AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    ‘Fingerprinting’ allows identification of a molecule by comparison of…

    Full specification wording

    Infrared spectroscopy: ‘Fingerprinting’ allows identification of a molecule by comparison of spectra.

    Below about 1500 cm⁻¹ an infrared spectrum becomes a dense pattern of overlapping absorptions produced by bending and by vibrations of the whole carbon skeleton.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 2 of 3
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Below about 1500 cm⁻¹ an infrared spectrum becomes a dense pattern of overlapping absorptions produced by bending and by vibrations of the whole carbon skeleton. Individual peaks there cannot reliably be assigned to single bonds, but the pattern as a whole is unique to one compound, which is why it is called the fingerprint region. Identification therefore works by matching: the spectrum of the unknown is compared with spectra of known compounds held in a database, and a match across the fingerprint region identifies the compound. This technique is highly effective for separating chain or positional isomers, which contain the same functional groups and so look alike above 1500 cm⁻¹ but differ in skeletal vibrations below it. NMR spectroscopy can also distinguish isomers.

    What earns the marks

    • Identifying the fingerprint region as below about 1500 cm⁻¹.
    • Saying the pattern there is unique to a particular compound.
    • Saying identification requires comparison with the spectrum of a known compound or a database.
    • Explaining that chain or positional isomers share functional groups, so the fingerprint region is used to separate them.

    Common mistakes and how to put them right

    • Trying to assign individual bonds within the fingerprint region, which is the job of the region above 1500 cm⁻¹.

    • Claiming a compound can be named from its fingerprint region with no reference spectrum.

    • Choosing high resolution mass spectrometry to distinguish isomers, which AQA sets as a wrong option.

    • Assuming all isomers have identical IR spectra above 1500 cm⁻¹ (functional group isomers will differ).

    Exam tips

    • Say "compared with a database or known spectrum"; identification by matching is the point of the region.
    • If two compounds are chain or positional isomers, the fingerprint region is a standard intended answer for distinguishing them.
    • Quote the boundary as about 1500 cm⁻¹ rather than an exact figure.

    Try the lesson

    The start of the Infrared spectroscopy: ‘Fingerprinting’ allows identification of a molecule by comparison of spectra. lesson, made from checked teaching notes for this specification point.

    Have a go first

    In infrared spectroscopy, what is meant by the fingerprint region of a spectrum?

    Listen

    Below about 1500 cm⁻¹ an infrared spectrum becomes a dense pattern of overlapping absorptions produced by bending and by vibrations of the whole carbon skeleton. Individual peaks there cannot reliably be assigned to single bonds, but the pattern as a whole is unique to one compound, which is why it is called the fingerprint region. Identification therefore works by matching: the spectrum of the unknown is compared with spectra of known compounds held in a database, and a match across the fingerprint region identifies the compound. This technique is highly effective for separating chain or positional isomers, which contain the same functional groups and so look alike above 1500 cm⁻¹ but differ in skeletal vibrations below it. NMR spectroscopy can also distinguish isomers. A strong answer earns marks for these points. identifying the fingerprint region as below about 1500 cm⁻¹. saying the pattern there is unique to a particular compound. saying identification requires comparison with the spectrum of a known compound or a database. explaining that chain or positional isomers share functional groups, so the fingerprint region is used to separate them. Exam tip: Say "compared with a database or known spectrum"; identification by matching is the point of the region.

    See it

    A top-down flowchart. The first node, 'Infrared spectrum', splits into two branches: 'Below about 1500 cm⁻¹: fingerprint region' and 'Above 1500 cm⁻¹'. From the fingerprint region branch, a node reads 'Dense pattern of overlapping absorptions', which leads to three nodes: 'Bending and vibrations of the whole carbon skeleton', 'Individual peaks cannot reliably be assigned to single bonds', and 'Pattern as a whole is unique to one compound'. The uniqueness node leads to 'Compare spectrum of unknown with spectra of known compounds in a database', then to 'Match across the fingerprint region identifies the compound', and finally to 'Separates chain or positional isomers, which share functional groups'.
    It shows how the fingerprint region below about 1500 cm⁻¹ gives a unique pattern that is matched against a database to identify a compound, including chain or positional isomers.

    Key words

    Say what each word means before you turn the card.

    fingerprint region

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    below 1500 cm⁻¹

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    unique pattern

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    comparison with a database

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    See every definition
    fingerprint region
    Below about 1500 cm⁻¹ an infrared spectrum becomes a dense pattern of overlapping absorptions produced by bending and by vibrations of the whole carbon skeleton.
    below 1500 cm⁻¹
    Individual peaks there cannot reliably be assigned to single bonds, but the pattern as a whole is unique to one compound.
    unique pattern
    The pattern as a whole is unique to one compound, which is why it is called the fingerprint region.
    comparison with a database
    The spectrum of the unknown is compared with spectra of known compounds held in a database, and a match across the fingerprint region identifies the compound.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first node, 'Infrared spectrum', splits into two branches: 'Below about 1500 cm⁻¹: fingerprint region' and 'Above 1500 cm⁻¹'. From the fingerprint region branch, a node reads 'Dense pattern of overlapping absorptions', which leads to three nodes: 'Bending and vibrations of the whole carbon skeleton', 'Individual peaks cannot reliably be assigned to single bonds', and 'Pattern as a whole is unique to one compound'. The uniqueness node leads to 'Compare spectrum of unknown with spectra of known compounds in a database', then to 'Match across the fingerprint region identifies the compound', and finally to 'Separates chain or positional isomers, which share functional groups'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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