Queston involving spectroscopy?

[Lan(r)12]

Why are IR and NMR always used together to determine a molecule’s structure. Why can NMR not be used alone to determine structure?

 

I know that nuclear magnetic resonance tells us what the carbon "skeleton", if you will, looks like and what types/locations of hydrogens there are...isnt that enough?

As far as I can tell, infrared spectroscopy only reveals the presence/absence of primary functional groups...why is this important...well I know why functional groups are important, but couldnt we just get by with NMR?

 

Is this situation akin to NMR being the drawing in balck and white, and IR being the color that makes it look better? lol

 

To me, IR seems superflous at best...but what do I know, thats why Im asking you all

Edited February 13, 2009 by Lan(r)12

[Kaeroll]

Hi there

 

NMR is indeed much more useful, but does not negate IR. IR vibration frequencies can tell us about, inter alia, bond strengths. These can be used to 'fingerprint' the kinds of (for example) carbonyl compound you have - is it an aldehyde or ketone? Amide, anhydride? etc. If your spectrum is of good quality, you can also use the aptly-named 'fingerprint' region to identify many compounds.

 

A very common application is to monitor growth or decay of peaks during a reaction. Recently my group carried out several LiAlH4 reductions. As the carbonyl peak is so strong, even minute amounts of remaining acid would show up on the FT-IR spectrum, which is much quicker and more convenient to run on-the-fly than an NMR spectrum.

 

Like IR, UV-vis spectroscopy is often sidelined; yet all I've done for the past 6 hours is monitor an oxidation reaction using UV-vis (recording the decay in a peak at 350 nm arising from CrO3).

 

I believe that IR also finds use in studies of stars and the like; by recording their IR spectra, organic molecules in clouds of gas in space can be found, for example. If you can find a way to run an NMR spectrum of a cloud of gas several lightyears away, I'll buy you a pint!

 

Hope this helps.

Kaeroll

[Lan(r)12]

...hmmm...

I see what you are saying...but it still seems like an afterthought...I guess thats why they are run together then, eh? lol

[DrDNA]

I know that nuclear magnetic resonance tells us what the carbon "skeleton", if you will, looks like and what types/locations of hydrogens there are...isnt that enough?

Generally no and you might be mixing 2D proton NMR with 2D carbon NMR (or maybe not?).

If you are referring specifically and only to 2D proton NMR, the information about the CARBON skeleton is limited.

Also please note that there are other types of NMR besides proton.

There is also Carbon, Nitrogen, Phosphorous, Boron (the ones I have used)........and many many others.

Check out: http://www-lcs.ensicaen.fr/pyPulsar/index.php/List_of_NMR_isotopes

for a complete list.

 

Also, there is 3D NMR (eg COSY, NOESY, etc) using at least two types of nuclei, which gives a LOT more information about molecular structure than 2D NMR.

 

As far as I can tell, infrared spectroscopy only reveals the presence/absence of primary functional groups...why is this important...well I know why functional groups are important, but couldnt we just get by with NMR?

 

Is this situation akin to NMR being the drawing in balck and white, and IR being the color that makes it look better? lol

 

To me, IR seems superflous at best...but what do I know, thats why Im asking you all

 

The methods are COMPLIMENTARY.

Which may be what you meant by color and black and white; or not?

 

In any event, you may be somewhat correct in that IR often gives less information than proton NMR, but IR can give information about specific functional groups; the identities of which that may be lacking or ambiguous in proton NMR alone, and it can be used to confirm other information.

But the level of complexity, cost and effort is also usually greater with most types of NMR.

The point is, I generally perform IR first, and then 2D proton NMR.

If it is a unique compound of unknown structure, and especially if it is difficult or impossible to crystallize, then 3D NMR is VERY helpful (and generally easier and less expensive that crystallization); for example a minuscule amount of a purified natural product.

I generally perform IR, then 2D proton NMR then 2D carbon NMR, in that order.

NOT just IR OR NMR. As I said above, they are complementary.

 

Plus, what Kaeroll said....

Edited February 13, 2009 by DrDNA

[Kaeroll]

Hi Lan®,

...hmmm...

I see what you are saying...but it still seems like an afterthought...I guess thats why they are run together then, eh? lol

Well, IR was around long before NMR (a comparatively young technique). NMR is more powerful, yes, but IR does have its niche (several of them). In my experience it does tend to be an 'afterthought' in characterisation - for a typical organic compound, my characterisation thought process tends to be along the lines of, "The proton NMR is good, and all the right peaks are there on my IR." So I do see your point. Though you can never have too much information to hand!

 

DrDNA - good points, though I guess to an extent a proton NMR (for a simple hydrocarbon) does reveal the carbon skeleton. I'm yet to run a 2D NMR, though I'm sure the day will come...

 

Kaeroll

[DrDNA]

Yes.

For very 'simple' hydrocarbons.

But outside of an undergraduate lab course, I suppose that one rarely has a need or a desire to do proton NMR on very 'simple' hydrocarbons.