Origin of Chemical Shifts
It is common to mention the frequency of an NMR instrument instead of its field. When someone says: I have in my laboratory a 100 MHz instrument, it means that a spectrometer where the protons precess with a frequency of 100 MHz (Lamor frequency) is available in the lab, Equation 1:
Where υ0 is the Lamor frequency, γ is the 1H’s gyromagnetic ratio (42.58 MHz/T), and B0 is the external field. In the example above, the spectrometer has a magnetic field of 2.35 tesla.
If the frequency for proton, in this case, is 100 MHz, why do we observe different frequencies for inequivalent protons in the same molecule? In a molecule, the presence of electrons generates local magnetic fields that slightly change the overall magnetic field experienced by different groups of protons. Those protons are said to be in a different local chemical environment or chemically non-equivalents. So, nuclei at different chemical environments (slightly different fields) will precess at slightly different frequencies; that is why we can observe many signals in an NMR spectrum. The range of chemical shift (δ) expected for 1H is in the order of 20 ppm, which represents a range of frequency of 2000 Hz in an instrument operating at 100 MHz (Equation 2).
Let’s see the above theory in practice: How many different frequencies do you expect to observe in a 1H NMR spectrum of the ethanol molecule (Figure 1)? To answer this question, you need to define how many different 1H groups the ethanol molecule possesses.
Figure 1: (a) Representation of the ethanol molecule and (b) highlight the sigma bond free rotation that allows some hydrogens to be chemically equivalent.
The ethanol has three different groups of hydrogens: the methyl protons (CH3), the methylene protons (CH2) and the hydroxyl proton (OH). Why are the three hydrogens of the methyl group considered chemically equivalent? That’s because the C-C bond has free and fast rotation, which means they are in the same local chemical environment. The same explanation can be extended to the two hydrogens of the methylene group. So, for the ethanol molecule, we will observe three signals with different frequencies in the NMR spectrum (Figure 2).
Figure 2: Simulation of the spectrum associated with the ethanol molecule using the software MNova.
The discussion never stops; stay tuned! The following blog will deal with the relation between the data stored in the time domain (FID) and in the frequency domain (spectrum). It will explain how the Fourier transformation works.
If you have any questions about chemical shift or about using benchtop NMR please don’t hesitate to reach out to us!