Macro view of NMR

Magnetism instead of transitions

NMR at the level of the sample rather than the nucleus requires a complete change of thinking and a lot of math. The focus on the main page will be the thinking part, the math can be accessed on separate pages via links. The interaction will be seen as a magnetic induction rather than photon absorption. A transmitter causes changes in the magnetic fields of a sample, which then induce currents in a detector coil which are picked up by a receiver. So understanding NMR at a macro level requires intimate knowledge of magnetic moments and induction.

First the sample has to be defined. Ethanol is a good starting sample for analysis. It is a simple molecule, a liquid at room temperature with good flow rate (low viscosity) and slow evaporation rate (low vapor pressure). The molecule has 6 hydrogens, 2 carbons and an oxygen (CH₃CH₂OH), and there is not much self-association at room temperature. The sample container will be a 5mm glass tube: picture of NMR tube

About 0.75ml of ethanol in the tube will make a perfect sample for introduction to NMR.

All matter that is charged also has a magnetic field. Why? Good luck with that. Technically, if a charge isn't moving it isn't supposed to have a magnetic field, but since movement is relative, everything is moving compared to something else, so everything that is charged has a magnetic field. At any rate, since every atom has positively charged protons in the nucleus and negatively charged electrons surrounding the nucleus, there is a net magnetic field around each atom. Similarly, there is a net magnetic field around every molecule since the atoms are bonded to each other via electrons which all have charge and thus magnetic fields.

In summary, in the NMR tube of ethanol there is a solution of tiny bar magnets.

On the benchtop, the bar magnets floating around in solution are randomly oriented, so there is no net magnetic field due to the ethanol molecules. If placed into a strong magnetic field however, some of the floating bar magnets will line up with the field, generating a net magnetic field. If magnetism was the only factor then the floating bar magnets would line up with the external field and that would be it, nothing more would happen. The key part about NMR is that these magnets are also spinning, so there is torque that causes a precession around the axis of the external field. This precession frequency is what is probed with a transmitter.