exactly correspond to the information given to us in the data booklet.
will VCAA exams have peaks that exactly correspond so you can be in no doubt of figuring them out?
or do you not look at chemical shift first, cuz thats what i do but some people have said look at environments and stuff like that.
tbh, it'd be better if someone could give me a crash course on NMR lol cuz im kinda struggling with it. as in, what should i look for first for the different types of questions they could ask. a lot of the stuff ive got get way to freaking complicated to understand halfway through, if you know what i mean...
so yeah, any help would be appreciated. cheeers.
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they will certainly correlate, but the principle theory is: the closer you are to an electronegative element [Oxygen], the higher your chemical shift.
what you should see first is what the environments are arragements are. the peaks and data correlating in the data booklet should be used as a checking mechanism rather than the identifying mechanism.
[i have some notes uploaded here on this topic... somewhere...]
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You should work with the peak splitting and integration first and then look at chemical shifts. I doubt that they'll just give you an nmr spec by itself and ask you to work out exactly what it is. You will more likely get more information (eg empirical/molecular formula, C NMR, IR, Mass spec) which will help you out.
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Crash course in NMR? Here you go...
What it does
- Determines structure of molecule: mostly in organic chemistry
- Uses energy in radio frequency (think of the spectrum with all the colours, UV light etc on it - energy with these frequencies is even less "energetic" than the microwaves in..microwaves...
- Energy is too low to cause electronic, vibrational or rotational transitions
- Instead causes change in spin of particles in the nucleus (aligns them either with or against a magnetic field)
- NMR measures the energy that is emitted when the nucleus "relaxes" to its regular spin state
How it works
1. Compound subject to powerful magnetic field
2. Radio waves shot at it in pulses
3. Instrument detects the amount of energy emitted when nuclei relax, printer spits out a spectrum
- The sample has to go in a spinning glass tube to make sure that it's exposed to a uniform amount of the magnetic field
- You can't dissolve the thing you're analysing in water, because H20 has two hydrogens which will also be measured on the spectrum and would make the graphs hard to read...instead, use D2O (don't worry about what it is - basically just a solvent that doesn't have any hydrogen in it so won't have any peaks on the graph)
How the graph is made
- Like the way you need to zero a set of scales, you need to have something to standardise the measurements. You would've heard of TMS... this is a substance that's analysed before anything else, for calibration, because it gives a single peak on the graph away from other ones that we normally analyse, so we call that "zero chemical shift"
- The difference in energy needed to change spin state in sample compared to TMS = “chemical shift”, measured in ppm
How to interpret the graphs
- First, look at the number of peaks. Each peak = different environment (if C NMR, each peak is a different C enviro, if proton NMR, each peak is a diff H enviro)
- Chemical shift is in the data booklet but it's just an approximation. Use it as a guide but don't absolutely trust it...
- The area under each peak is proportional to the number of H or C atoms in each enviro (so if there are 6 hydrogens that belong to a CH3 that is next to a CH2, the "CH3 peak" will have area = 6
- Normally the area is shown as just a number at the top of the curve, like a label. Don't confuse area with number of peaks caused by splitting
- No. peaks caused by splitting = n+1 (where n=no. H/C atoms on NEIGHBOURING atom)
- CAREFUL: the H in OH groups doesn’t split peaks of adjacent H atoms, nor is its own peak split.
Hope this helps!
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a few points to add:
There are high and low resolution NMR spectrums.
Low resolution can apply to 13C and 1H, whereas High res. only applies to Proton [1H] NMR.
Splitting only occurs in high resolution NMR, i.e. only high res 1H NMR [not low res 1H NMR]
splitting is caused by spin-spin coupling [dont need to know this], however, only differeing environments can cause splitting. adjacency groups with the same chemical environment will not cause splitting.
splitting can occur on both sides: e.g. CH2=CH-CH3, the bolded hydrogen is affected by neighbours on both sides [which both have different chemical environment]: 5+1 = 6 peaks, or a sextet split.
area under the peak may also be referred to as "integration trace"
only atoms with an odd number of nucleons can be used in NMR [otherwise the spin would cancel each other out]
shielding: in -OH especially, oxygen blocks inteference of nuclearmagnetic fields because of its electronegativity, hence the H in OH does not split, and does not contribute to splitting of adjacent groups
also because of this electronegativity, the adjacent groups' chemical shifts will be elevated [i.e. the higher the chemical shift, the closer it is to electronegative element]
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wth, D2O is water, just 2H instead of 1H so it doesn't cause interference
you is whack!
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Or heavy water (deuterium to be precise).
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wth, D2O is water, just 2H instead of 1H so it doesn't cause interference
you is whack!
why whack? she's not wrong.
however, a more common solvent is CDCl3.
Cl
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Cl-C-2H
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Cl
It is used because most compounds will dissolve in it, it is volatile and therefore easy to get rid of, and it is non-reactive and will not exchange its deuterium with protons in the molecule being studied. It is also "silent" in the NMR and will not show peaks to interfere with the analysis of the compound of interest.
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okay. cheers for the info guys, im gonna do a few practice questions tomorrow and hope for the best, lol
got another question tho, do we really need to know the principles of NMR? i havent come across any question that asks us anything on it, its just interpreting spectra.
so principles, do we need to know it or dont bother?
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the study design and vcaa errata does say that the emphasis is both on the interpretation AND principles (of all the chromatography+spectroscopy stuff in general, not NMR specifically), so i guess ur best to learn it? vcaa arent very specific in terms of what we need to know and not
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another one :p
will the VCAA exams definitely tell us what the area of the peaks are? as in the numbers on top of the peaks, or atleast the ratio in which the peaks are..?
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The numbers on the top are ratios of each other.
VCAA will probably (very) tell us the area of the peaks, because I don't think any of us has been taught to do integrator traces