Chemical reaction and shift in magnetic susceptibility sign I'm trying to find an easy experiment involving a chemical reaction in which two paramagnetic reactants A and B react to give a product which is diamagnetic (or the reverse). The idea is to show that A is paramagnetic, B is paramagnetic, but A+B --> C is diamagnetic.

Or, another experiment: an experiment in which a change in temperature changes paramagnetic to diamagnetic (or the reverse). I've seen such an experiment but cannot remember the details. All I can say is that there was a color change and, at the same moment, a paramagnetic/diamagnetic shift
Seems to me any reaction involving the recombination of two radicals leads to a closed-shell diamagnetic species (which oxygen is definitely not!). I can't think of any other examples immediately. You could make singlet oxygen, which is diamagnetic, from paramagnetic triplet oxygen by irradiation, but then there is only one reactant. 

If you heat a paramagnetic substance enough so that the thermal energy can overcome the energy that is gained by the alignment of your magnetic moments, I believe the substance will not exhibit paramagnetism in a magnetic field above that temperature. So that seems doable. 

Edit: Actually, I found an example: ozone is diamagnetic and can be made from paramagnetic oxygen and an oxygen radical. So that could be a possible reaction where you have two paramagnetic reagents combining into a diamagnetic product.