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).