killswitch
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Goddamned NMR
I know this sounds juvenile, but when deprived of computerized tools that do the work for me, interpreting H-NMR+c-NMR+IR multi-spec problems, even
with a chart of the frequency signatures of common functional groups right next to me, is kicking my ass. Fischer projections? No problem. Chiral
chemistry? Easy. Twisting things around in my head in 3D is something I've practiced a lot.
But I'll be damned if some squiggly lines on paper can't send me into conniptions.
Not that I can't do it, but I'm under time pressure now that wasn't there before and my old (absence of) methods isn't cutting it. What's the best
process of elimination to arrive at the speediest answer if you are given A: the 1H NMR, B: the 13C NMR, C: the IR spectra, and D: the chemical
formula?
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smaerd
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Check the IR for functional groups.
Interpret the H-NMR accordingly. If its a typical organic class all you need to worry about is where the peaks are, how many splits there are, and how
many times that peak occurs. Remember the functional groups that shift the peaks.
Usually that and the mass spec or chemical formula, is all I need for simple molecules. I wish there was an easier way to line it out but that's
really all there is too it. That and a bit of practice.
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ScienceSquirrel
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Your best guide is familiarity.
Simple molecules are easily solvable from looking at the spectra is you know what you are looking for.
Look at the molecular formula first. If there are oxygen, nitrogen atoms, etc, try and guess at likely funtional groups, confirm these with the IR
spectrum.
Look at at the degree of unsaturation, For example say it is C8H802, deduct C6H4 from that and you are left with C2H402. That comes down to either a
methoxy benzaldehyde or methyl benzoate.
Use the other spectra to confirm your structure.
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kristofvagyok
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Try SDBS (pectral Database for Organic Compounds), it could be found here: http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_to...
Also, there are several groups what looks really characteristic on H1-NMR and if you know the integrals what fits to the peaks then it could be
logically found out. E.g. ethyl, isopropyl, allyl aromatic, 1,4, 1,3 1,2 disubstitued aromatics have really characteristic form on a spectra. Also if
you have an idea from the H1-NMR than the C13 could confirm it. And if you got a molecular formula then it can't be a problem(:
I have a blog where I post my pictures from my work: http://labphoto.tumblr.com/
-Pictures from chemistry, check it out(:
"You can’t become a chemist and expect to live forever." |
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DJF90
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First off, I'll ask if this is based on homework or exam questions. If so, you're only going to be dealing with the spectra of fairly simple compounds
e.g. ethyl acetate. From the molecular formula and the IR, deduct possible functional groups and/or identities of the potential solutions. Then try
and confirm this by NMR' look for a peak that would be characteristic to the compound you think it is (if at all possible... e.g. anisole will have a
peak at about 3-4ppm for the methoxy CH3 (singlet, 3H), yet some regioisomer of cresol (methylphenol) will have a CH3 peak at a lower shift (say,
2ppm?, singlet, 3H), and possibly (if visible) a 1H broad peak for the (exchangable) phenolic proton. An isopropyl group is easy to spot with a 6H
singlet somewhere between 1 and 2ppm, and a 1H septet around 3.5ppm (assuming its in the form of an isopropyl ester or ether or something). Aldehydes
have a distinctive 1H (singlet) at around 9-10ppm. A peak you may observe at 7.26ppm in the form of a singlet of whatever intensity is the chloroform
solvent (or rather, traces of CHCl3 in CDCl3). Water shows up at 1.55ppm.
If you're in a research setting, then I have a couple of suggestions. First off, if its one of your products, you should know roughtly what to expect.
From what you know of the structure and the MS you should be able to deduct what has happened (e.g. unexpected double addition of some reagent, or
obtained product is a dimer, or unexpected incorportation of say bromide into the product; a friend had this problem opening an epoxide with some
RMgBr and CuCN!!). In the case of halogen, this should be easily seen by isotope peaks. Being that you now know whether the formula is what you want
for the desired product or not, you can go about assigning your NMR. If you want to fully characterise your material (and assign all peaks apart from
unresolved multiplets), you'll need a COSY, HSQC and HMBC, as well as the 1H NMR and 13C NMR. This was standard practice in the research group I was
in.
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