How Vibrational\nNotations Can Spoil Infrared Spectroscopy:\nA Case Study on Isolated Methanol

Unraveling methanol’s infrared spectrum has challenged\nspectroscopists\nfor a century, with numerous loose ends still to be explored. We engage\nin this exploration based on experiments of isolating single methanol\nmolecules in solid argon and neon matrices. We report infrared spectra\nof methanol in its natural isotopic composition and with partial and\nfull deuteration. These experiments are accompanied by calculating\nwavenumbers involving anharmonicity and mode-coupling based on the\nvibrational configuration interaction approach. This allows for an\nunambiguous assignment of all fundamentals and resonances in the mid-infrared\nspectrum. An increasing degree of deuteration lifts resonances and\naids in assigning bands uniquely. It also becomes evident that different\nnotations typically used in chemistry or physics to describe molecular\nvibration from spectroscopy fail to describe the spectra appropriately.\nWe highlight the shortcomings and suggest a more elaborate analysis\nusing Sankey diagrams to unambiguously identify spectral features.\nConsequently, we demystify debated resonances occurring from various\nstretches and deformations of the methyl group.