Solving Nmr Problems

NMR can be a challenging part of organic chemistry.As you evaluate a problem, following a set procedure can help to solve problems systematically. How many neighboring protons do those CH2 protons have?

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For the purpose of this question, I assume you mean solving the structure of a compound based on the NMR shifts, a common problem type.

This is the way I solve these problems, and hopefully it will help.

Now it makes a lot of sense because we calculated an HDI of one indicating their (mumbles) double bond present and we need to account for an oxygen in our molecular formula. So we're saying this signal is due to those two protons.

So now we have accounted for all 10 protons using our integration values. And that's in the region for a proton next to a carbonyl. All right, let's color these protons in here magenta.

Being deshielded because they're next to the carbonyl. So these protons in blue here, these three protons are giving us this signal.

Six minus one is five so we will expect five neighboring protons. And 42.2 divided by 27 is once again pretty close to 1.5. So if we multiply one by two, then we get two protons. 28.4 divided by 27, that's pretty close to one. Electronegative groups move to the down field (left; increase in ppm).Unsaturated groups shift to downfield (left) when affecting nucleus is in the plane of the unsaturation, but reverse shift takes place in the regions above and below this plane.And so if we have five Carbons here, the maximum number of Hydrogens we could have is two N plus two, where N is equal to five. Immediately, that makes me think about a double bond might be present in this molecule or one ring. For this signal, there's an integration value of 27. You divide all four integration values by the lowest one. Remember, these are just the relative numbers of protons giving you these signals but you can't have 1.5 protons giving you a signal. So these two neighboring protons for the Metal group must be these two protons right here. All right, so let's think about the red protons again. We know that these five protons are giving us this complicated signal over here. They're not being deshielded as much as the two signals in this direction. From the signal, we know that these Metal protons are next to two neighbors and so we must have a CH2 next to that metal. It works for this example so we're gonna go with it because all we care about is getting the structure of our molecular here. Here's one and here's two, so this makes sense. Let's go back to these five protons with this complex signal, all right. We don't have to worry about the slightly different environments. Two neighboring protons, that must be these two neighboring protons. Let's color coordinate here just to make sure everything makes sense. So we have these protons and we have these protons. We'd expect three neighbors because we have four peaks. So three neighbors, so here's the next door carbon. And then this is also a next door carbon but there are no protons on this carbon. For example, you have eight carbons you have to worry about. One, two, three, four, five, six, and then we get seven and then we get eight. And if you count all those up, we have our ten Hydrogens accounted for too. All right, this one was a little bit easier than the previous example.


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