At first, IR & NMR can seem very overwhelming with the crazy peaks, but once you get the hang of it you'll find it relatively easy. I'll give you a rundown on how to read these in general, but keep in mind that the MCAT probably won't be as in-depth.
What you need to know for IR:
1. First, you'll have to memorize some typical IR peaks, which you can find here:
http://www.chem.ucla.edu/~webspectra/irtable.html.
Focus on main/important peaks like alcohols, carboxylic acids, & carbonyls (you can group ketones, esters, amides, etc into a big carbonyl group. You won't have to differentiate between those on the MCAT.)
2. Nomenclature:
Broad (b) peak means the peak looks like a big, wide U shape. Sharp (s) peak means it looks like a V or a needle. Those are the main characteristics of peaks.
Don't worry about bending or stretching. That's likely beyond the MCAT.
How to read IR:
1. Peaks below 1300 cm-1 should be ignored. This is the footprint region and is relatively unreliable
2. Look for the most common/obvious peaks:
Alcohols - always around 3300 and crazy broad. Can't miss it. Super common IR peak
Carboxylic acids - center around 3000 and even crazier broad.
Carbonyls - around 1600-1700 and always very sharp and obvious.
3. Look at the multiple choices you have. Eliminate those that didn't have the obvious peaks you spotted. Again, the MCAT won't go very in-depth into IR, so you should be good by just memorizing some big peaks. If you find a practice question, I can show you step by step how to solve it.
What you need to know for NMR:
There's two main types of NMR: Hydrogen (H) NMR and Carbon (C) NMR.
View attachment 177890<---Ethanol
1. CNMR has single, sharp peaks that represent every carbon unless there's symmetry in the molecule. That means if two carbons are mirror images of each other in a molecule, the two peaks will show as one. In the case of ethanol, you would have two carbon peaks.
2. HNMR has splitting. This is indicative of how many hydrogens are next the the hydrogen you're looking at The formula for how many peaks it splits into is (1 + n). "n" is the number of hydrogens next to the hydrogen you're looking at.
For instance, there are 2 hydrogens on C2, which is directly adjacent to C1. This means C1 will have (1 + 2) = 3 split peaks. This makes it a "triplet."
Hydrogens are ONLY split by hydrogens on carbons. So in ethanol, C2 would NOT be split by the hydrogen on Oxygen. Therefore it would only be split by the 3 hydrogens on C1, making it (1 + 3) = 4 --> a quartet. Also, hydrogens on non-carbon atoms will not show any splitting. So the hydrogen on oxygen here will be a singlet.
Because of its splitting, HNMR is hugely useful for finding out a structure.
View attachment 177893
NMR
3. Where a peak appears along the x-axis depends on how "deshielded" that carbon or hydrogen is. Deshielded means the electrons have been stripped from hydrogens, due to the presence of an electronegative atom nearby. Below is an HNMR of ethanol. Oxygen is very electronegative, so this "deshields" C2, the carbon adjacent to it. However, it leaves C1 fairly untouched because it is much farther away from oxygen. Thus below, you see that C2 has a higher numerical shift than C1.
View attachment 177892<---H
From here, you just need to try more and more practice problems and carefully study the solutions to understand the reasoning for problem solving.
Again -- if you have any specific examples to show me, I'd be happy to work through it step by step.
Let me know if anything's is unclear or if you have more specific questions!
🙂