How Vibrational Notations Can Spoil Infrared Spectroscopy: A Case Study on Isolated Methanol

Unraveling methanol's infrared spectrum has challenged spectroscopists for a century, with numerous loose ends still to be explored. We engage in this exploration based on experiments of isolating single methanol molecules in solid argon and neon matrices. We report infrared spectra of methanol in its natural isotopic composition and with partial and full deuteration. These experiments are accompanied by calculating wavenumbers involving anharmonicity and mode-coupling based on the vibrational configuration interaction approach. This allows for an unambiguous assignment of all fundamentals and resonances in the mid-infrared spectrum. An increasing degree of deuteration lifts resonances and aids in assigning bands uniquely. It also becomes evident that different notations typically used in chemistry or physics to describe molecular vibration from spectroscopy fail to describe the spectra appropriately. We highlight the shortcomings and suggest a more elaborate analysis using Sankey diagrams to unambiguously identify spectral features. Consequently, we demystify debated resonances occurring from various stretches and deformations of the methyl group.