Magnetic field effect in CH3F nuclear spin conversion

We predict the influence of an external homogeneous magnetic field on nuclear spin conversion in molecules. The effect arises either from the Zeeman splittings or from the magnetic shielding of the molecular nuclei. The magnitude of the effects in CH3F molecules is estimated. The Zeeman effect can be large. The shielding effect is found to be relatively small at reasonable magnetic fields, but it is shown that the magnitude of the effect can be enhanced by using an alternating magnetic field at a resonant frequency.