Diagonal and off-diagonal anharmonicity in hydrogen bonds:Direct detection by infrared spectroscopy in the gas phase

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Chemistry is mostly about bond breaking and rearrangement of atoms, whereas spectroscopy and molecular dynamics still focus to a large extent on the harmonic oscillator which does not allow for such processes. In this project, we will move away from the harmonic picture for hydrogen bonds in the gas phase in three experimental directions: by studying overtones of hydrogen bond donor stretching vibrations, by characterizing librational motions orthogonal to the hydrogen bond direction, and by investigating the electrophilic approach to CC multiple bonds. Essential goals are the poorly known experimental anharmonicity constants in hydrogen bonds along high frequency stretching and low frequency librational coordinates as well as the subtle effects induced by OH docking to C=C double bonds. Besides spectral shifts, the less accessible pronounced intensity effects due to vibrational charge dislocations shall be addressed. Reasons for a lack of experimental data in this field include unfavorable transition moments, multidimensional couplings, feeble light sources and difficult detection conditions. Our recent work indicates that these limitations can now be overcome for several model systems. One may thus hope to understand in a quantitative way the perhaps simplest organic hydrogen bond, the one between two methanol molecules. A 50% discrepancy between experiment and state of the art theory for the change of the donor vibration in the hydrogen bond waits to be explained. If successful, the project will provide theoretical approaches to the hydrogen bond vibrational dynamics with detailed experimental benchmarks, allowing them to accurately judge the underlying potential energy hypersurfaces. Vice versa, theoretical results can foster the experimental studies planned in this project. A quantitatively new perspective on more than 40 years of calculated spectral red shifts could emerge.