1H NMR: How Many Signals?
Last updated: February 8th, 2022 |
How Many Unique 1H NMR Signals In A Molecule?
How many signals will appear in the proton (1H)NMR spectrum of a molecule? If the answer were as straightforward as just counting the number of protons, then we wouldn’t bother asking the question, now, would we?
The purpose of this post is to help you figure out the answer to these types of questions. The plan is to start with the most absurdly simple examples and then slowly work our way up to more complicated ones. We will see that protons will have equivalent 1H NMR signals (chemically shift equivalent) if they can be superimposed on each other through three
- reflection through a mirror plane in the molecule
- rotation of the molecule itself [inversion center]
- fast bond rotation (which has the effect of “blurring” the signals together, averaging them out)
Table of Contents
- Bromoacetylene and Acetylene
- Chemical Shift Equivalence
- Identifying Planes of Symmetry
- Disubstituted Benzene
- Chemical Shift Equivalence Through Bond Rotation
- More Examples
Bromoacetylene is about the simplest possible example of a molecule that has one proton, and therefore exactly one peak in a 1H NMR spectrum (with a chemical shift of 2.33 in CCl4 [ref]).
Acetylene itself (C2H2) has two protons. Does that mean it has two signals?
No. Despite doubling the number of protons, the correct answer is still ONE signal.
Why is that?
Recall that the “chemical shift” of a proton NMR signal represents the shielding of a hydrogen nucleus by its surrounding electrons.
The two protons in acetylene are in identical electronic environments. Note that we can flip acetylene 180 degrees, or rotate it along its axis, and the two protons are completely indistinguishable. This means that the two protons will have the exact same chemical shift. [Note – it will integrate to two] .
We call this chemical shift equivalence.
When two or more protons are chemically equivalent, they will contribute to the production of a single signal at one specific chemical shift in a 1H NMR spectrum. For example, if three hydrogens on a molecule are chemically equivalent, instead of producing three different signals in the spectrum, only one signal will be produced to represent all three.
This chemical shift equivalence is a result of the hydrogens’ nuclei being interchangeable through operations of symmetry (planes of symmetry) or rapid intramolecular processes (bond rotation or tautomerization).
Hydrogens with this equivalence may be either homotopic (identical) or enantiotopic (equivalent in achiral solvents).
[The number of peaks we observe in an NMR spectrum will correspond to the number of protons that are in different environments. ]
Let’s expand this further with the simplest possible alkene, ethene (a.k.a. “ethylene”). Ethylene contains four hydrogens. How many signals would we observe?
The answer remains “one”. Four protons, but just one peak in the 1H NMR spectrum.
Taking a look at ethylene, we can see two apparent planes of symmetry that cut the molecule vertically and horizontally in half. Performing symmetry operations along these axes confirm that its four hydrogens are interchangeable. The four nuclei reside in identical intramolecular environments, making them chemically equivalent. Thus, the performance of a 1H NMR experiment will produce only one signal in the spectrum to represent all four hydrogens at a single chemical shift. [NMR spectrum here].
So far we have looked at an alkyne and alkene. Now, let’s take a look at an alkane like methane.
Methane is a highly symmetrical molecule that contains many planes of symmetry–13 to be exact. By performing just a few of these symmetry operations, such as rotating about the four rotational axes shown above, we can see that all four of methane’s hydrogens are interchangeable. The four equivalent protons will produce one signal at a single chemical shift on the spectrum.
How about benzene? Six protons. How many signals?
Benzene is another compound that contains many planes of symmetry. Through mirror planes and multiple rotational operations, all six of benzene’s hydrogens are found to be homotopic. These, too, will produce only one signal in a 1H NMR spectrum.
Let’s apply and expand a bit. Based on what we have learned so far, how many signals would each molecule below produce?
Look for planes of symmetry.
They each produce 1! This can be confirmed just by looking at the basic mirror planes represented in each of the molecules. Further confirmation is possible by performing more symmetry operations along their rotational axes.
How about a few more complex examples?
There are three isomers of dichlorobenzene. Ortho-dichlorobenzene, meta dichlorobenzene, and para dichlorobenzene. How many peaks in each?
These molecules do, in fact, contain mirror planes and rotational axes. However, containing a plane of symmetry does not automatically deem all hydrogens equivalent. Hydrogens may still be unique despite containing symmetry depending on their neighboring atoms.
Some substitution patterns result in planes of symmetry, some do not.
Now, how about chloroethylene?
Chloroethylene is not a very symmetrical molecule. It lacks a plane of symmetry that would make any of its hydrogens interchangeable. Each of its protons is in a unique environment and will produce its own signal in a 1H NMR spectrum. Not all alkene hydrogens will always be identical.
Let’s take a look at some more alkanes.
We saw that methane produced one signal because of its rotational symmetry. How about ethane?
We can can see that through ethane’s mirror plane that its hydrogens are equivalent, but this is just a function of the way I’ve drawn it here. , but recall that there is rotation about the C–C bond of the molecule. Do ethane’s conformations affect the number of signals produced in its 1H NMR spectrum?
The answer is no! While the spinning of the methyl groups does occur in ethane, the speed of rotation is fast enough relative to the NMR “shutter speed” that the hydrogens become equivalent and blur together like the spinning of a bicycle wheel or blades of a fan. The signal produced is composed of the average of the hydrogens in different conformers. For ethane’s case its conformers can be ignored for 1H NMR purposes here. Only one signal will be produced in its spectrum.
This equivalence through rapid bond rotation applies to other alkanes, as well.
As we can see in the table above, basic alkane chains contain planes of symmetry that bisect the molecules straight down the middle. Both this symmetry and rapid bond rotations contribute to the equivalence of alkyl hydrogens. The number of signals these molecule’s produce in a 1H NMR spectrum can be determined just by counting the number of distinct hydrogens on one side of the plane of symmetry.
Applying the information we have learned, let’s try a few examples.
We can see that even though these molecules have an abundance of protons, their symmetry greatly reduces the number of signals a 1H NMR experiment would produce. Using operations of symmetry to find proton equivalence can be greatly simplifying!