Ch 13 - NMR Chemical Shift

Chapter 13: Spectroscopy

Chemical Shift

An NMR spectrum is a plot of the radio frequency applied against absorption.
A signal in the spectrum is referred to as a resonance.
The frequency of a signal is known as its chemical shift, d

The chemical shift in absolute terms is defined by the frequency of the resonance expressed with reference to a standard compound which is defined to be at 0 ppm. The scale is made more manageable by expressing it in parts per million (ppm) and is indepedent of the spectrometer frequency.

It is often convienient to describe the relative positions of the resonances in an NMR spectrum. For example, a peak at a chemical shift, δ, of 10 ppm is said to be downfield or deshielded with respect to a peak at 5 ppm, or if you prefer, the peak at 5 ppm is upfield or shielded with respect to the peak at 10 ppm. The terms shielded and deshielded will be explained later.

upfield and downfield regions of nmr spectra

Typically for a field strength of 4.7 T the resonance frequency of a proton will occur around 200MHz and for a carbon, around 50.4MHz. The reference compound is the same for both, tetramethysilane (Si(CH₃)₄).

QUESTIONS

What would be the chemical shift of a peak that occurs 655.2 Hz downfield of TMS on a spectrum recorded using a 90 MHz spectrometer? ANSWER
At what frequency would the chemical shift of chloroform (CHCl₃, δ = 7.28 ppm) occur relative to TMS on a spectrum recorded on a 300 MHz spectrometer? ANSWER
A 1 GHz (1000 MHz) NMR spectrometer is being developed, at what frequency and chemical shift would chloroform occur? ANSWER