Monohydrate complexes of organic molecules at low temperature in vacuum isolation are natural benchmark systems for the ability of electronic structure theory and vibrational models to describe (micro-)solvation. The talk will focus on the OH stretching fundamentals and discuss a recently discovered anharmonic resonance which is systematically observed in ketone monohydrates and perhaps other hydrate clusters, whenever the OH fundamental falls in a narrow window slightly above 3500 cm^-1. It is suggested to be of 1-3 Darling-Dennison character, involving the water bend overtone and a water libration (b2lib) [1]. This study was made possible by a new gas recycling concept for jet FTIR spectroscopy [2], which allows to study costly isotopes and chemicals and to use expensive carrier gases.

The b2lib observation provides the starting point for a concerted effort to reliably model water OH stretching modes, when hydrogen-bonded to different organic molecules, whether resonance perturbed or not. In the HyDRA blind challenge starting this year [3], theory groups are invited to suggest all kinds of ab-initio, density functional, semiempirical, empirical, machine learning or other approaches to predict water OH stretching wavenumbers for a test set of 1:1 organic hydrate complexes. For this, they can build on an initial training set [4] and the spectroscopy community is invited to extend this training set into a growing data base. Progress will be discussed at an International Bunsen Discussion Meeting in Göttingen next March [5]. After completion of this initial, rather focussed HyDRA blind challenge next year, it is hoped that further research groups will extend the effort to wider ranges of hydrogen bond strength, to other spectroscopic observables, and to other perspectives or solvents. In the end, the most consistent and most economic tools to assist experimental spectroscopy in predicting key observables for hydrogen-bonded clusters might be identified for the benefit of our community.

If time permits, systematic benchmarking for the vibrational spectroscopy of alcohols [6] and vicinal diols will also be discussed.            

[1] Taija L. Fischer, Till Wagner, Hannes C. Gottschalk, Arman Nejad, Martin A. Suhm, J. Phys. Chem. Lett., 2021, 12, 138–144.

[2] Hannes C. Gottschalk, Taija Fischer, Volker Meyer, Reinhard Hildebrandt, Ulrich Schmitt, Martin A. Suhm, Instruments, 2021, 5, 12.

[3] Hydrate Donor Redshift Anticipation, https://qmbench.net/challenge.htm

[4] Training Set Submissions, https://qmbench.net/hydra_phaseI_sub.htm

[5] International Bunsen Discussion Meeting, 2022, https://www.uni-goettingen.de/en/645339.html

[6] Robert Medel, Martin A. Suhm, Phys. Chem. Chem. Phys., 2021, 23, 5629–5643.