Carboxylic acid aggregation: From anharmonic benchmarks to chemical imaging

• Applicant: Professor Dr. Martin Suhm
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 388861488

• Final Report Year: 2022

Final Report Abstract

The complex formed by two formic acid molecules is one of the best studied model systems for hydrogen bonds. It is the most elementary organic compound with two such particularly symmetric molecular linkers. This symmetry requires the combined use of infrared and Raman spectroscopy in supersonic jet expansions if one wants to understand its vibrational dynamics. The project succeeded in characterizing all molecular framework vibrations, raising hope for an understanding of the rather chaotic energy flow after exciting the particularly fast OH stretching vibrations which modulate the hydrogen bonds. Of particular interest was the splitting of the formic acid vibrations into in-phase and out-of- phase motions of the binding partners, indicative of how they synchronize their motion in the complex. Furthermore, the arrangement of a single formic acid molecule around a pair of such molecules was systematically characterized. It yields information on further aggregation towards the condensed phase, but also on a metastable dimer connected by a single hydrogen bond which was so far only detected under embedded conditions. These quantitative experimental findings are now available to theoretical chemistry, to test their electronic structure and nuclear dynamics tools for a realistic hydrogen bonded complex. First methodical strengths and weaknesses have already been uncovered, others are under investigation in different theory groups. This benchmarking process is also the topic of a local research training group, whose start was supported by the present project. It involves the variation of molecular residues attached to the carboxylic acid group which is so typical in chemistry and helps to distinguish accidental from systematic simulation success. About 150 years after the first observations of anomalous gas phase aggregation, our experimental understanding of formic acid dimer and trimer has once more made substantial progress. It is now the turn of numerical quantum chemistry to demonstrate how well it can model the molecular reality.

Publications

• Dinitrogen as a Sensor for Metastable Carboxylic Acid Dimers and a Weak Hydrogen Bond Benchmarking Tool. J. Phys. Chem. A. 122 (2018) 2933-2946
  Sönke Oswald, Enno Meyer, Martin A. Suhm
  
• Vibrational Exciton Coupling in Homo and Hetero Dimers of Carboxylic Acids Studied by Linear Infrared and Raman Jet Spectroscopy. J. Chem. Phys. 149 (2018) 104307
  Katharina A. E. Meyer, Martin A. Suhm
  
• Carboxylic Acids Under Vibrational Scrutiny: Experimental Reference Data to Benchmark Quantum Chemical Calculations, Dissertation, 2019, Universität Göttingen
  Katharina A. E. Meyer
  
• Concerted Pair Motion Due to Double Hydrogen Bonding: The Formic Acid Dimer Case. J. Indian Inst. Sci. 100 (2020) 5–19
  Arman Nejad, Martin A. Suhm
  
• Shifting formic acid dimers into perspective: vibrational scrutiny in helium nanodroplets, Phys. Chem. Chem. Phys. 22 (2020) 9637-9646
  Katharina A. E. Meyer, Julia A. Davies, Andrew M. Ellis
  
• CC-stretched formic acid: isomerisation, dimerisation, and carboxylic acid complexation, Phys. Chem. Chem. Phys. 23 (2021) 17208–17223
  Katharina A. E. Meyer, Arman Nejad
  
• Slow monomer vibrations in formic acid dimer: Stepping up the ladder with FTIR and Raman jet spectroscopy, J. Chem. Phys. 155 (2021) 224301
  Arman Nejad, Katharina A. E. Meyer, Franz Kollipost, Zhifeng Xue, Martin A. Suhm