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# Chemical shift

The video explains how to calculate chemical shifts in NMR spectroscopy using tetramethylsilane (TMS) as a reference. It emphasizes the importance of TMS's shielding effect and how it affects the frequency of proton absorption. The video also demonstrates how to maintain constant chemical shift values across different spectrometer frequencies. Created by Jay.

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• How to calculate observed shift for TMS? How do you get 2181 value?
• The machine measures the observed shift from TMS for you.
In practice, the machine prints out the spectrum.
If you see a peak at δ = 7.27 ppm, you can calculate the shift from the formula
shift = δ × spectrometer frequency = 7.27 × 10⁻⁶ × 300 × 10⁶ Hz = 2181 Hz.
• What is on the Y-axis? is it Intensity or Absorbence?
• The y-axis represents the intensity of the NMR signal.
• Why is the observed shift from TMS different for different spectrometers. Shouldn't delta E be the same?
• The formula for shift is slightly wrong, as it's supposed to be reported in ppm.
shift(compound) = frequency(compound) - frequency(reference)/frequency(reference)
(1 vote)
• Sorry, the video is correct.
The formula for chemical shift is
δ = (ν(compound) – ν(reference))/spectrometer frequency × 10⁶
= observed shift/spectrometer frequency × 10⁶
• I thought modern spectrometers send out a bunch of different frequencies? How does that correlate with just one given frequency, e.g. 300Mhz?
Don't quite understand why the spectrometer in this case only has one fixed frequency.
• How did you get the observed shift from TMS values?
• When you send a pulse of many frequencies, only those corresponding to the frequency of excitation of the equivalent protons will be "returned". This frequency is different than the one you get from TMS (which is pretty much the lowest, therefore they made it the reference), so, you may compare the shift in returned frequency between any equivalent proton and TMS.
• What is 300MHz or 60MHz spectrometer?
• Shouldn't C(CH3)4 be used as the reference? because carbon has a much smaller radius and so it would be more shielded than TMS.
(1 vote)
• Si is less electronegative than C, so it is more electron-donating, making H more shielded in TMS.