Nuclear spin conversion, which involves the interconversion of molecular states differing in nuclear spin symmetry, is fundamental to molecular spectroscopy, low-temperature kinetics, astrochemistry, and quantum control. Traditional theoretical models primarily focus on spin–spin (SS) and spin–rotation (SR) interactions, which explain conversion in rigid molecules through weak internal magnetic couplings. However, the full dynamics of molecules with large-amplitude internal motion have not been captured. This study introduces the spin–torsion (ST) interaction as a mechanism for spin isomer conversion, in which the magnetic field generated by internal rotation couples to nuclear spins. We apply this mechanism to methanol $\left({\mathrm{CH}}_3\mathrm{OH}\right)$, a prototypical internal-rotor molecule, and demonstrate that ST provides a non-negligible contribution. The most important level pair for the methanol spin isomer conversion induced by ST is identified as the $(J^{\prime }=13,$ $K^{\prime }=0,$ ${\sigma }^{\prime }=1)–(J=13,$ $K=1,$ $p=1)$ of the ground torsion state $v_t=0$, with a pronounced configurational selectivity in the torsional coordinate. The nuclear ST contribution to the conversion rate is $4.0\times {10}^{-4}{\text{s}}^{-1}$.

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• Atomic & molecular structure
• Fine & hyperfine structure
• Molecules