Giant K-doubling and in-plane/out-of-plane mixing in the asymmetric methyl-bending bands of CH₃SH

In analyzing high-resolution spectra of the methyl-deformation bands of methyl mercaptan recorded at the Canadian Light Source synchrotron, we have encountered interesting interactions between certain levels of the ν₄ in-plane asymmetric CH₃-bending mode and its ν₁₀ out-of-plane bending partner below. The origin of the K = 0A ν₄ substate is just 0.2 cm⁻¹ higher than that of the K = 2A ν₁₀ substate, while the K = 0E ν₄ origin is only 0.035 cm⁻¹ below the K = 2E ν₁₀ origin. These very close accidental near-degeneracies lead to substantial perturbations in the spectrum. For the former, the A⁺/A⁻ asymmetry K-doublet coupling rules are such that the A⁻ component of the 2A ν₁₀ doublet interacts and mixes strongly with the 0A⁺ ν₄ levels whereas the 2A⁺ component is unaffected. The 2A⁻ levels are pushed rapidly downwards by the coupling creating an extremely large apparent K = 2A asymmetry splitting. We call this “giant K-doubling” by analogy with a comparable phenomenon seen for methanol. The 0A⁺ ν₄ state, in turn, is perturbed upward and passes through the descending K = 1A⁺ ν₄ state between J = 22 and 23, leading to distinct local perturbations near the level-crossing. The 0E ν₄ and 2E ν₁₀ coupling produces a correspondingly strong repulsion and mixing between those two substates, and gives rise to a forbidden K = 0 ← 3E intermode sub-band in the spectrum via intensity borrowing.