London vs. Keesom and Debye forces: From FTIR cluster spectroscopy of ambivalent alcohol complexes towards intermolecular energy balances

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London dispersion forces can tip the balance between different hydrogen bond docking sites of an alcohol molecule to a multifunctional acceptor molecule. By selecting systems with low barriers between energetically nearly degenerate docking sites and by working in a supersonic jet environment, very subtle energy differences can be detected and quantified with linear infrared spectroscopy. By studying a large number of systems with varying dispersion anchors, fortuitous error cancellation can be ruled out and subtle deficiencies of quantum chemical methods in providing balanced descriptions of all intermolecular forces can be uncovered in a systematic way. By choosing chemically similar docking sites, distorting effects from anharmonic zero point energy can be minimized. By including chiral donor and acceptor molecules, chirality recognition effects mediated by dispersion forces can be studied as well. The project concentrates on carbonyl lone pair and alkene π bond face choices, along with the oxygen/π competition, which was in the focus of the first funding period. Improved nozzle and sample preparation designs will be explored. Intense experimental cooperation with UV/IR and microwave experts in the priority programme is planned. Besides testing quantum chemical predictions for intermolecular interactions, such theoretical methods will be used to separate and visualize the London dispersion contribution of substituents in cooperation with theory groups. In the end, the new concept of intermolecular energy balances to probe London dispersion interactions in the gas phase at low temperature will complement the popular intramolecular torsional balances in solution by providing energy-focused information free of bulk solvent influence.