Transmission spectra of solid methanol and its isotopologues recorded at 10 K, highlighting the CH₃ deformation region. Modes are labelled δ for the in-plane bend. δas and δs denote the asymmetric and symmetric components, respectively, with the vibrating group given in parentheses. For CH₃OH, the dashed rectangle marks the CH₃ deformation region. Corresponding peak positions are listed in Table 5. Spectra are vertically offset for clarity.