Can magnetic fields affect a chemical reaction? This question was asked recently in an interview and I just said that "Yeah, like if the reaction involves ions or paramagnetic species". But the interviewer went on and asked me to elaborate on HOW will the reaction change, and I was stumped. 

So what I am asking here is what will happen if I allow a chemical reaction to take place in a live MRI machine?
Here is a nice review paper on the subject: Rodgers, C. T. Magnetic field effects in chemical systems. Pure Appl. Chem. 2009, 81 (1), 19–43. DOI: 10.1351/PAC-CON-08-10-18.

From the abstract:


  Chemical reactions that involve radical intermediates can be influenced by magnetic
  fields, which act to alter their rate, yield, or product distribution. These effects have
  been studied extensively in liquids, solids, and constrained media such as micelles. They  may be interpreted using the radical pair mechanism (RPM). Such effects are central to the field
  of spin chemistry of which there have been several detailed and extensive reviews.