Spin-orbit coupling allows anomalous transitions in spectra involving states of the same degenerate multiplicity, due to the relaxation of the spin selection rule ΔΣ = 0. This effect is discussed for doublet-doublet transitions in asymmetric top molecules. All the nonvanishing and independent transition moments, including those responsible for the anomalous transitions, are determined by symmetry and time reversal arguments. Rotational intensities are given for transitions between case (b) near-symmetric top eigenstates. © 1974.
di Lauro, C. (1974). Spin-allowed and forbidden rotational intensities in doublet-doublet transitions in asymmetric top molecules. Journal of Molecular Spectroscopy, 51(2), 356–362. https://doi.org/10.1016/0022-2852(74)90064-2