The Power of NMR: The Beginnings

Nuclear magnetic resonance (NMR) spectroscopy involves the excitation of nuclei by electromagnetic radiation in the radio-frequency range of the electromagnetic spectrum. For a nucleus to absorb energy from radiowaves in this way, it must have the quantum mechanical property of spin. A spinning nucleus, such as that of the hydrogen atom, will adopt one of only two possible states when placed in a magnetic field. (In NMR, the hydrogen nucleus is often referred to as a proton, and is given the abbreviation ¹ H.) As the strength of the magnetic field is increased, there is a proportional increase in the energy 'gap' between these two states, as is shown in Fig 1. We can predict the resonant frequency at which any spinning nucleus will absorb energy from radio-frequency radiation (ν), as it jumps from the lower energy state to the upper state, by using the fundamental equation: 

ν = γB₀/2π       (i)

where  B₀ is the magnetic field which the nucleus experiences, and ν is a proportionality constant, which is specific to that nucleus.