Nmr & ir

[enTropeeeeeeeee]

I'm reading NMR & IR via book,wiki, etc...However, I can't seem to understand the theory behind why things work. I want to really understand why things shift up/down field and why corresponding peaks form.

Frustrated atm, so I definitely appreciate any help!

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[tttgo]

haven't learned IR spec, but we did nmr in ochem1 last semester. For proton nmr, the different peaks are caused by hydrogens in different environments. You start by looking at the positions of all the H's and either number or letter each one that has a different environment. http://www.chem.ucalgary.ca/courses/351/Carey/Ch13/ch13-nmr-5.html
The c2cl2 here has H's in 2 environments, so 2 peaks appear. Then, there's spin coupling. This is due to H's from dif environments that are 3 bonds or less (maybe 4? don't remember) away. The H in green is a quartet because it is coupled to 3 H's that are close by. Look up the N+1 rule. The ones in blue shouw up as a doublet because of the green one (1+1=doublet). The upfield/downfield shift is affected by what electronegative atoms they're next to. I think being close to an F would shift that H more to the left because it's electrons are shielded. F also deshields more than Br because of electronegativity. There's more complex coupling like doublet or quartets, but I'm not sure if that's covered on the mcat. You can also try freelanceteach's video's, since the guy expains it pretty well. I think he only does nmr and not ir.

 

[babystepsCA]

I'm reading NMR & IR via book,wiki, etc...However, I can't seem to understand the theory behind why things work. I want to really understand why things shift up/down field and why corresponding peaks form.

Frustrated atm, so I definitely appreciate any help!

For IR, it basically measures the vibration pattern of the bonds in a molecule.
You probably still remember in physics they talked about the vibration of a spring. The vibration of a bond is somewhat like that. And if you remember that the vibration frequency is proportional to (k/m)^(1/2), where k represents the strength of the spring and m is the mass, you can understand IR in a similar way. In fact, if the numbers are given, you can calculate the IR spectrum from them, and that is exactly what those softwares do.
Let's come back to MCAT. For example, let's pretend someone asks in which order is the frequency of the stretch vibration mode of C#C, C=C, and C-C? So, obviously m is the same, triple bond (#) is the strongest in terms of bond energy, followed by double bound, therefore, the frequency (or equivalently wavenumber) is C#C > C=C > C-C, which is exactly the case in IR. Another example, C=O, C=N, C=C, well they are quite similar in the strength of the double bond, but in terms of mass, O>N>C. So in IR, C=O has higher wavenumbers, and C=C has lower ones, just as you may have seen on wiki. You may ask why there are overlaps on the wiki figure? Very good question, because the bond strength will be affected by the substituents on the atoms. if you have, for example, a conjugated double bond, attached to the C=O, that will cause the C=O double bond less strong as in isolated ones, because of delocalization of the pi electrons, and therefore, leading to a shift of the C=O stretch toward the lower wavelength side. Such effect can also be seen with an electronegative atom attached to the carbonyl, that is why the C=O stretch in ketone is higher than in ester, amide, etc. Similarly you can expect the C=O stretch can be affected by hydrogen bond formation of the carbonyl oxygen. So if you have a benzaldehyde, and a o-hydroxyl benzaldehyde, which one would you expect having a higher C=O stretch wavenumber in IR? I guess you already know by now. Same reason, hydrogen bonding with solvent, or coordination with metal, or anything that may affect the bond strength, may have impact on the IR, causing the peak shift, larger or smaller.

Regarding NMR, tttgo gave a fair explanation, especially the link within that post is worth reading. The key word would be the shielding, which causes the different chemical environment of each and every proton. Let me know if you need further help.

Good luck and have fun!

 

[enTropeeeeeeeee]

I thought of a better way to ask this question. In H-NMR, things that are deshielded produces a shift downfield (left). What does deshielding mean?

The axis always says ppm, but parts per million of what?

 

[ModernAlchemist]

I thought of a better way to ask this question. In H-NMR, things that are deshielded produces a shift downfield (left). What does deshielding mean?

The axis always says ppm, but parts per million of what?

Here's some helpful notes I took from a lot of different sources. Hope they help!:

"Nuclei with odd atomic numbers exhibit nuclear spin, which creates a magnetic field around the nucleus, much like a small magnet would create. When these small magnets are placed in an external magnetic field, they must orient their magnetic fields with or against the external magnetic field. "With" means that the north pole of the nucleus points towards the south pole of the external magnet. The nucei oriented with the external magnetic field are in a lower energy position than those oriented against the field. The stronger the magnetic field, the greater the energy difference between these states. If a photon having an energy that exactly matches that energy difference should strike a low energy nucleus, the nucleus is inverted to the high energy state. Such a nucleus is said to be in resonance. An NMR spectrometer uses a constant frequency photon source and changes the magnetic field until resonance is detected.

If there were no electrons around the nucleus, all protons would exhibit resonance at the same field strength, but electrons shield protons from the external magnetic field. Protons that are shielded from the external magnetic field by electrons require a stronger external field in order to exhibit resonance. Electron withdrawing groups deshield protons allowing them to experience resonance at a lower magnetic field strength. On the NMR graph, the larger numbers (increasing to the left), indicate a larger magnetic field strength, so to the left is called downfield."

So, deshielding means that the electron withdrawing groups are taking away electron density from that hydrogen. This reduces the field experienced at the nucleus and decreases the freqency required for the absorption.

Parts per million, as I understand it in this instance (not a physicist haha), is a measure of the difference in frequency between the reference frequency and the frequency of the signal that signifies the chemical shift, which is actually supposed to be dimensionless. It's expressed in parts per million because that is typically the order of the shift (10Hz-->10MHz).