Vibrational Spectroscopy and Intermolecular Interactions
Even stable molecules interact with each other, and these interactions often represent the first step to reactivity. More importantly, they shape the appearance and physical properties of matter. This is relevant in both the life and the materials sciences. Because non-covalent interactions are usually subtle, they are quite challenging to describe accurately. Fairly high levels of quantum chemistry are often needed, although it is tempting and illustrative to decompose the interaction into electrostatics, polarization, Pauli repulsion and London dispersion contributions. Our group concentrates on determining the total interaction and its effect on conformation, on supramolecular complex formation, and on molecular vibrations, by vibrational spectroscopy.
Molecular vibrations are not only sensitive to the interaction of molecules, but also to the shape of their Born-Oppenheimer potential energy hypersurface. Anharmonic effects can be important, even for zero point vibrational motion and definitely for high frequency hydride stretching modes as well as low frequency intermolecular vibrations.
By measuring vibrational spectra (using direct absorption [128] and Raman scattering) for selected small molecular and supramolecular systems, one can experimentally probe the non-covalent forces and the anharmonic nature of the potentials and compare to theoretical predictions. We often do this for the purpose of testing the performance of theoretical models [168, 173, 174, 177, 178, 179, 182, 191]. We establish experimental benchmarks to validate or disqualify these theoretical methods for applications to larger systems. If experiment agrees with theory, this can be due to fortuitous error cancellation or for a good reason. To rule out the former, it is necessary to vary the investigated systems in systematic ways [184]. This is a strength of chemistry, which we explore extensively. We thus put vibrational spectroscopy at the service of theoretical chemistry [154]. To maximize the theory-experiment interaction, new instruments may have to be developed [186] and these instruments have to be characterized thoroughly [187].
In recent years, we were able to address a range of questions in this field (answers to be found in the cited papers from the publication list):
What is the experimental binding energy of the simplest doubly hydrogen-bridged organic dimer - that of formic acid [122]? How can one visualize this molecular aggregation [159] and induce isomerisation [171, 182]?
At which length does a linear alkane or other polymer segment start to prefer a hairpin structure over a stretched conformation - due to non-covalent interactions between the chain ends [124, 134, 135, 147, 156, 161]?
How does energy jump from one OH stretching mode to the next in a water cluster or formic acid dimer - due to excitonic coupling of equivalent hydride bonds? How does it flow into other vibrations of the molecular assembly [183]? How do hydrogen atoms jump on a femtosecond timescale [127, 131, 165]? Can they delocalize between non-equivalent minima [192]?
Can one disentangle the different anharmonic contributions to the potential hypersurface of a simple intermolecular organic hydrogen bond - that of methanol dimer [136, 137, 139, 149]? Is (an)harmonic theory able to describe very weak hydrogen bonds to dinitrogen [155, 158, 163, 166]? Or strained hydrogen bonds between two neighboring OH groups [189]?
How comes that alcohols and ketones attract each other in the gas phase, but demix in the solid phase [140]?
How does an alcohol interact with an alkene [141, 145, 164, 170]?
How do the simplest peptide models and sugar models aggregate [123, 125, 153]?
How can one control almost at will whether a solvent molecule prefers the ether oxygen or the π system of an aromatic ether - tipping an intermolecular scale? Can one design other kinds of intermolecular balances [143, 144, 148, 151, 152, 153, 157, 158, 160, 162, 164, 169, 172, 176, 181, 188]?
How can one control and detect the relative chirality of flexible molecules by aggregation [126, 150, 167, 179, 180, 190]?
(gif created with molden)
Is there anything special when radicals interact with solvent molecules [185]?
To answer these and other questions, we had to develop unique supersonic jet spectrometers, which prepare molecules and molecular clusters at low temperature without environmental influence.
Our research is embedded into a national priority program on London dispersion interactions.