Understanding Fermi resonances behind the complex vibrational spectra of the methyl groups in simple alcohol, thiol, and their ethers

Vibrational spectra of the methyl groups in CH₃SH, CH₃SCH₃, CH₃OH, and CH₃OCH₃ monomers were measured to analyze their complex features as a result of bend/umbrella-stretch Fermi resonance. Multiple bands were recorded between 2800–3000 cm⁻¹ associated with the vibrations of the methyl groups in CH₃OH, and CH₃OCH₃. On the other hand, the corresponding spectra of CH₃SH and CH₃SCH₃ are relatively simpler with main peaks between 2900 to 3000 cm⁻¹. Theoretical ab initio anharmonic algorithm using six normal modes on a methyl group with potential energy surface at CCSD(T)-F12/aug-cc-pVTZ quality is able to account for all the experimentally observed features across these four species. In CH₃SCH₃ and CH₃SH, all three C-H stretch fundamentals are above 2900 cm⁻¹, but the fundamental of umbrella mode is lower than 1350 cm⁻¹, thus only the overtone states of two bending modes are able to borrow sufficient intensities. In CH₃OCH₃ and CH₃OH, umbrella modes have frequencies similar to the bending modes and the C-H stretch fundamentals are red-shifted to 2850–2950 cm⁻¹ leading to more complex vibrational features due to better energy-matching condition.