NMR Spectroscopy

Introduction

A number of nuclei possess magnetic properties. This means that if they are subjected to a strong magnetic field they will either:

• align themselves in the same direction of the magnetic field (a low-energy state)
• align themselves in the opposite direction of the magnetic field (a high-energy state)

If they are subjected to a range of radio wave frequencies each nucleus will absorb the frequency corresponding to the difference in energy between its low and high-energy state and then switch from one to the other.

It is possible to detect this absorption and convert it into a spectrum. The frequency absorbed by the nuclei depends on the type of nucleus and how the electrons are arranged around the nucleus. Therefore, this information can be used to determine structural features of a molecule. This technique is known as nuclear magnetic resonance (nmr) spectroscopy.

There are a number of types of nmr spectroscopy. The type used depends on the nucleus being looked into. However, for A level you only need to know about two of these techniques:

• Proton nmr spectroscopy: this looks at the absorption of radiation by the nuclei of hydrogen atoms (¹H) and, therefore, can be used to determine the number of hydrogen atoms in a molecule and how they are arranged.
• Carbon-13 nmr spectroscopy: this looks at the absorption of radiation by the nuclei of carbon-13 atoms (¹³C) and, therefore, can be used to determine the number of carbon atoms in a molecule and how they are arranged.

Proton nmr spectroscopy

Analysing a proton nmr spectrum you can deduce a number of important pieces of information.

Hydrogen atoms which are identical all absorb radiation at the same frequency. This means that they will all contribute to the same peak. Therefore, the number of different peaks will tell you how many different environments of hydrogen atoms there are in a molecule.

The number of hydrogen atoms of that type can be worked out by looking at the area under each peak which is related to the intensity of the absorption. The relative intensity of the peaks, known as the integration factor, is usually given as a simplest whole number ratio. This is because it is not always so easy to deduce from the spectrum.

The frequency of the radiation absorbed is dependent on the environment of the hydrogen atom. It provides information on the position of those hydrogen atoms in the molecule in relation to other carbon atoms and functional groups. As the actual frequency of the absorption is hard to measure, the frequency is measured relative to a standard instead. Therefore, what is measured is the difference between the frequency absorbed by the sample atoms and the frequency absorbed by the standard. It is expressed as a fraction of the frequency absorbed by the standard and is known as the chemical shift (?):

? = (f(sample) – f(standard)) / f(standard)

Usually, ? is a small value (generally equal to 1 – 10 x 10^-6). Therefore, it it expressed in parts per million (for example, 3.0 x 10^-6 = 3.0 ppm). The closer the hydrogen atoms are to the electronegative atoms, the greater the chemical shift tends to be. However, it is also relatively high for hydrogen atoms which are close to arene and alkene groups. Hydrogen atoms which possess a large chemical shift are said to be deshielded while those with a low chemical shift are said to be shielded. By definition, the peak which results from the standard always has a chemical shift of zero.

Most peaks which appear single are in fact a number of peaks positioned very close together. How each peak splits is dependent on the total number of hydrogen atoms on adjacent atoms. The level of splitting is given by (n + 1) in which n is equal to the number of hydrogen atoms on adjacent atoms.

If a hydrogen atom possesses no hydrogen atoms on adjacent carbon atoms then the peak is a single peak which is called a singlet. If n = 1 then the peak only has one split and is a doublet. The names given to the peaks are outlined in the table below.

The term given to the process in which adjacent hydrogen atoms split peaks is coupling. Hydrogen atoms which are involved in hydrogen bonding never possess split peaks (so they will always be singlets) and never contribute to the splitting of other peaks.

Below is an example of a proton nmr spectrum:

From the horizontal axis you can read the chemical shift and from the size and shape of the peaks it is possible to deduce the integration factor and the splitting. This information can then be used to work out the structure of the molecule in a sample.

It is hard to read the integration factor directly. Therefore, this information is usually provided above the peak in question. Otherwise, pre-integrated nmr spectra are used. As it passes each peak the line above the spectrum shifts vertically. The size of this vertical shift is in proportion to the integration factor of the peak. The ratio of the shifts at each peak is the integration factor. You have to work out the integration factor of each peak using the integration line only.

Carbon-13 nmr spectroscopy

Carbon-13 nmr spectroscopy works in a similar way to proton nmr spectroscopy but is much simpler.

As before, carbon atoms which are identical all absorb radiation at the same frequency. This means that they will all contribute to the same peak. Therefore, the number of different peaks will tell you how many different environments of carbon atoms there are in a molecule.

You cannot deduce the number of carbon atoms present in each environment from the area under the peaks because it does not correspond to the intensity of the absorptions exactly.

The frequency of the radiation absorbed is dependent on the environment of the carbon atom. It provides information on the position of those carbon atoms in the molecule in relation to other atoms and functional groups. As with proton nmr spectra, it is measured using chemical shift.

As the peaks in the carbon-13 nmr spectrum are not split you cannot found out about the number of adjacent carbon atoms present in each carbon environment.

Below is an example of a carbon-13 nmr spectrum:

Preparing samples for proton nmr analysis

A number of factors need to be taken into consideration when preparing a sample for proton nmr analysis.

Choosing a suitable solvent

Before a sample can be analysed it needs to be dissolved in a solvent. However, a lot of solvents contain hydrogen atoms which would interfere with the spectrum produced by the sample. Therefore, a solvent must be used that contains no hydrogen atoms. Popular choices include CCl₄ and CDCl₃. D stands for deuterium, a hydrogen isotope which consists of one neutron in its nucleus. As it possesses no magnetic properties it will not interfere with the proton nmr spectrum.

Choosing a standard

A standard must also be used to calibrate the spectrum. It is then possible to measure the chemical shift in relation to this standard. The most commonly used standard is tetramethylsilane (TMS). The advantages of this standard are:

• It is composed of 12 identical hydrogen atoms which produce an easily identifiable single, singlet peak.
• It contains an Si electropositive atom which releases electron density onto the H atoms. This means that the hydrogen atoms are highly shielded so the peak is usually at a much lower frequency to those present in the majority of organic molecules. Therefore, it does not interfere with other peaks and is easily identifiable.
• It is both cheap and non-toxic.

It is common for this large singlet to be erased from a spectra before it is produced.

Determining structure using proton nmr spectra

The information that can be taken from proton nmr spectra to deduce the structure of organic molecules can be split up into four parts.

Number of peaks

Each peak is correspondent to a set of identical hydrogen atoms. Therefore, the peak number equals the number of hydrogen atom types present.

Some molecules only possess one type of hydrogen atom and so their spectrum only gives one peak. For example, ethane and propanone.

Some molecules possess two types of hydrogen atoms and so their spectrum gives two peaks. For example, butane and methanol.

However, most molecules give more than two peaks. For example, ethanol gives three peaks and butanal gives four peaks.

Integration factor

The integration factors is an indicator for how many identical hydrogen atoms correspond to a particular peak. Molecules which provide the identical numbers of peaks can be distinguished by their different integration factors which are different for each peak. For example, methylpropanal and butanone both give three peaks. However, the integration factors for these peaks are different:

• methylpropanal = 6:1:1
• butanone = 3:3:2

Chemical shift

The environment around the H atom, and in particular its related proximity of electronegative atoms, dictates the chemical shift. The closer the hydrogen atoms and electronegative atom are the higher the deshielding and the larger the chemical shift. For instance, hydrogen atoms found in alkanes usually have a low chemical shifts (? = 0 – 2). However, if the H atoms are attached to O atoms in carboxylic acids then the chemical shifts tend to be high (? = 10 -12).

Hydrogen atoms which are not within one carbon atom of a functional group will always have a chemical shift which lies between 0 and 2 ppm.

The table below is a summary of important chemical shifts for hydrogen atoms in common environments which are close to a functional group.

Using chemical shift data you can figure out the environments responsible for particular peaks and identify functional groups and the position of other H atoms on the molecule.

Coupling

As long as they are are involved in hydrogen bonding, hydrogen atoms on adjacent carbon atoms will affect the absorption of radiation by hydrogen atoms. These hydrogen atoms lead to splitting of peaks according to the (n + 1) rule.

This information can be used to figure out the position of hydrogen atoms in difference environments in relation to one another. For example, take a CH_3- group. The peak will have an integration of factor 3. However, how the peak is split is dependent on the hydrogen atoms which are adjacent to the carbon atom and either a triplet, doublet or singlet will be observed.

When dealing with coupling two general points should be taken into consideration:

• A triplet which has an integration factor of 3 and a chemical shift of between 0 and 2 along with a quartet which possesses an integration factor 2 strongly suggests that CH₃CH_2- is present.
• A singlet peak which possesses an integration factor of 1 strongly suggests that a O-H bond is present.

Determining structure using carbon-13 nmr spectra

Although they provide less information, carbon-13 nmr spectra are also useful in determining the structure of organic molecules.

Number of peaks

Each peak is correspondent to a set of identical carbon atoms. Therefore, the peak number equals the number of carbon atom types present.

Some molecules only possess one type of carbon atom and so their spectrum only gives one peak. For example, ethane and benzene.

However, most molecules give two or more peaks. For example, butane gives two peaks, propene gives three peaks, and but-1-ene gives four peaks.

Chemical shift

The environment around the carbon atom, and in particular its related proximity of electronegative atoms, dictates the chemical shift. The closer the carbon atoms and electronegative atom are the higher the deshielding and the larger the chemical shift. For instance, carbon atoms found in alkanes usually have a low chemical shifts (? = 0 – 40). However, if the C atoms are attached to O atoms with a double bond then the chemical shifts tend to be high (? = 160 -220).

Carbon atoms which are not directly attached to a functional group (in other words, they only have single bonds to carbon or hydrogen atoms) will always have a chemical shift which lies between 0 and 40 ppm.

The table below is a summary of important chemical shifts for carbon atoms in common environments which are close to a functional group.