Concept: Concept: General NMR Features11m
Now we're going to discuss a very powerful analytical tool called nuclear magnetic resonance or NMR. While there's many different types of NMRs that we could learn, we're going to start off with the one that's most important for this course and that's called proton NMR.
Proton NMR is an instrumental method that is going to allow us to identify and distinguish protons in slightly different electronic environments. We're going to use magnetic fields to actually generate magnetic fields around these atoms and see what the strength of the magnetic fields are that we get back from them.
I'm actually going to leave the entire scientific explanation of how nuclear magnetic resonance works up to your professor or up to YouTube because you could definitely spend a good 10 – 15 minutes learning all about that and that's not really the most important part. What I'm going to really focus on is how to read it, how to understand it and what you need to know to pass your exam. That's what we do here.
Let's go ahead and jump right into what the spectrum looks like. As you see this spectrum has units that you have to get familiar with. It has shapes you have to get familiar with. Let's just talk about the basics in terms of navigating the spectra.
Well, first of all, we're going to see that on the x-axis, we have this unit of ppm. Ppm stands for parts per million and it's really just an arbitrary unit of measurement that we use for this. Just some scientist decided that he wanted to measure the magnetic response of these atoms or these nuclei through parts per million so that's what we go with. Notice that it starts at zero and it usually goes up to – here I have it about to 11. It actually usually ends around 13. 13 – 14, somewhere around there, you're going to get your entire spectrum.
Now notice that we have these words at the top that are kind of our navigation words. It's important that you are able to associate these. First of all, we have the words downfield and upfield. Notice that upfield is close to zero and downfield is close to the high number 13. Now this might make sense while I'm telling you right now, but you'd be surprised once you shut off this video how confusing that could be. It's like up-field is that towards the bigger number or the smaller number. We have to figure out a way to memorize that because I want you to make sure that you have that for the rest of your life, kind of a life-changing definition.
Let's talk about something else which is shielded and deshielded. Shielded is a word that we're going to discuss in a second. Deshielded would be the opposite of that word. I'm going to define what it means to be shielded in a second, but notice that downfield and deshielded happen to be the same direction of the spectrum. If you say something is downfield, that's the same as saying it's deshielded. If you say it's upfield, that's the same thing as saying it's shielded.
The way I like to remember this is – I'm sorry, I'm a guy, double D's. You got your downfield, you've got your deshielded. Super inappropriate and it worked. Now you're going to remember that. Now you know kind of if you can remember which direction they go, then you're fine. You've got everything else.
Let's go ahead and talk about just some general features of the NMR spectrum. First of all, there actually is no natural zero for NMR spectrum. The zero is basically going to be a molecule that is our test molecule that we run all of the other molecules against. That test molecule, the one that we use as our zero, is going to be called TMS. That stands for tetramethylsilane. It's basically a silicon molecule that has four methyl groups around it. That happens to be – remember that we said zero is around shielded. That happens to be an extremely shielded molecule. We'll discuss what that means.
Basically, electrons are what shield protons from the effects of NMR. Basically, what that means is that the more electrons that I can have around my hydrogen, the less it's going to experience the magnetic field that I am producing from the NMR. The more electrons, the less it experiences it, the more shielded it is. The more stripped of electrons it is, the less electrons it has, the more it's going to feel that magnetic field and it's going to actually result as a consequence of that and the more downfield or deshielded it's going to be.
One way you can think of it is that the further downfield your proton is, the more – getting man, super inappropriate – the more naked the proton is. You're going in the direction of your double D's, now you're naked. I don't even know where this is going. I didn't even plan that, by the way.
But basically, what I'm trying to say here is imagine that you're going out into the middle of Michigan winter or something like that. It's bitter cold. You have a big wool coat. You're not going to really feel the cold that much. That would be kind of the idea of being shielded. You're not going to feel that cold, so you would kind of show up in the shielded area, kind of like the tetramethylsilane. It's very shielded. It's got all these methyl groups. But if all of the sudden I were to take off that big, wool coat and all I have is boxers on – I've given you a great visual now – I'm going to feel super cold and I'm going to result much more downfield or deshielded.
In general, the less electrons I have around me or the thinner my coat is, the more downfield I'm going to result.
Now we understand kind of the shifting of the right to left idea here., but it turns out that there's a lot more information that we can gain than just that. There's actually four different types of information that we get from a proton NMR spectra. We're going to introduce what those four types of information are.
The first type of information that we're going to be able to learn is we're going to learn the number of signals, the total number of signals. The total number of signals is going to describe how many different types of hydrogens are present. Because, basically, let's say that you have three different peaks or let's look at our example here. This one has four different peaks. Notice that it has peak A, peak B, peak C, and peak D, or signal A through D. All of these protons are wearing different layers of clothing. You could imagine that proton A is the guy with the big heavy jacket and proton D is the one that he's in the process of stripping down and the other guys are somewhere in between.
What that's telling us is we have four distinct types of protons on this molecule. Just by telling me that, that's already a lot of information. It tells me how many different types of hydrogens or protons are present.
Then we've got the chemical shift. The chemical shift is the actual part that's the parts per million. That's going to tell me how shielded or deshielded the actual protons are. So now I'm going to know that there's four different types of protons and this one has a shift of four, this one has a shift of two, that's going to tell me kind of what kind of functional groups they're attached to. See where we're going with this? It's already telling me a lot of information.
But there's more than that. We're also going to be able to use the height of the signal. It turns out that on this sample NMR that I showed you, notice that some of them are really short and some of them are really tall? That's actually for a reason. It turns out that the ones that are taller, the ones that take up more area under that curve, that's going to represent that there's actually more of that type of hydrogen.
Just for example, taking A versus B, notice that I have A and B next to each other, I could safely say that there's more of the coated hydrogens, the wool coat, than the ones that are B, the ones that have slightly less clothing on or are slightly more deshielded because there seems to be a lot more area underneath that curve than there is below B. Get what I'm saying?
It turns out that we call that the relative ratio of hydrogen. Basically, it tells us that we have more of one type of hydrogen and less of another. Again, that's a lot of information.
But finally, we have spin splitting or what's called multiplicity. Spin splitting is that final piece of information that really just helps us determine the structure of a molecule and what it does is it describes to us how close or how far the different molecules are from each other.
Now spin splitting is represented by the fact instead of just being one peak, notice that a lot of these have multiple little peaks in them. For example, D, the guy that's most stripped down, appears to have four little tiny peaks in it. Those are what we call the multiplicity or the splits. That's going to tell us what type of hydrogens he's next to.
Tons of information here. What we're going to do now is we're going to spend lots of time going through every single piece of this so that you really understand. We're going to do every single piece of this in depth so that you can take one of these NMR spectra and understand perfectly what's going on. At least that's the goal. Let's go ahead and move on to the next topic.
The carbon NMR chemical shifts of diethylmethylamine, piperidine, propan-1-ol, and propanal follow. Determine which spectrum corresponds to each structure, and show which carbon atom(s) are responsible for each absorption.
(a) 25.9, 27.8, 47.9
(b) 12.4, 41.0, 51.1
(c) 7.9, 44.7, 201.9
(d) 10.0, 25.8, 63.6
Where would you expect to find the 'H NMR signal of (CH3)2Mg relative to the TMS signal? (Hint: Magnesium is even less electronegative than silicon.)
Each signal produced in an NMR spectra has 3 variables. Those variables are ___ .
A) Signal strength, signal splitting and signal height
B) Signal strength, signal shift and signal height
C) Signal shift, signal splitting and signal strength
D) Signal shift, signal splitting and signal integration
Which of the following statements regarding NMR spectroscopy is wrong?
a) NMR signals towards the left of the spectral chart correspond to larger chemical shifts.
b) Chemical shifts are larger when the frequencies of the radiation which induces the nuclear transitions are higher.
c) Chemical shifts are larger when shielding effects are greater.
d) A hydrogen signal splits into n+1 peaks by spin-spin coupling when the number of equivalent hydrogen atoms on adjacent atom(s) is n, and no other neighbouring atoms are involved.
NMR stands for ____________________ ____________________ ____________________ .
The following questions pertain to the charting of NMR spectra. MATCH a term to each description below. Place the letter of the term in the blank to the right of the description.
b. high-field or upfield side
d. delta δ
e. low-field or downfield side
f. chemical shift
g. specific absorption
i. When looking at an NMR chart the right-hand part of the chart is the ____.
ii. The exact place on the chart at which a nucleus absorbs is called its ____.
iii. The calibration standard for 1H and 13C NMR is:
iv. The NMR charts are calibrated using an arbitrary scale that is divided into ____ units.
Which of the following is true about the location of signals in a 1H NMR spectrum?
a) It indicates the number of protons in the signal.
b) It indicates the electronic environment of neighboring protons.
c) It indicates the number of different protons.
d) It indicates the electronic environment of absorbing protons.
e) It indicates the number of neighboring protons.
The coupling constant (J) for bromoethane is 7 Hz when the 1H NMR spectrum is acquired at 250 MHz. What is the coupling constant between these protons when the spectrum is acquired at 500 MHz?
a) 14 Hz
b) 7 Hz
c) 3.5 Hz
d) 21 Hz
e) none of these
Which of the following is not a common solvent used for acquiring a 1H NMR spectrum?
Which aspect of 1H NMR spectrum gives information about other nearby protons?
a. number of different signals
b. degree of shielding
c. intensities of peaks
d. splitting of signals
e. degree of deshielding