AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    The use of the δ scale for recording chemical shift.

    Full specification wording

    Nuclear magnetic resonance spectroscopy (A-level only): The use of the δ scale for recording chemical shift.

    Chemical shift is quoted on the delta scale in parts per million.

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

    • Point 2 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Chemical shift is quoted on the delta scale in parts per million. Zero on that scale is set by tetramethylsilane, added to the sample, and every other signal is reported as its distance from that peak divided by the operating frequency of the spectrometer. Dividing by the operating frequency is the important part, because it makes delta a ratio: a sample run on a low-field machine gives the same delta values as one run at high field, so the Data Booklet tables work for every instrument. Proton shifts run from roughly 0 to 12 ppm, carbon shifts from roughly 0 to 220 ppm. By convention the scale increases to the left, so a deshielded nucleus is described as downfield and tetramethylsilane sits at the right-hand edge.

    What earns the marks

    • Stating that tetramethylsilane is the standard and is assigned a shift of zero.
    • Chemical shift being measured in parts per million.
    • Explaining that dividing by the spectrometer frequency makes the value independent of the field used.
    • Reading a shift against the Data Booklet table and naming the environment it corresponds to.

    Common mistakes and how to put them right

    • Quoting a chemical shift in hertz rather than in parts per million.

    • Saying delta values change when a stronger magnet is used.

    • Treating tetramethylsilane as the solvent rather than as a reference added in small amount.

    • Reading the proton table when the spectrum is a carbon spectrum, or the reverse.

    Exam tips

    • Always write ppm after a shift; a bare number is not a chemical shift.
    • When a shift is given, find its row in the Data Booklet before writing anything, and quote the environment the table names.
    • Remember which end is which: a peak far to the left of the spectrum is strongly deshielded.

    Try the lesson

    The start of the Nuclear magnetic resonance spectroscopy (A-level only): The use of the δ scale for recording chemical shift. lesson, made from checked teaching notes for this specification point.

    Have a go first

    On the δ scale used in NMR spectroscopy, what is the chemical shift of the reference standard tetramethylsilane (TMS)?

    Listen

    Chemical shift is quoted on the delta scale in parts per million. Zero on that scale is set by tetramethylsilane, added to the sample, and every other signal is reported as its distance from that peak divided by the operating frequency of the spectrometer. Dividing by the operating frequency is the important part, because it makes delta a ratio: a sample run on a low-field machine gives the same delta values as one run at high field, so the Data Booklet tables work for every instrument. Proton shifts run from roughly 0 to 12 ppm, carbon shifts from roughly 0 to 220 ppm. By convention the scale increases to the left, so a deshielded nucleus is described as downfield and tetramethylsilane sits at the right-hand edge. A strong answer earns marks for these points. stating that tetramethylsilane is the standard and is assigned a shift of zero. chemical shift being measured in parts per million. explaining that dividing by the spectrometer frequency makes the value independent of the field used. reading a shift against the Data Booklet table and naming the environment it corresponds to. Exam tip: Always write ppm after a shift; a bare number is not a chemical shift.

    See it

    A top-down flowchart. The first box reads 'Tetramethylsilane added to sample' and points to 'Assigned shift of zero'. That points to 'Signal position measured as distance from TMS peak', which points to 'Divide by operating frequency of spectrometer'. This points to 'Chemical shift on delta scale in ppm'. From that box three arrows lead to three boxes: 'Same delta values on low-field and high-field machines', 'Proton shifts roughly 0 to 12 ppm' and 'Carbon shifts roughly 0 to 220 ppm'. A fourth arrow from the same box leads to 'Scale increases to the left; deshielded is downfield'.
    It shows how a signal's distance from the tetramethylsilane peak is divided by the spectrometer frequency to give a chemical shift in ppm on the delta scale.

    Key words

    Say what each word means before you turn the card.

    delta scale

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    parts per million

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    tetramethylsilane reference

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    independence of field strength

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    See every definition
    delta scale
    The scale on which chemical shift is quoted, in parts per million, increasing to the left.
    parts per million
    The unit in which chemical shift is measured on the delta scale.
    tetramethylsilane reference
    The standard added to the sample, assigned a shift of zero, sitting at the right-hand edge.
    independence of field strength
    Dividing by the operating frequency makes delta a ratio, so a sample run on a low-field machine gives the same delta values as one run at high field.

    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 box reads 'Tetramethylsilane added to sample' and points to 'Assigned shift of zero'. That points to 'Signal position measured as distance from TMS peak', which points to 'Divide by operating frequency of spectrometer'. This points to 'Chemical shift on delta scale in ppm'. From that box three arrows lead to three boxes: 'Same delta values on low-field and high-field machines', 'Proton shifts roughly 0 to 12 ppm' and 'Carbon shifts roughly 0 to 220 ppm'. A fourth arrow from the same box leads to 'Scale increases to the left; deshielded is downfield'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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