Chapter 29 – Infographic descriptions

29.6a Analytical Chemistry – Infrared (IR) Spectroscopy

Commonly referred to as IR spectroscopy, this technique allows chemists to identify characteristic groups of atoms (functional groups) present in molecules.

The fingerprint region: 1500cm^-1 to 500cm^-1. The fingerprint region of the spectrum contains a complex set of absorptions, which are unique to each compound. Though these are hard to interpret visually, by comparison with references they allow identification of specific compounds.

Infrared frequencies make up a portion of the electromagnetic spectrum. If a range of infrared frequencies are shone through an organic compound, some of the frequencies are absorbed by the chemical bonds within the compound. Different chemical bonds absorb different frequencies of infrared radiation. There are a number of characteristic absorptions which allow functional groups (the parts of a compound which give it its particular reactivity) to be identified.

In the table above, = (equal sign) means double bond.  ≡ (identical to sign) means triple bond.

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29.7a Mass spectrometry and a guide to interpreting mass spectra

Mass spectrometry is an analytical technique that allows us to measure the masses of atoms and molecules. The most important peak in a mass spectrum is the molecular ion peak, which can be used to determine the mass of the molecule, but fragment icons can also provide information on chemical structure.

How mass spectrum works:

1. A small sample of the substance to be analyzed is added to the mass spectrometer.
2. The mass spectrometer ionizes the sample. This can be done win a number of ways, including with a laser, applying a voltage to a liquid sample spray, or firing electrons at a gaseous sample. Some molecules fragment into smaller ions.
3. A mass analyzer separates ions based on their mass/charge ratio. This can be done in a number of ways using an electric and/or magnetic field.
4. Ions hit the detector and it converts them into a signal, amplifies it, and records it.
5. The signal is output as a mass spectrum.

In the table above, = (equal sign) means double bond.

A selection of common fragment ions seen in mass spectra are shown above, along with their masses. Note that the structures shown are general representations, and it can also be possible for isomeric structures (those with the same constituent atoms, but a different structure) to cause the peaks in spectra. There are many more fragments possible that those shown, but knowledge of these fragments should suffice to interpret spectra of most simple molecules.

Read more about “Mass spectrometry and a guide to interpreting mass spectra [New tab]” by Andy Brunning / Compound Interest, CC BY-NC-ND

29.9a Analytical Chemistry – A Guide to Proton Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance (NMR) is a commonly used technique for organic compound structure determination. In ¹H NMR, applying external magnetic field causes the nuclei spin to flip. The environment of the proton in the molecule affects where the signal is seen on the resultant spectrum.

Key: C means carbon atom. H means hydrogen atom. *H means hydrogens producing signal. O means oxygen atom. N means nitrogen atom. R means rest of organic molecule. Ar means aromatic ring. X means halogen atom. – (minus sign) means single bond. = (equal sign) means double bond.  ≡ (identical to sign) means triple bond. Note these are typical values only, and vary depending on the solvent, the temperature, and presence of other functional groups.

Spin-spin coupling patterns in NMR spectra

Hydrogen nuclei themselves possess a small magnetic field, and can influence the signal seen for hydrogen on neighbouring carbon atoms. This is known as spin-spin coupling. The number of signals the original signal is split into is equal to the number of hydrogens on neighbouring carbon atoms plus one, according to the patterns shown in the table below. The area underneath the peaks indicated the number of hydrogen atoms responsible for each signal.

Read more about “Analytical Chemistry – A Guide to Proton Nuclear Magnetic Resonance (NMR) [New tab]” by Andy Brunning / Compound Interest, CC BY-NC-ND

29.10a A Guide to 13-C Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance (NMR) is a commonly used technique for organic compound structure determination. In ¹³C NMR, applying an external magnetic field causes the nuclei spin to flip. The environment of the carbon atom in the molecule affects where the signal is seen on the resultant spectrum.

Key: C means carbon atom. *C means carbon causing signal. H means hydrogen atom. O means oxygen atom. N means nitrogen atom. R means rest of organic molecule. Ar means aromatic ring. X means halogen atom. – (minus sign) means single bond. = (equal sign) means double bond.  ≡ (identical to sign) means triple bond. Note these are typical values only, and vary depending on the solvent, the temperature, and presence of other functional groups.

¹²C 99%, ¹³C 1%

Only 1% of carbon atoms are carbon-13, atoms which have one more neutron that carbon-12. NMR doesn’t work for carbon-12, as its nucleus doesn’t have a ‘spin’. The frequency required to ‘flip’ a carbon-13 nucleus is around a quarter of that required to flip a hydrogen nucleus in H-NMR. As the probability of two adjacent carbons in a single molecule being carbon-13 atoms is very low, no splitting of peaks is seen, unlike in H-NMR.

Read more about “A Guide to 13-C Nuclear Magnetic Resonance (NMR) [New tab]” by Andy Brunning / Compound Interest, CC BY-NC-ND

Attribution & References

Compound Interest infographics are created by Andy Brunning and licensed under CC BY-NC-ND

Except where otherwise noted, content on this page has been created as a textual summary of the infographics used within our OER. Please refer to the original website (noted below each description) for further details about the image.