Ch 13 - NMR basics

Chapter 13: Spectroscopy

Basic Principles

In NMR, electromagnetic (EM) radiation is used to "flip" the alignment of nuclear spins from the low energy, spin aligned state to the higher energy spin opposed state.

The energy required for this transition depends on the strength of the applied magnetic field (see below). The energy difference between the spin states is quite small and corresponds to the radio frequency range of the EM spectrum.

As this diagram shows, the energy required to cause the spin-flip, DE, depends on the magnetic field strength at the nucleus. With no applied field, there is no energy difference between the spin states, but as the field increases, so does the separation of energies of the spin states and therefore so does the frequency required to cause the spin-flip.

DE' > DE since H_o' > H_o

The basic arrangement of an NMR spectrometer is shown to the left. The sample is positioned in the magnetic field and excited via pulsations in the radio frequency input circuit. The realigned magnetic fields induce a radio signal in the output circuit which is used to generate the output signal. Fourier analysis of the complex output produces the actual spectrum. The pulse is repeated as many times as necessary to allow the signals to be identified from the background noise.