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2 November, 03:33

Using a 300 MHz NMR instrument:

a. How many Hz downfield from TMS is a signal at 2.5 ppm?

b. If a signal comes at 1200 Hz downfield from TMS, at what ppm does it occur?

c. If two peaks are separated by 2 ppm, how many I-AZ does this correspond to?

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  1. 2 November, 06:46
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    a. 750Hz, b. 4.0ppm, c. 600Hz

    Explanation:

    The Downfield Shift (Hz) is given by the formula

    Downfield Shift (Hz) = Chemical Shift (ppm) x Spectrometer Frequency (Hz)

    Using the above formula we can solve all three parts easily

    a. fspec = 300 MHz, Chem. Shift = 2.5ppm, 1MHz = 10⁶ Hz, 1ppm (parts per million) = 10⁻⁶

    Downfield Shift (Hz) = 2.5ppm x 300MHz x (1Hz/10⁶MHz) x (10⁻⁶/1ppm)

    Downfield Shift = 750 Hz

    The signal is at 750Hz Downfield from TMS

    b. Downfield Shift = 1200 Hz, Chemical Shift = ?

    Chemical Shift = Downfield shift/Spectrometer Frequency

    Chemical Shift = (1200Hz/300MHz) x (1ppm/10⁻⁶) = 4.0 ppm

    The signal comes at 4.0 ppm

    c. Separation of 2ppm, Downfield Shift = ?

    Downfield Shift (Hz) = 2 (ppm) x 300 (MHz) x (1Hz/10⁶MHz) x (10⁻⁶/1ppm) = 600 Hz

    The two peaks are separated by 600Hz
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