Memorizing NMR/IR shifts: helpful tricks?

[Little Etoile]

Good god, there are so many (30 by my count). How in the world do you memorize them all? Any ticks, hints, suggestions? Trends that we can use?

It just seems like a ridiculous and trivial thing to have us memorize. It would be like memorizing the boiling points of substances. Why have us do one and not the other? 😛

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[Libtech]

Memorize only the Aldehyde/Alcohol/Ketone/Alkyl and maybe the Amine. (Standard functional groups in each category)

Personally I memorize the aldehyde + alcohol + alkyl and never had to use more than that on any of the practice tests or the real thing.

 

[Dirt]

Yeah for IR I only memorized alcohols and carbonyls.

For NMR, I think it is more helpful to know why things show up like they do, i.e. what groups cause greater shielding and such. If you are given a compound, say isopropyl alcohol, you should be able to put the peaks in the proper order, but I don't think you need to know what the actual chemical shifts are.

 

[copperfrog09]

Helpful Tricks? don't bother. 1700 is a carbonyl and 3500 is an alcohol. You're set!

 

[cateyezz]

I agree with copper...i am an orgo tutor and i felt like my whole head worth of orgo was only about 1/3 (not even) used on the exam.....the only thing i saw was stereochemistry and IUPAC naming....but yea i would just bother with the carbonyl and alcohol peaks in the IR and for the H-NMR, just know the multipilicity...i mean how to read it...good luck!

 

[tardyturtle]

IR:
Broad peak around 3000 - OH
Weak to medium sharp peak above 3000 - NH
Pair of weak to medium sharp peaks above 3000 - NH2
Other crap around 3000, especially medium-strong - CH
1715 - Carbonyl
1650 - Alkene
Strong at 1100 - C-O (often difficult to tell with all of the other peaks here)

HNMR
12+ - Carboxylic Acid
10-12 - Aldehyde
7-8 - Aromatics
1-2 - Alkyl
Everything else (alcohol, amine, adjacent to oxygen or halogen) I assign by default using splitting and integration.

CNMR
180+ - Aldehydes
120-150ish - Aromatics
100-150 - C=C
90-100 - CtriplebondC
40-60ish - Attached to O, N, Halogen
The CNMR numbers I just thought up of... they're just rules of thumb.

I also keep in mind that a carbonyl or electronegative atom adjacent shifts the peak downfield with NMR (to a higher chemical shift; to the left).


Honestly though, there's a little room for play in these numbers, so just get the general idea about where things are. As long as you don't mistake a broad OH peak in an IR for carbonyl, or look for aromatics at 4 ppm on the NMR, you should be fine.