ABSTRACT
Elementary particles such as protons, electrons and neutrons exhibit behavior that can be explained in terms of a spinning motion for which the angular momentum is quantized. This property is called spin. There are two different ways of looking at nuclear spin. One way relies strictly on quantum mechanics, and the other on classical physics. The latter is referred to as the "spinning top" model. In any ensemble of quantum oscillators, the populations of the available quantized states are determined at thermal equilibrium by the relative energy of those states, with lower energy states being more populated. In many forms of spectroscopy, the energy difference between states is large enough so that the lowest energy state, or ground state is almost completely populated at thermal equilibrium, with negligible population of excited states. The true value of resonance spectroscopy (NMR) lies in the ability of the spectroscopist to detect coupling between nuclear spins, and to delineate coupling pathways.
