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Hydrogen -1 NMR

Hydrogen -1 NMR

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Have you ever wondered how scientists figure out what compounds are made of? Imagine you have two test tubes filled with mystery compounds. Sure, you can figure out the relative mass of ions with time-of-flight spectroscopy or add Tollens reagent to check for an aldehyde. But what if you want to know the exact structure of the samples? That's where hydrogen-1 NMR spectroscopy comes in.

Hydrogen-1 NMR spectroscopy is a fancy way to figure out the structure of a molecule. It tells us about the number of hydrogen atoms in each environment and how many hydrogen atoms are in adjacent environments. It's like a secret decoder ring for molecules!

Certain nuclei have a property called spin, which helps us identify different functional groups in molecules. Hydrogen-1 NMR takes this to the next level by giving us information on the exact structure of the molecule. In this article, we'll learn all about hydrogen-1 NMR, including spin-spin coupling, integration traces, and the n+1 rule. By the end, you'll be able to compare hydrogen-1 NMR to carbon-13 NMR and use hydrogen-1 spectra to figure out the structure of a molecule. So let's dive in and become molecular detectives!

How does hydrogen-1 NMR work?

Hydrogen-1 NMR works in just the same way as carbon-13 NMR. However, whilst in carbon-13 NMR we examined carbon-13 atoms, in this technique we look at hydrogen-1 atoms. Like carbon-13 atoms, these have an odd mass number and so have spin, meaning they show up in NMR spectra.

Hydrogen-1 atoms have one proton and no neutrons in their nucleus, giving them a net spin of 1/2
Hydrogen-1 atoms have one proton and no neutrons in their nucleus, giving them a net spin of 1/2

To quickly recap, NMR is a technique that helps us figure out the structure of molecules. Carbon-13 NMR uses a rare isotope of carbon, while hydrogen-1 NMR uses the most common hydrogen isotope. This makes obtaining hydrogen-1 spectra much easier.

To get a hydrogen-1 spectrum, we dissolve our sample and add in a reference molecule called TMS. Then, we apply radio waves to the solution, which causes some hydrogen atoms in the sample to flip to their antiparallel, spin-opposed state. This creates a spectrum that shows chemical shift, a property related to resonance frequency. By comparing chemical shift values to a data table, we can determine the environment of the hydrogen atoms in our sample.

In hydrogen-1 NMR, we can use solvents like deuterated chloroform, which is based on regular chloroform but has its hydrogen atoms replaced with deuterium. Deuterium has an even mass number, so it doesn't have spin. This helps us get a clearer spectrum.

This means that it won’t show up on the spectrum. Deuterium

The energy needed for an atom to flip is known as its magnetic resonance frequency. It varies depending on the atom’s environment - all the other chemical groups surrounding it. Atoms that are better-shielded from the magnetic field by electrons have a lower resonance frequency, and therefore lower chemical shift, than those less well-shielded. This means that hydrogen atoms bonded to electron-releasing groups such as methyl have lower chemical shift values than those bonded to electronegative groups like oxygen. For a more detailed look into NMR, check out Understanding NMR and Carbon -13 NMR.

Interpreting hydrogen-1 spectra

Now that we’ve revisited how we carry out NMR, we can look at how we analyse the spectra it produces. To fully understand these spectra, we need to consider environment and chemical shift values, as well as these new terms. Integration traces. Spin-spin coupling. The n+1 rule. Singlet, doublet, triplet, and quartet.

Environment

When we talk about an atom's environment in hydrogen-1 NMR, we're referring to all the other atoms and chemical groups attached to it. The number of peaks on a spectrum tells us how many different environments the hydrogen atoms we're looking at are in.

For example, in ethanol, the three hydrogen atoms circled in red are in the same environment because they're attached to the same carbon. The two hydrogens circled in blue are in a different environment than the red ones, but they're in the same environment as each other because the carbons they're attached to are bonded to the same chemical groups. Finally, the green-circled hydrogen atom is in its own unique environment. By analyzing the number of peaks and their chemical shift values, we can start to piece together the structure of a molecule and figure out where different functional groups are located. This is the power of hydrogen-1 NMR spectroscopy!

Ethanol: the different hydrogen atoms are circled according to their environments
Ethanol: the different hydrogen atoms are circled according to their environments

However, if we look at propan-2-ol, there are hydrogen atoms from multiple different carbon atoms all found in the same environment. This is because the carbon atoms are both bonded to exactly the same groups. In this case, each carbon atom is bonded to two hydrogen atoms and a group.

Propan-2-ol. Again, the hydrogen atoms are circled according to their environments

If a molecule is symmetrical, it will have hydrogen atoms in the same environment.

Chemical shift

Chemical shift is a property that relates to the magnetic resonance frequency of a nucleus - the energy required to flip it from its parallel to its antiparallel state. It's measured in parts per million (ppm) and varies depending on the environment of the hydrogen atoms we're looking at.

Hydrogen atoms in different environments have different chemical shift values because they're shielded differently from the external magnetic field. Electrons shield nuclei, so an atom bonded to an electron-releasing group like the methyl group is better shielded and has a lower resonance frequency and a lower chemical shift than one bonded to an electron-withdrawing group. By comparing the chemical shift values on a spectrum to those in a data table, we can infer the environments of the hydrogen atoms in our molecule. This helps us determine the structure and functional groups present, which is crucial for understanding the properties and behavior of molecules in various applications.

A hydrogen-1 NMR spectrum for ethanol
A hydrogen-1 NMR spectrum for ethanol

Look at the above spectrum for ethanol. The small right-hand peak is given by TMS, our reference compound. The next peak along has a value of about 1.2. Looking at our data table below, we can work out that this peak belongs to hydrogen atoms in a methyl group, . The next peak has a value of around 3.4. It belongs to hydrogen atoms on a carbon atom that is attached to an oxygen atom, as this gives values in the range 3.1-3.9 ppm. The leftmost peak has a value of about 4.8, and represents the hydrogen atom in ethanol’s  group.

A data table for hydrogen-1 NMR spectroscopy

Hydrogen-1 spectra show much lower chemical shift values than carbon-13 spectra. This is because its bonded electron pair is much closer to its nucleus than carbon’s bonded pair, so hydrogen’s nucleus is much better shielded from the external magnetic field.  We know from above that this gives atoms a lower resonance frequency, and thus lower chemical shift values.

Integration traces

In hydrogen-1 NMR spectra, the peaks are directly related to the number of hydrogen atoms in each environment. The area under each peak is proportional to the number of hydrogen atoms present. This means that a taller peak represents more hydrogen atoms in that particular environment than a shorter peak.

To determine the number of hydrogen atoms in each environment, the computer creates an integration trace - a line placed over the top of the spectrum that goes up in steps. The relative height of each step tells us the ratio of the number of hydrogen atoms in each environment. By measuring these heights, we can calculate the ratio and infer the number of hydrogen atoms in each environment. For example, let's look at the NMR spectrum for ethanol. The peak at around 1.2 ppm has a taller integration step than the others, which tells us there are more hydrogen atoms in that environment. This corresponds to the three hydrogen atoms circled in red in the molecule diagram we discussed earlier. The integration steps for the peaks at 3.6 ppm and 0.9 ppm are shorter, indicating only two and one hydrogen atoms in those environments, respectively. This matches with the blue and green-circled hydrogen atoms in the molecule diagram. Overall, the integration trace helps us determine the ratio of hydrogen atoms in different environments and provides a more accurate way to interpret the NMR spectrum.

The integration trace is shown in red
The integration trace is shown in red

To make life easier, the computer often also places a number above each peak. This also tells you the ratio of hydrogen atoms in each environment - it saves you from having to measure each step of the integration trace! In the example above, we can see that methanol has three hydrogen atoms in one environment, two hydrogens in a second environment and one hydrogen atom in a third environment.

Spin-spin coupling

If we zoom in a little closer to a hydrogen-1 NMR spectrum, we notice something a little bit odd. Take a look at the spectrum for ethanol, for example.

The hydrogen-1 spectrum for ethanol
The hydrogen-1 spectrum for ethanol

In NMR spectra, some peaks are split into a number of smaller peaks due to spin-spin coupling, also known as spin-spin splitting. Spin-spin coupling provides information about the number of hydrogen atoms on the neighboring carbon atom to the one responsible for the peak we are studying. These are called hydrogen atoms in adjacent environments. If there are n hydrogen atoms on neighboring carbons, the peak will split into n+1 smaller peaks.

For example, let's look at the NMR spectrum for ethanol again. We know that ethanol has three different hydrogen environments: the methyl group (-CH3), the methylene group (-CH2-), and the hydroxyl group (-OH). The peak for the methyl group at around 1.2 ppm is split into a triplet, with two smaller peaks on either side of the main peak. This is because the hydrogen atoms on the neighboring methylene group (-CH2-) cause the peak to split into three peaks (n+1, where n is 1).

Similarly, the peak for the methylene group at around 3.6 ppm is split into a quartet, with three smaller peaks on either side of the main peak. This is because the two hydrogen atoms on the neighboring methyl group (-CH3) cause the peak to split into four peaks (n+1, where n is 2).

The hydroxyl group (-OH) does not cause any splitting because there are no hydrogen atoms on the neighboring carbon atom. By analyzing the splitting pattern, we can determine the number of hydrogen atoms on the neighboring carbon atom and gain more information about the molecule's structure.

The different environments in an ethanol molecule
The different environments in an ethanol molecule

Look at the hydrogens circled in red, all part of a methyl group. They all belong to the same environment, so produce just one peak. Now look at the adjacent carbon atom. It contains two hydrogen atoms. There are two hydrogen atoms in an adjacent environment. Therefore, n=2. If we use the n+1 rule, we can predict that the methyl group peak will split into 2+1=3 smaller peaks. Looking at our graph, we can see that this is what actually happens.

Let’s take the carbon atom on the right now. Its hydrogen atoms produce the middle peak. Look at all the groups attached to it. There is just one attached carbon atom, our methyl group from above. The methyl group has three hydrogen atoms, so n=3. Using the n+1 rule, we can predict that this peak will split into 3+1=4 smaller peaks.

The smaller peaks all have names, shown in the table below.

A table showing names for smaller peaks that arise from spin-spin coupling
A table showing names for smaller peaks that arise from spin-spin coupling

You are correct about the further rules concerning spin-spin coupling. If there are no hydrogen atoms attached to any neighboring carbons, n=0, which means the peak won't split, forming a singlet. Spin-spin coupling only occurs if the neighboring hydrogen atoms are in different environments than the ones we are looking at. We call these equivalent hydrogens. If there are multiple neighboring carbon atoms with attached hydrogen atoms, we count n as the total number of hydrogens. The alcohol group always forms just one peak, a singlet, and has no effect on adjacent carbons.

To determine the structure of a molecule from its NMR spectrum, we need to analyze the splitting pattern of the peaks. For example, let's say we have an unknown molecule with three different hydrogen environments. We record its NMR spectrum and see that one peak is a singlet, one peak is a doublet, and one peak is a triplet.

The singlet peak corresponds to hydrogen atoms in an environment with no neighboring hydrogen atoms, such as a methyl group. The doublet peak means that there is one neighboring hydrogen atom, and the triplet peak means that there are two neighboring hydrogen atoms. From this information, we can deduce that the molecule has a methyl group, a methylene group, and a methine group. The splitting pattern also tells us the number of hydrogen atoms on the neighboring carbon atoms, allowing us to determine the molecular structure more accurately. Overall, analyzing the splitting pattern in NMR spectra provides valuable information about the molecular structure and arrangement of hydrogen atoms in a molecule.

The hydrogen-1 spectrum for an unknown molecule
The hydrogen-1 spectrum for an unknown molecule

What can we infer from this spectrum? It can be helpful to make a table.

Let’s start at the peak with a chemical shift of 1.2. Its integration trace value is 3, so it has 3 hydrogen atoms. It must be a methyl group. Because it is a triplet, its neighbouring carbon must have 2 hydrogen atoms attached. The next peak along is a quartet, so it must have a total of 3 hydrogen atoms on neighbouring carbons. It has a trace value of 2 so has just two hydrogen atoms itself. From the data table earlier, we can see that its chemical shift value of 2.2 means it is .

The final peak is a singlet and has a shift value of 10.5. This means it must be part of a carboxylic acid group, . Remember that the  group, also found in carboxylic acids, always produces a singlet. Let’s put this molecule together. We have two ends: a carboxylic acid group and a methyl group. In the middle we have some sort of group with  . Our molecule is propanoic acid.

Propanoic acid with its hydrogen-1 NMR spectrum. The peaks are labelled accordingly
Propanoic acid with its hydrogen-1 NMR spectrum. The peaks are labelled accordingly

is a powerful tool for identifying and characterizing molecules. By providing information about the number and arrangement of hydrogen atoms in a molecule, hydrogen-1 NMR helps researchers determine the molecular structure and can also aid in the identification of unknown compounds.

In addition to its use in structural determination, hydrogen-1 NMR has a variety of applications in fields such as medicine, food science, and environmental analysis. For example, in medicine, hydrogen-1 NMR is used in magnetic resonance imaging (MRI) scans to diagnose and monitor diseases. In food science, hydrogen-1 NMR can be used to identify and quantify specific metabolites and contaminants in food products. In environmental analysis, hydrogen-1 NMR can help identify and analyze contaminants in water and soil samples. Overall, hydrogen-1 NMR is a versatile analytical technique with a wide range of applications. Its ability to provide detailed information about molecular structure and composition makes it an essential tool for researchers in many different fields.

Hydrogen -1 NMR

What is Hydrogen-1 NMR used for?

Hydrogen-1 NMR is mostly used to identify molecules and work out their structure.

What are the principles of H NMR?

Hydrogen-1 atoms have an odd mass number, meaning they show a property called spin. This means they are affected by an external magnetic field. By analysing their behaviour in such a magnetic field, we can find out what chemical groups the hydrogen-1 atoms are a part of and work out the structure of their parent molecule.

Is proton NMR the same as hydrogen NMR? 

Yes. Hydrogen NMR and proton NMR spectroscopy are different names for the same technique.

What is hydrogen resonance? 

When nuclei with spin are placed in an external magnetic field, they either line up parallel to the magnetic field, or antiparallel to the field. The parallel state is much more energetically stable than the antiparallel state, but if you give the nucleus enough energy, it can flip to the antiparallel state. This is known as magnetic resonance. Hydrogen resonance is therefore the name of the process where hydrogen nuclei flip from their parallel to their antiparallel state. 

Why is hydrogen used in NMR?

Hydrogen is used in NMR because it has an odd mass number. This means it has spin and is affected by external magnetic fields.

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