In 1923, Lewis wrote that the study of spectra and magnetism are the two best ways to learn about chemical bonding. This was quite perceptive of him, because during my PhD, finished almost 90 years later, I used exactly those methods to study chemical bonding. Some types of spectra are discussed here. In this section, we will describe the magnetic evidence Lewis used. You are familiar with normal magnets, like the ones used to stick restaurant menus to fridges. In this section, however, we are more interested in the magnetic properties of molecular materials, rather than metallic or semi-ionic solids. Most materials are non-magnetic. The molecular materials that are magnetic still wouldn't stick to your fridge, because they only really act magnetic in a magnetic field. Normally, each molecular magnetic has a random direction because the interactions between the molecules are weak. (Just like how molecular materials melt at lower temperature than metals or rocks.) If you put them in a magnetic field, if they are cold enough, they will start to line up with the field. If they are too hot, they will continue to move randomly. This is called paramagnetism.
Let's start by reviewing electric dipoles. An electric dipole is something that has separated
electrical charges. For instance, an HF molecule or a water molecule
have an electric dipole moment defined as
μ = q d
where q is the partial charge on each end of the molecule and d is the
distance between the charges. Generally, solvents with large dipoles
have high dielectric constants. In Coulomb's
law, the dielectric constant D reduces the force between charges.
| F = | kQq |
| Dr2 |
Although you can have a point charge, magnets are always dipoles (have a North and South pole). Just like solvents and other materials have dielectric constants, they also have magnetic permeabilities.
How do you measure magnetic moments or magnetic dipoles in molecules? The old way is to put the molecular material between the poles of a big magnet and see what forces are present. For instance, if you put a paramagnetic material (that is cold enough) between the N and S magnetic poles, the individual magnetic moments in the molecules will line up with the field and be attracted by the field. Thus if you weigh a paramagnetic material in a magnetic field, it will be heavier than if you weigh it outside the magnetic field. On the other hand, most materials are diamagnetic, and get lighter in magnetic fields, because they are repelled by the field. So the easiest way to see if a material is magnetic is to weigh it with and without a magnetic field. Now there is a fancier method also, called SQUID, which is too complicated to explain here.
Lewis correctly realized that molecules with an odd number of electrons (such as NO, with 7+8=15, but not CO, with 6+8=14) have some unusual properties. They are paramagnetic, usually strongly colored, and quite reactive. The huge majority of molecules have even numbers of electrons, and are diamagnetic. Thus, paramagnetism seems to be a result of unpaired electrons, while diamagnetism is a result of paired electrons. Transition metal compounds are often colored and magnetic; most compounds of C,H,O,N etc (such as in your body) have little color, are diamagnetic, and not so reactive. Basically, almost all molecules have all their electrons in pairs.
I won't explain how magnetometry via Superconducting Quantum Interference Devices works. However, I used it often during my PhD, and I will describe the process, because it's funny. You very carefully make a very pure sample of the material you want to measure, which in my case I did in a glove-box which protected the material from oxygen and moisture. I reached into the box with attached rubber gloves to make the samples. I put nitrile gloves on top of the rubber gloves on the inside of the box, because the rubber was always dirty (I also wore cloth gloves before putting my hands into the rubber gloves, so they wouldn't stick). I put the sample (carefully weighed, since I needed the exact mass for the analysis) in a thumbnail-sized plastic bag, roled up the bag to seal it, and put it into a gel capsule like you might have used to take medicine. Sometimes I covered it with melted wax. Then I stuck the gel capsule into the middle of a drinking straw of a particular brand. Then I put the straw into a bottle, taped the lid on, and rushed across town on the train, where I stuck the straw onto a stick and sent it down into a very expensive (millions $) machine. In a gel capsule, in a straw.