Enhancement of nuclear spin transitions as a resonance effect of isotope substitution

Mixing of different components of the total nuclear spin in the rovibrational states of isolated molecules is extremely weak. It has only been observed in hyperfine spectra for few systems, including S2Cl2, SiF4, PH3, and SF6. We perform variational calculations of nuclear quadrupole interactions in the rotational spectra of S2Cl2 and CH2Cl2 and analyze the effects of breaking the molecular symmetry by isotopic substitution of one of the chlorine atoms. This symmetry breaking significantly enhances the mixing of nuclear spin states and produces a distinct spin polarization pattern with opposite spin orientations on the different isotopes. This enhancement arises as a resonance effect driven jointly by differences in isotopic masses and nuclear quadrupole coupling constants and gives rise to electric and magnetic dipole transitions between states with different relative orientations of the nuclear spin.