H NMR Table - Online Tutor, Practice Problems & Exam Prep

1H NMR Chemical Shifts

Now that we know how to solve for the total number of signals in a proton NMR, it's time to move on to our second piece of information. That's going to be the chemical shifts of each signal. The chemical shift indicates the exact electrochemical generalization we can make. What we say is that electronegative groups, remember electronegative, just something that likes to pull electrons away, will pull the electrons away from the nuclei deshielding them, making them more naked so they're going to experience the NMR more. The shifts, the chemical shifts are going to increase or move downfield. The number will get bigger as the protons become more deshielded. Now before we get into some very specific shifts that you need to know, we're just going to look at a general trend. As a general trend, what you're going to see is that as our functional groups get more and more electronegative, the bigger the numbers get. Now remember, all of this is in reference to TMS, which is our reference molecule that has by definition a shift of 0 because that is our reference point. Now we're seeing all of these things in order, they get more and more deshielded as you go down the list. What we see is that alkanes, so this is just an alkane, are actually the most shielded organic molecules because there's no electronegative things pulling electrons away. It's very, very shielded. C's are going to shift at the lowest number around 1 to 2. Then we have triple bonds. Triple bonds are going to come next. That's alkynes. Alkynes are going to come next at about 2.5, still relatively shielded. Then we have this very broad group called ZCH. Now what does Z stand for? Z is just going to be something electronegative that's next to it. Z could stand for an atom. It could stand for a group that is electronegative. I'm just going to be putting EN for something electronegative. Now this has a range anywhere from 2 to 4 because it really depends on what that Z is. If it's something super electronegative like fluorine, then it's going to be closer to 4. If it's something less electronegative, then a little bit less. Then we've got alcohol and amine. Now these have a very broad range of 1 to 5. There's such a broad range, in fact, that I consider these useless in terms of chemical shift because it's so variable that the difference between 1 through 5 is really huge on NMR. Remember I told you that NMR really only goes like 13? So 1 through 5 is like a third of the entire spectrum. That being said, it's not very helpful if I have a peak at 2 or if I have a peak at 5. It doesn't tell me much because I don't really know what it is. I'm really just going to not even focus on those shifts because they're pointless because they're so wide. They don't really give us a lot of information. Then we have alkenes which result higher than the 2 other hydrocarbons at 4.5 to 6. Now I'm not going to be able to get into the theory of why alkenes are higher than alkynes, but just know that it actually has to do with a very technical explanation dealing with molecular orbitals and the shapes of the molecular orbitals around the double bond will actually deshield the hydrogens more than the shapes of the orbitals on a triple bond. So that's why. Okay? If you're really interested, Wikipedia it, right? So Benzene. Benzenes are getting more deshielded. If you think about it, Benzene has a lot of electrons inside. It's got those 3 triple bonds, the Clutch logo. Right? So there's a lot of electrons in there. It's pulling electrons kinda towards itself. So benzene's going to be higher, 6 to 8. And now this is where things start to get really deshielded. Aldehyde and carboxylic acid. One thing they have in common is that both of those H's, I'm just going to draw the carboxylic acid, they're next to carbonyls. Now remember, a carbonyl has a very strong dipole pulling away from it. On top of that, carboxylic acid has another strong dipole with the oxygen pulling away from it. All that's to say that well, I didn't actually mean to draw it exactly there. I meant to draw it here that the oxygen is pulling electrons away from the H. All that is to say that carboxylic acids and aldehydes are going to have the most deshielded or the most naked protons because you can imagine that basically this hydrogen here doesn't have any electrons around it because all the electrons are getting sucked up by the carbonyl, sucked up by the oxygen. So that thing is like butt naked. And that's going to our highest peak possible is carboxylic acid at 13. Now the specific chemical shifts that you need to know for your class, for your specific classroom is really going to be up to your professor. There's absolutely no way for me to know on my end exactly which shifts your professor thinks is important and which ones they say you don't need to worry about. In fact, double check. Your professor might even give you a sheet on your exam that has all these shifts already written out for you. So you don't have to memorize them at all. Okay? But as always, I'm going to go over them just in case. Okay? And honestly, even if your professor told you don't need to memorize these shifts, I still would recommend watching this part of the video because this can help you a lot when we do structure determination later on in this course. Let's go ahead and just go through some more specific ranges. First of all, we're just going to start at the bottom. Remember that our bottom is TMS. Now we talked about alkanes being in the 1 to 2 range. It turns out that alkanes actually follow a pattern that the more substituted the alkane, the more R groups it has, the higher it's going to result. That means that your professor may want you to be able to tell the difference between a primary alkane and a tertiary. Typically a tertiary one could come out like at 1.8 whereas a primary one might come out like at 1.1. See? It's like little differences here and there. And then a secondary would be somewhere in the middle like 1.4. All of these ranges are definitely variable. It doesn't have to be exactly that. But I'm just trying to illustrate how as you move towards tertiary, you actually get a little bit more deshielded. Now we move on to our ZCH range in which we said that it's anywhere between 2.4 and it really depends on what Z is. Well, that's actually really important. We have to learn some specific Zs here and I want you to know these. First of all, the most electronegative things that we can put next to a hydrogen are either fluorine or oxygen. Both of these guys are actually going to result in a peak of just about 4. If you ever see like an OCH that's common like an ether, that usually results right around 4. 3.94, etc. The next one I want you to be aware of is kind of the middle ground which is nitrogen and iodine. Now if you think about it, think about your periodic table. In terms of electronegativity, it goes FCL br I. Your electronegativity goes up as you get closer to fluorine. That means that I would expect that iodine would be the least shifted alkyl halide and that's exactly the case. Iodine and nitrogen come out about 3 with the other alkyl halides as you can see, BR and CL coming out somewhere in between. So we could theorize it's something like 3.2 and 3.5. You know, just something in the middle. Not for you to memorize. Just for you guys to grasp and understand the general picture. Now this part is actually the most important part in terms of I think in terms of your long term knowledge. Some additional Zs that we like to talk about are benzene rings, carbonyls, and allyl groups. These are groups that are going to be able to pull electrons towards themselves. Now I put that they're all roughly around 2. So if you just wanted to remember that they're all about 2, that's fine. But they do kind of go in order. For example, a benzene will typically be around 2.3, whereas a carbonyl will usually be around 2.1 whereas an allyl will usually be about 1.9. So they're all in the range of 2 but there is kind of a hierarchy of benzene and carbonyl being higher than allyl. Just keep in mind that really it depends on what the Z is is going to tell you where it's on this 2 to 4 spectrum. Now going down to the triple bond, triple bond results anywhere from 2.5 to 3. So that whole range is kind of fair game for the triple bond. What do we have for the double bond? These ranges are really just the same as the one I stated before, 4.5 to 6 for a double bond. For a benzene ring, anywhere from 6 to 8. For aldehyde, anywhere from 9 to 10. And for benzene, for carboxylic acid, anywhere from 10 to 13. There you have it. Really all I did in this section was I went more into depth on the Z portion and more into depth on the alkyl version because of the fact that it really kinda depends on the types of Zs you have or the types of alkyl groups you have. Awesome. One last thing, I had the alcohols and amines at the bottom. This just gives you a visual of how large that swath is and how really useless it is. We don't really use this part of the graph. We don't really worry about it because it doesn't tell us much information. Now we're going to do a practice problem. What I want you guys to do is use the definition. It's like an open book exam to look at this molecule and tell me what you think the ranking would be in terms of shielded to deshielded. It says order the following 5 protons in order from most deshielded. So that means highest number 2 most shielded. So that means the lowest number. Go ahead and start at the highest parts per million, the highest chemical shift and then we can go ahead and work our way down to the lowest question.

Ranking Shielded Protons

So first, I'm just going to go over the rankings in order and then I'll go over each individual chemical shift just to reinforce what we just learned. So our highest number, our most deshielded, is going to be H5. Because H5 is on a double bond, it has the highest range as you can see above. It's going to go from H5. Then what's the one with the next highest number? That's actually going to be H2 because H2 is directly attached to a carbon that has fluorine, the most electronegative atom on it. That would be an example of ZCH. So that one's going to come in next. So H5 was a double bond. H2 is ZCH. Okay. So then after that, what I'm going to get is H3 because H3 is on a triple bond and remember that a triple bond results a little bit lower than a fluorine would result, then what we would get is H4 because if you notice, H4 is actually allyl. This would be an example I'm just going to erase this and move it up a little bit. Now allyl is actually on the lower side of Z but it still counts. That's allyl. And then finally, we would end up with H1 at the bottom because it's nothing. All it is just CH. It's just an alkane. Hopefully that made sense so far.

Now let's go over the exact chemical shifts. Now, something that I neglected to mention in the previous example when I showed you guys the spectrum is that chemical shifts are actually denoted by a Greek symbol. And this Greek symbol is the lowercase delta. So, lowercase delta is the same thing as saying parts per million. Okay? So I'm just going to put all of the different parts per million here. What I would expect for H5 is that it's somewhere between 4.5 and 6. We'll just give it a shift of 5. What we would expect for H2 is that it's on the top range of ZCH because there's a Fluorine present, so I'm going to say that one's 4, around 4. Then for H3, since it's on a triple bond, it's somewhere between 2.5 and 3. We'll just give it a chemical shift of 3. Make it easy. Now for H4 because it's allyl, this is going to be around 2. Remember, I said that usually they're a little bit below 2. So you know what? Even though I could make it easy, I could say 2. I'm just going to be a little bit more accurate. I'll say that it's like 1.9 because usually it's right below 2. Okay? And then finally we have H1 which it's secondary, so it's actually probable not going to be 1.0. It's probably going to be a little bit more in the middle, probably something a little bit more like 1.4. Okay? So I got a little bit tricky at the end. Sorry if I'm blocking that a little bit. I'm not trying to trick you guys or anything. I'm just trying to show you guys kind of the way this works, the general trend. And that a lot of times you are going to see little fluctuations with these values. And the biggest point isn't to argue over 3.94. It's to memorize and understand kind of the general idea of what's going on. One more note about these values that I taught you. It might be in your best interest to learn these values because the sheet that you get on your exam in case your professor is just giving you these shifts, right? There are some professors that really don't care for you to memorize. They just give them to you. You might not understand them the same way that you understand them here, on the sheet that you get. Sometimes, it's just better to learn it anyway and then you can go into your exam more confident. So I'll leave that one up to you, guys. But anyway, that being said, let's go ahead and move on to the next topic.

Which of the labeled protons absorbs energy most upfield in the 1H NMR?

Hey guys. Let's take a look at the following practice question. So here it says, "Which of the labeled protons absorbs energy most upfield in the proton NMR?" Remember, we're dealing with two types of ideas. We have upfield, which means that our signal is close to 0. And then here we have downfield, which means our number is closer to 12 or 13. We're accustomed to seeing NMR spectrums usually from 0 to 10. But remember, our carboxylic acid goes beyond that. It's off-field because it's so high. It's usually between 11 and 13. Here, I'm saying from 0 to 13 in terms of our signal. Remember, it'll be in PPM or Delta. Now, how do we know if something is upfield versus downfield? Remember, if you're downfield, that means you are more deshielded. The electrons are not as tightly held near your proton, near your hydrogen as they normally would be. But why is that? That happens because our hydrogen or proton is close to an electronegative group. Who are the electronegative groups? We say that the electronegative groups that we're close to, that deshield us, take away our electrons are phosphorus, oxygen, nitrogen, sulfur, bonds or halogens, or benzene. The closer you are to one of those groups, the more deshielded you are. They strip away your electrons, so when the NMR machine is flipped on, its magnetic field hits your nucleus harder. Remember, your electrons shield your nucleus from the effect of the NMR spectrum on the spectrometer. Here, if you're upfield, then it's the exact opposite. You're more shielded, which means you're farther from these electronegative groups. You're farther from phosphorus, halogens, or benzene. Since you're farther away from them, they're not going to take away your electrons from your protons. Your electrons will be more closely tied toward your nucleus, and they will protect your nucleus from the NMR machine. When I flip it on, I'm not going to be knocked down by the magnetic field. So my signal number will be smaller. Here, we want most upfield. So we want to be the farthest away from these electronegative groups. That means I want to be the farthest away from this oxygen, which is electronegative, and I want to be farther away from this benzene ring. If we take a look, which set of protons are the farthest? The answer would be E. These methyl hydrogens here are the farthest away from the oxygen and the farthest away from the benzene. They would be the most shielded, most upfield, most protected and they would be our answer.

Which of the labeled hydrogens will be most de-shielded? 

• A. A
• B. B
• C. C
• D. D
• E. E

Which compound possesses a hydrogen with the highest chemical shift for its 1H NMR signal? 

• A. A
• B. B
• C. C
• D. D

Here’s what students ask on this topic:

What is the significance of chemical shifts in proton NMR?

Chemical shifts in proton NMR are crucial because they provide information about the electronic environment surrounding the protons in a molecule. Electronegative groups deshield protons, causing downfield shifts (higher ppm values). For example, alkanes are the most shielded (1-2 ppm), while carboxylic acids are the most deshielded (10-13 ppm). Understanding these shifts helps in determining the structure of organic compounds by correlating the degree of deshielding with the electronegativity of adjacent groups.

How do electronegative atoms affect chemical shifts in proton NMR?

Electronegative atoms, such as fluorine or oxygen, pull electron density away from nearby protons, deshielding them. This deshielding effect causes the chemical shifts to move downfield (higher ppm values). For instance, a proton adjacent to a fluorine atom will have a higher chemical shift compared to one adjacent to a less electronegative atom. This trend helps in identifying the presence and position of electronegative groups in a molecule.

What are the typical chemical shift ranges for different functional groups in proton NMR?

Typical chemical shift ranges for various functional groups in proton NMR are as follows: Alkanes (1-2 ppm), Alkynes (2-3 ppm), ZCH (2-4 ppm, where Z is an electronegative group), Alkenes (4.5-6 ppm), Benzene rings (6-8 ppm), Aldehydes (9-10 ppm), and Carboxylic acids (10-13 ppm). These ranges help in identifying the types of functional groups present in a molecule.

Why are alcohol and amine chemical shifts considered less useful in proton NMR?

Alcohol and amine chemical shifts are considered less useful in proton NMR because they have a very broad range (1-5 ppm). This wide range makes it difficult to pinpoint the exact nature of the proton environment, as the shifts overlap significantly with other functional groups. Consequently, these shifts provide limited information for structure determination.

How does the substitution level of alkanes affect their chemical shifts in proton NMR?

The substitution level of alkanes affects their chemical shifts in proton NMR. More substituted alkanes (e.g., tertiary) are more deshielded and thus have higher chemical shifts compared to less substituted ones (e.g., primary). For example, a primary alkane might have a shift around 1.1 ppm, while a tertiary alkane could be around 1.8 ppm. This trend helps in distinguishing between different types of alkane protons.