Measured g-factor of a free-radical changing as a function of something. But I cant work out what?

I'm looking at EPR of a paramagnetic organic molecule in solution that has a very stable free-radical signature with gfactor ~2.0035. All good. Except when watching one particular experiment I saw that the g-factor was drifting [the zero-crossing was slowly moving to the right over the course of 24hrs, i.e. the g-factor was dropping]. The Vpp was not changing. I am happy to believe that the molecule was self-aggregating. I would be even happier if there was a theory that related g-factor to particle size (e.g. via tumbling rate) because then I could estimate aggregation by looking at the gfactor. And that is where I get stumped. Are there any theoretical papers that discuss rotation effects on organic free radicals?


Or maybe my EPR machine is just drifting :-(


