IR Spectroscopy

[Mister Significant]

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Hello all,

Can two different compounds share the same IR Spectra? I got docked down a couple points in my General Chemistry lab for putting that they could. I was thinking along the lines of how Benzene vs. Napthla would share the same aromatic C-H bends, or how since alkanes of differing size would all share the same C-H(sp3) vibrations.

 

[TheBiologist]

the fact that you think benzene and naphthalene would have the same IR spec shows you might need to review what the molecular basis of the spectrum is; that is, what is it that causes the spectrum you're reading. knowing this you should know only 2 identical compounds could share the EXACT same spectrum

it sounds like you are saying that SOME characteristics might be shared among molecules. but the exact same spec could not be

 

[Mister Significant]

the fact that you think benzene and naphthalene would have the same IR spec shows you might need to review what the molecular basis of the spectrum is; that is, what is it that causes the spectrum you're reading. knowing this you should know only 2 identical compounds could share the EXACT same spectrum

it sounds like you are saying that SOME characteristics might be shared among molecules. but the exact same spec could not be

I'm looking at both Spectra right now and only difference is in the intensity of the aromatic C-H peaks. I was taught it measures the bond vibrations (O-H, C-H). In Orgo lab we use it just to check functional groups, was taught that anything lower than 1500 cm-1 (footprint reigion) was not reliable. I don't believe its very practical to use an IR to distinguish a hexane from a heptane.

 

[TheBiologist]

I'm looking at both Spectra right now and only difference is in the intensity of the aromatic C-H peaks. I was taught it measures the bond vibrations (O-H, C-H). In Orgo lab we use it just to check functional groups, was taught that anything lower than 1500 cm-1 (footprint reigion) was not reliable. I don't believe its very practical to use an IR to distinguish a hexane from a heptane.

practical no but in theory they are different

btw, did u ask the professor about this?

 

[dr_kateb]

I'm looking at both Spectra right now and only difference is in the intensity of the aromatic C-H peaks. I was taught it measures the bond vibrations (O-H, C-H). In Orgo lab we use it just to check functional groups, was taught that anything lower than 1500 cm-1 (footprint reigion) was not reliable. I don't believe its very practical to use an IR to distinguish a hexane from a heptane.

It’s not. You’re gonna learn Nuclear Magnetic Resonance spectroscopy (NMR) later on. Spoiler, it’s a pain in the ass. NMR will tell you the orientation of the functional groups, IR just tells you which are present.

 

[aldol16]

It’s not. You’re gonna learn Nuclear Magnetic Resonance spectroscopy (NMR) later on. Spoiler, it’s a pain in the ass. NMR will tell you the orientation of the functional groups, IR just tells you which are present.

NMR won't distinguish hexane from heptane well since you're gonna get a lot of aliphatic absorptions (some of which can be from plasticizer contaminants, etc.). Straight proton NMR also likely won't tell you "orientation." If you run a COSY or a NOESY experiment, then you can tell orientation quite well.

 

[aldol16]

There's a region of the IR spectrum known as the "fingerprint region." That will be the "fingerprint" of unique molecule. Also, the other absorptions can be similar but not identical across molecules. This is because if you get into the quantum mechanics of it all, you will see that IR stretch is determined by the force constant of each bond and each bond will have a different force constant.

 

[Magus5454]

NMR won't distinguish hexane from heptane well since you're gonna get a lot of aliphatic absorptions (some of which can be from plasticizer contaminants, etc.). Straight proton NMR also likely won't tell you "orientation." If you run a COSY or a NOESY experiment, then you can tell orientation quite well.

I disagree with this. Proton would show 4 equivalent for heptane and 3 equivalent for hexane.

 

[aldol16]

I disagree with this. Proton would show 4 equivalent for heptane and 3 equivalent for hexane.

I'm not saying that their spectra won't be different. I'm saying that practically, it will be hard to distinguish because the aliphatics will all appear in upfield regions, where a lot of the minor contaminants come in (e.g. plasticizers). Once you do NMR for a few years, you'll understand that the aliphatic region is insanely difficult to distinguish in terms of proton coupling. Usually we just look at the downfield shifts, the key shifts that we would expect from our molecule, and then as long as the integrations for the aliphatics come out right, we don't worry about them too much. We also tend not to meticulously assign signals in the aliphatic region to specific protons because each molecule will generally have a lot of aliphatics that will be hard to distinguish without NOESY or COSY spectra.

This is of course advanced NMR techniques that would not be accessible to undergrads and so we generally tend to simplify things in courses. Just don't be surprised when your post-doc or PI tell you to not focus on the aliphatic region when you're interpreting an NMR and instead do other things to confirm like high-res mass spec.

 

[Magus5454]

And this is a forum about the MCAT, undergrad organic is all that is expected. 🙂 my professor actually went beyond what I've seen the normal organic class get into. I also did a lab for pchem about Raman spectroscopy, which I thoroughly enjoyed writing as you were able to get into stretch and bending of bonds

 

[aldol16]

And this is a forum about the MCAT, undergrad organic is all that is expected. 🙂 my professor actually went beyond what I've seen the normal organic class get into. I also did a lab for pchem about Raman spectroscopy, which I thoroughly enjoyed writing as you were able to get into stretch and bending of bonds

Sure, and I'm saying that the people who are writing this aren't going to show you a spectrum of heptane and make you assign all the protons. That's not how it works in real life and that's not how it works on the MCAT. The people who write the MCAT are experts in the field who are going to be more interested in whether you know what the spectrum of chloroform looks like compared to methane and why that is so. IR is very similar to Raman in that it measures bond stretching - in fact, we often use IR and Raman in complementary ways because certain modes you see on IR you don't on Raman and vice versa so the mere existence of a spectrum can be diagnostic in some cases.

Very few of us chemists use IR anymore nowadays except those into really hard-core PChem stuff. Nowadays NMR technology is sophisticated enough to diagnose the vast majority of compounds. One notable exception would be gaseous compounds or those that are difficult to force into solution phase. Still, IR is generally considered by chemists as not very useful in most instances. The standard for organic chemistry reporting is generally NMR + high-res mass spec.

 

[Magus5454]

And i get that, but most undergrad courses in organic still teach IR. And i know the difference between Raman and IR. I took my first organic classes at a CC so the most we had was a GC and an IR for lab, no nmr/mass spec/ etc. Which is funny as im sitting at work next to two ICPs and other gadgets

 

[aldol16]

And i get that, but most undergrad courses in organic still teach IR. And i know the difference between Raman and IR. I took my first organic classes at a CC so the most we had was a GC and an IR for lab, no nmr/mass spec/ etc. Which is funny as im sitting at work next to two ICPs and other gadgets

Yes, we still teach IR because it's a good way for students to think about quantum mechanics at a practical level. We don't teach it because it's practically useful. Whenever an undergrad came into my lab, we had to help them unlearn many of the things they learned in chemistry class because it's just not practical in real research. So it all boils down to a simple thing - do you want to be the guy who knows everything in the book or do you want to be the guy who can apply it all? Two different skill sets.

At the university where I taught, we still do IR labs but we heavily emphasize NMR in the curriculum because that's what's most important in the future. Even for pre-meds, NMR is just the same thing as MRI. Very relevant for whatever you go into. Especially now with these special NMR techniques, you can really get the fine structural details from the NMR.

 

[Magus5454]

Yes, we still teach IR because it's a good way for students to think about quantum mechanics at a practical level. We don't teach it because it's practically useful. Whenever an undergrad came into my lab, we had to help them unlearn many of the things they learned in chemistry class because it's just not practical in real research. So it all boils down to a simple thing - do you want to be the guy who knows everything in the book or do you want to be the guy who can apply it all? Two different skill sets.

At the university where I taught, we still do IR labs but we heavily emphasize NMR in the curriculum because that's what's most important in the future. Even for pre-meds, NMR is just the same thing as MRI. Very relevant for whatever you go into. Especially now with these special NMR techniques, you can really get the fine structural details from the NMR.

Right, we learned NMR, and why things happened, we just didn't have access to one.

 

[deleted804295]

Since this is in the MCAT section I wanted to revive it. I'm currently doing content review for MCAT and I'm working on IR and UV.

Do we have to memorize IR and UV peaks???

This never happened in class. We had a chart for every exam and every lab.

I've memorized the simple ones like carbonlys and OH but one of my practice questions while studying for the MCAT had an amine peak I had to list???

 

[Magus5454]

Since this is in the MCAT section I wanted to revive it. I'm currently doing content review for MCAT and I'm working on IR and UV.

Do we have to memorize IR and UV peaks???

This never happened in class. We had a chart for every exam and every lab.

I've memorized the simple ones like carbonlys and OH but one of my practice questions while studying for the MCAT had an amine peak I had to list???

Which amine peak? The N-H near 3k?