The size of molecules of interest varies from small organic molecules, to biological molecules of middle size, and even to some macromolecules such as nucleic acids and proteins. Yoder, Charles D.
For larger nuclei, it is not immediately evident what the spin should be as there are a multitude of possible values. Nuclear spin angular momentum was first reported by Pauli in 1924 and will be described here.
The number of nuclei in each spin state can be described by the Boltzmann distribution. We now understand why the nucleus has a magnetic moment associated with it.
A full theoretical explanation for a single atom was developed by Bloch into a set of equations known as the Bloch equations. Where N upper and N lower represent the population of nuclei in upper and lower energy states, E is the energy difference between the spin states, k is the Boltzmann constant 1.
Spins configurations according to applied magnetic field. The projection of the vector on the z axis arbitrarily chosen , takes on discretized values according to m, where.
Now we are getting to the crux of NMR, the use of an external magnetic field.
There are some rules that the nuclei do follow with respect to nuclear spin. However, to most people spin seems like an abstract concept. Schaeffer,Introduction to multinuclear NMR: Those spins which align with the magnetic field are lower in energy, while those that align against the field are higher in energy. Each energy level is populated by the spins which have the same angular momentum.
During the NMR experiment several things happen to the nucleus, the bulk magnetization is rotated from the z axis into the xy plane and then allowed to relax back along the z-axis.
This is due to the fact there is no macroscopic equivalent of what spin is.
Jeremy K. Table 1. The atomic nucleus black of 2 H.
However, for those people who have taken an introduction to chemistry course have seen the concept of spin in electrons. For more information about satellite transitions please look at quarupole interactions. Bovey, Lynn Jelinski, Peter A.