chapter  2
37 Pages

31P Nuclear Spin Relaxation

ByDouglas C. McCain

Nuclear magnetic resonance (NMR) spectra provide information about the static or time-averaged properties of a sample. The chemical shifts, coupling constants, and peak intensities may be used to identify molecules and to measure their relative concentrations. An isolated P nucleus in a static magnetic field will exist in one of two quantum states, α or β. A magnetic field oscillating at frequency vP induces transitions between states α and β. Such a field is produced in an NMR spectrometer by radiofrequency (rf) power flowing through a coil that encloses the sample. Spin-lattice relaxation involves an exchange of energy between the spins and a thermal reservoir, but free exchange is greatly hindered by the fact that spins respond to magnetic forces, while energy flows through the lattice mainly by way of electrical or mechanical forces. The behavior of P nuclei is correctly described by quantum mechanics, but as is so often the case, the resulting description is difficult to visualize.