Question

Write a short note on two scales of measurement of chemical shift?​

Answer 1

In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of a molecule. Chemical shifts are also used to describe signals in other forms of spectroscopy such as photoemission spectroscopy.

Answer 2

Short note on two scales of measurement of chemical shift are

• The NMR spectra is displayed as a plot of the applied radio frequency versus the absorption.
• The applied frequency increases from left to right, thus the left side of the plot is the low field, downfield or deshielded side and the right side of the plot is the high field, upfield or shielded side.
• The concept of shielding will be explained shortly.

• The position on the plot at which the nuclei absorbs is called the chemical shift.
• Since this has an arbitrary value a standard reference point must be used.
• The two most common standards are TMS (tetramethylsilane, (Si(CH3)4) which has been assigned a chemical shift of zero, and CDCl3 (deuterochloroform) which has a chemical shift of 7.26 for 1H NMR and 77 for 13C NMR.
• The scale is commonly expressed as parts per million (ppm) which is independent of the spectrometer frequency.
• The scale is the delta (δ) scale.

• The delta scale is the frequency of the signal minus the freuqnce of the standard.
• This is then divided by spectrometer frequency.
• This answer is then multiplied by 10 to the sixth.

• The range at which most NMR absorptions occur is quite narrow.
• Almost all 1H absorptions occur downfield within 10 ppm of TMS.
• For 13C NMR almost all absorptions occurs within 220 ppm downfield of the C atom in TMS.

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