At times, solving an NMR problem leads to two or more plausible structures satisfying the given data. We have seen that ¹³C NMR is usually decoupled and therefore, there is no splitting of signals which limits the information we can get as to how many hydrogens are connected to a carbon atom. Even combining ¹H and ¹³C NMR spectra may not give definite evidence for choosing only one structure.

This is where a technique called DPET (distortionless enhancement by polarization transfer) becomes very useful. All it does (and that’s a lot and very handy) is differentiate the carbons based on the number of hydrogens it is bonded to.

So, instead of simply saying hey this is a carbon, and this is another one, it tells us if it is a C, CH, CH₂, or a CH₃. Isn’t that nice?

Let’s see how we get this information in DEPT.

Depending on the carbon type, the signal in DEPT can be pointing up or down while being at the same ppm values as in the regular ¹³C NMR. Another possibility in DEPT is the lack of a given signal. Here is the summary of DEPT signals:

 

DEPT-90 and DEPT-135 are different types of DEPT experiments and we won’t go over the mechanisms here but rather use this data as it is. The aim of this article is to explain the application of DEPT in solving NMR spectra.

 

As an example, let’s see this (stimulated) ¹³C NMR combined with the DEPT experiments:

 

 

Notice how the ppm values are retained but depending on the signals in DEPT we can tell if the carbon is a C, CH, CH₂ or a CH₃ group.

 

In case you needed, here are the chemical shift values for ¹³C NMR:

 

 

 

Show Me a Good Example of DEPT NMR Problem

Let’s discuss a specific NMR problem where the final structure is only determined using the DEPT data.

The proton and carbon NMR spectra of a compound with the formula C₅H₉Br are shown below. The DEPT experimental results are also provided in the table.

Purpose a plausible structure based on the data provided.

 

I went over the steps for solving NMR problems with lots of examples which you can find here but for now let’s quickly apply those and see what we get.

First, determine the hydrogen deficiency index. Replacing the Br with an H we get C₄H₁₀ which corresponds to one degree of unsaturation. Therefore, the compound has a double bond or a ring.

 

Now, looking at signal at about 4.7 ppm in the proton, and the ones above 100 on the carbon, we know that it must be a double bond rather than a ring.

Next, look at the signal splitting in ¹H NMR; two triplets indicate a -CH₂-CH₂– fragment which is connected to Br on one end since it is downfield (3.3 ppm). And the other CH₂ must be connected to the double bond since the signal is still more downfield than if it was a regular alkyl group.

I’ll put this table for ¹H NMR shifts for a reference:

 

So, let’s put down the groups we have so far:

 

Two of these X groups must be hydrogens because of the integration of the signal at ~4.7 ppm.

The other X group is a methyl group which we can deduce from the integration.

 

And now the interesting part realted to DEPT. There are three combinations of putting two hydrogens and a methyl group on the double bond:

 

 

All of these would be good candidates based on the data from the proton and carbon NMR. But only the last structure matches the data from the DEPT experiments which indicate the presence of three CH₂ groups (three negative signals in DEPT-135):

 

 

I do want to mention that the structure of a double bond can be analyzed using the J coupling values and a powerful NMR spectrometer will give a resolution good enough to exclude the other candidates based on the coupling. However, DEPT makes things easier without the need for a lot of complicated analysis.

 

Check Also

• NMR spectroscopy – An Easy Introduction
• NMR Chemical Shift
• NMR Chemical Shift Range and Value Table
• NMR Number of Signals and Equivalent Protons
• Homotopic Enantiotopic Diastereotopic and Heterotopic
• Homotopic Enantiotopic Diastereotopic Practice Problems
• Integration in NMR Spectroscopy
• Splitting and Multiplicity (N+1 rule) in NMR Spectroscopy
• NMR Signal Splitting N+1 Rule Multiplicity Practice Problems
• ¹³C NMR NMR
• DEPT NMR: Signals and Problem Solving
• NMR Spectroscopy-Carbon-Dept-IR Practice Problems