Violation of nuclear-spin symmetry in collisions of symmetric tops

Evidence of nuclear-spin symmetry violation in molecular collisions is presented. Measurements of the ortho-para conversion rates in 12CH3F embedded in an gaseous oxygen or nitrogen shows that oxygen speeds up the nuclear spin conversion by 2×10−4 s−1/Torr contrary to nitrogen which slows down the conversion by 1×10−4 s−1/Torr. The difference is attributed to direct collisional transitions between the ortho and para states in CH3F due to the large oxygen magnetic moment, which is much smaller in the case of nitrogen.