ManBearPig
Diamond Member
I think yes, but just wanna make sure.
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Do all molecules have different IR and Carbon 13 spectra?
- Thread starter ManBearPig
- Start date
Babbles
Diamond Member
IR measures different types of bonds, it is not necessarily the specificity of each type of molecule.
Meaning you can determine if your unknown has double bonds, aromatic rings, amine groups and so forth but you could not necessarily tell the difference between isomers. Also I don't believe you could tell the difference of, for example, you have an aromatic ring with a ethyl group hanging off of it or if it was 20 carbons long - if they are all single bonds then the IR spectrum will just show you a pattern indicative of a molecule containing single carbon-carbon bonds as well as that aromatic ring.
Meaning you can determine if your unknown has double bonds, aromatic rings, amine groups and so forth but you could not necessarily tell the difference between isomers. Also I don't believe you could tell the difference of, for example, you have an aromatic ring with a ethyl group hanging off of it or if it was 20 carbons long - if they are all single bonds then the IR spectrum will just show you a pattern indicative of a molecule containing single carbon-carbon bonds as well as that aromatic ring.
ManBearPig
Diamond Member
bumpooooo
Babbles
Diamond Member
Also your question regarding carbon 13 doesn't make sense. Do you mean Caron-13 NMR? In that case you are essentially just determining how many unique carbons you have in the molecule.
If you combine many of these tools you can eventually discern the structure of your molecule, but you may not necessarily be able to do it using only one tool
If you combine many of these tools you can eventually discern the structure of your molecule, but you may not necessarily be able to do it using only one tool
Suspicious-Teach8788
Lifer
Mass spectrometry ftw. It's weakness is isotopes though.
Babbles
Diamond Member
Not really. . . in fact different isotopic abundances help in describing unknown (or known) compounds.
For example about 70% of Chlorine is m/z 35 and ~30% is m/z 37. So when you see a doublet peak with a difference of 2 and the second one is about a third of the size of the previous one then that is indicative of a mono-chlorinate compound.
Different numbers of chlorine will yield different patters due to the isotopes.
Similarly you can do this with bromine who has an abundancies about 50/50 of m/z 79 and 81. Therefore if you see two equal peaks spaced apart by two then you can be reasonably sure that you have bromine present.
Also a mixture of chlorine and bromine will provide different spectra, all based on their isotopes.
Deutroated and carbon-13 containing compounds are used like crazy as internal standards since the different isotopes will create different molecular weights while maintaining the same chemistry as the parent compound.
A magnetic sector instrument, as well as a Q-TOF, can easily detect molecular fragment weights down to the fraction which is used to give accurate mass. This means that different isotopes can be easily determined.
Hell arguably determining isotopic differences is a fundamental benefit of mass spectrometry.
It's been a few years since I took organic and analytic chemistry, but I do believe that if you have the IR spectra AND C13 NMR chemical shifts you can delineate between all compounds, with the exception of isomers.
Oh, and the number of atomic symbol avatars posting in here is great. 🙂
Oh, and the number of atomic symbol avatars posting in here is great. 🙂
ManBearPig
Diamond Member
so each atom DOES have a different IR spectra, right? No two are the same. Same goes for carbon 13 (NMR).
gotcha...babbles you are a chemistry champion my friend
gotcha...babbles you are a chemistry champion my friend
uclaLabrat
Diamond Member
Of course.
I just want to add: Mass Spec and IR are more of a formality these days. Most people just use proton and carbon NMR to characterize the compounds, and most favor proton over carbon. Both are useful, as you can tell the difference between different diastereomers using a proton spectrum and the coupling constants (usually), and other techniques such as COESY or NOESY allow you to get more information still. Enantiomers are impossible to tell apart without chiral shift reagents, and the most useful technique there is to just use X-ray diffraction, or chiral HPLC is the compound is a liquid.
Brainonska511
Lifer
Can you measure IR for a specific atom?
IR is measuring vibrations of atoms (bond stretching/bending). You don't have bond stretching in free atoms, so there should be no IR. If you have a compound, say propylene (CH3CHCH2), each set of bonds will have different stretching frequencies because of the nature of the bond and the effects of the atoms alpha to the stretching.
I don't remember enough about NMR to comment on that though.
ManBearPig
Diamond Member
oops, i meant molecule, sorry!
uclaLabrat
Diamond Member
Nope. Mass spec is fairly useless. You can get the empirical formula, and hazard a guess as to structure from the fragmentation pattern, but for connectivity and stereoisomers, you need NMR or X-ray.
Babbles
Diamond Member
Mass spec and IR are a bit more than a formality. Until I was laid off I use to be a project manager where one of my responsibilities was doing characterizations under FDA GLPs and the typical thing was to use MS or MS/MS, FTIR (Fourier Transform InfraRed Spectroscopy), TGA (ThermoGravimetric Analysis), and/or DSC (Differential Scanning Calorimetry) and sometimes we would outsource it for NMR (H+).
When most people say X-Ray Diffraction (XRD) they are talking about XRD Crystallography which only works molecules that forms crystalline structure, e.g. quartz and cryistobalite.
You can use GC to separate chiral compounds, and the compound does not need to be liquid; it needs to be soluble in the mobile phase and be suitable for the detector.
Mass Spec will not be able to resolve the difference; they are isomers and have the same molecular structure. What you can do, though, is seperate it via chromatography - e.g. Gas Chromatography (GC) or LC (liquid chromatography). Once seperated in mobile phase the two chiral compounds will elute at different retention times and then can be detected by MS (or FID, ECD, PID, or whatever you want that is appropriate).
This is woefully ignorant. In fact I can only think that you must still be in college and are simply unaware of how the analytical chemistry industry actually works.
In fact the American Society of Mass Spectrometry (ASMS) is arguably the largest professional chemistry society in the world (outside of the ACS). It just about dwarfs any other conventions of other related societies (e.g. PittCon).
Not each atom, it is each bond-type that gives off a different spectrum.
I was laid off, but I have been doing analytical chemistry professional for ten years. My area of expertise (if you want to call it that) is GC/MS (gas chromatography/mass spectrometry) but I managed a group that did the FTIR and I also managed studies using a shit ton of different instruments so I am relatively familiar with them.
Don't you dare ask me any wet chemistry (i.e. pH) type of question or any reaction chemistry stuff and my inorganic is pretty weak!
uclaLabrat
Diamond Member
People use mass spec and IR because it's cheap. If you're screening for known compounds and have profiles, like in QA/QC, it's awesome. For $50-$100K, a quadrupole GC-MS can't be beat if you're looking for known compounds, and beats the hell out of dropping $1-2 mil on an NMR, plus full time staff. If you're doing structure elucidation of unknown compounds, mass spec and IR are required for characterization, but they don't tell you dick about the structure. I don't do QA/QC, I'm an organic grad student trying to figure out what the hell I made.
For the most part, yes. BUT often the differences between very similar molecules are so subtle that they are lost in the signal "noise" and you don't see them.
Basically, there are two ways to use the various spectral tool results. One is that each tool is able to detect very clearly certain details of a molecule's structure, but not all of them. So you use several tools and combine the details revealed by each into one large collection of charcteristics. From that you usually can figure out the real molecular structure. Sometimes not completely, though, so you use what you do know so far to decide what other tools to use to fill in the missing bits of information.
The other way is to look at every detail of the spectal output of one tool and compare it to a massive library of spectra of known pure compounds. You are looking for an exact match, which almost never happens. But a really good match may tell you which molecule you do have. This process is possible now with large computer systems using large databases of spectral data and sophisticated indexing and searching software. It still may only narrow down the possibilities to a very few, and not just to one compound. And it has a problem if your sample actually contains a few contaminants that add "peaks" or whatever to confuse the comparison. That's why you never get a perfect match.
A combination of both fundamental approaches is the best strategy. Using the first round of results to narrow the field of possibilities, then choosing how to answer a few remaining questions, always works.
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