chemrox
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instrumental methods of analysis
When I was an UG the wisdom was after getting an imperical formula via MS you then deduced structure using IR and NMR ... (before the heyday of NMR I
understand everone was using IR to work out the structures.) Anyway, I went to Berkeley where the way to work out structure was MS ... period. I
never got that good at it and still relied on NMR. I still like IR it's cheap and I have one now. I also have UV-vis but haven't gotten familiar
with it as structural tool. I almost got an NMR through a surpus outlet but the bidding for it left me in the dust so maybe 'almost' isn't the word.
I can do a lot more with IR than I can with MS but feel that's my training or lack of MS training in particular.. all about the way the molecule
breaks up in the field.
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JohnWW
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If you are entitled to the password to the References section, you will find in it, on the Organic Chemistry ebooks thread, links for my recent
uploads of comprehensive and general organic chemistry textbooks and handbooks. These include some on instrumental and spectrometric organic analysis,
and spectrometric identification of organic compounds.
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Nicodem
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What you need for a proper characterization of a new organic compound is at least 1H & 13C NMR, MS, IR, mp or bp, and CHN elemental analysis as
the imperial analytical tool (if the compound is liquid at room temperature then you are allowed to give a HRMS measuring of the mass peak instead of
elemental analysis). Few journals will get satisfied with anything less. You can not determine a new structure solely on MS, just as you can't do it
by any of the other methods alone, except with single crystal XRD spectroscopy.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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JohnWW
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In addition, if it is believed to have double bonds, especially if it is believed to have conjugated double bonds and/or is colored, it may be worth
doing the UV-visible spectrum. If it is thought to be a stable free-radical compound, or a complex of a transition metal having an incomplete "d"
shell, then electron spin resonance spectrometry (ESR) and mesurement of the paramagnetic moment would be applicable.
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Antwain
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My personal opinion is that IR is more useful only than UV/Vis for elucidation. It becomes more useful if you have an existing spectrum to compare it
to. Having said that, for the relative cost compared to NMR or MS, IR is by far the most cost effective. The mass spec at ANU is a high res one, so it
gives you an empirical formula for the molecule (based on the fact that the mass of 1H not= (1/12)C not= (1/16)O etc). Gotta love that mass-energy,
that what you get for being made of stardust.
So I think high res mass spec is the post powerful...... But it depends on what you want to know. If you have some idea whats there because you know
how it was made then MS may not tell you anything you didn't already know. As johnWW says, even my least favorite - the old UV/Vis - can actually
still be useful in the modern lab.
So:
connectivity and bonding - MS, NMR, IR
Crystal structure - Xray
Bonding oddities such as electron transitions or oxidation states of metals - IR, UV/Vis, Voltametry. Especially the voltametry combined with IR or
UV/Vis
Probably heaps more techniques I haven't used yet.
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jokull
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Sometimes is difficult to achieve or to get access to all the instruments you need to fully determine a chemical structure. With MS maybe the most
important information you can get is the MW, and if you have the time and patience you can assume the chemical structure.
I agree with all you concerning the use of IR, since it is a tool I always use together with MS. However, the suitability of each different technique,
you may choose depends (for me) on:
1. Phase of the sample.
2. Purity.
3. Atoms expected within the molecule.
Nowadays, if you design a molecule then you can simulate its IR, Raman, UV, NMR, XRD and even the MS molecular ion; and when you analyze it you can
compare and conclude faster.
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trilobite
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| Quote: | Originally posted by chemrox
When I was an UG the wisdom was after getting an imperical formula via MS you then deduced structure using IR and NMR ... (before the heyday of NMR I
understand everone was using IR to work out the structures.) Anyway, I went to Berkeley where the way to work out structure was MS ... period.
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Very interesting. To my knowledge there are isomers that can not be distinguished with MS alone, some cases of aromatic substituents in different
positions I think. Surely they've had to use some other methods as well? MS and NMR seem to complement each other. Symmetrical molecules for example
give overlapping signals with NMR but in that case the fragmentation might very well give a clue that leads to proper interpretation of NMR.
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chemrox
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That was a little over the top of me. I should say that for the uses I usually would seek IR/NMR they were getting information using MS. How could
you distinguish chiral compounds with MS? Also, @Nicodem, I'm assuming its an organic compound and any hetero atoms would be N or S .. possibly P.
Further that one knows what the reactants were so there are posibilities to choose among rather than doing an unknown. Shulgin's analytical lab used
GC-MS, NMR and IR for pretty much everything but he could assume the unknowns were medicinal in nature. In geochemistry we used a lot of XRF which
was super for elements.
Wow! Thanks everyone for enlightening me about what's available today and where to get more information.
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