Product operator (NMR)
In the various fields of nuclear magnetic resonance, the product operator mathematical formalism is often used to simplify both the design and the interpretation of often very complex sequences of radio frequency electromagnetic pulses applied to samples under study. Basically, it is a short hand mathematical construct, a set of equations, that is used in place of more complex, although equivalent, matrix multiplication. The formalism uses a rotating frame of reference, in which the central irradiation frequency, say 800 MHz, is fixed on the X- or Y-axis, and the magnetic field, by convention, points towards the postive Z-axis. By convention, I and S indicate magnetic vectors associated with protons or heteroatom, respectively. Subscripts are used to indicate the axial orientation of the magnetic vector. At equilibrium, the net proton magnetic vector is thus Iz.