HyDRA Challenge
Starting date: February 2021 (Phase I)
Blind challenge start: 24.08.2021 (Phase III)
Submission deadline: 23.02.2022
The HyDRA challenge for theoretical spectroscopy
Hydration is arguably one of the most important phenomena in physical chemistry, on a planet where water is the most abundant molecule. There are different ways to approach this complex phenomenon. Here, a molecular bottom-up approach is addressed. How does a single water molecule change when it attaches to an organic solute? Its most prominent and solvate-sensitive antenna is the hydrogen-bonded OH stretching fundamental OHb, which has a high visibility in the infrared spectrum. The non-bonded OHf mode also contains important vibrational coupling information. Furthermore, the hindered rotations and translations associated with the coordination of a water molecule provide detailed information on the restoring forces of the hydrogen bond.
Experimental accessibility has to be taken into account when making a choice among these probes of hydration. Hindered intermolecular motions are best addressed by matrix isolation (e.g. https://doi.org/10.1021/acs.jpca.9b01497) and the OHf mode suffers from spectral overlap and a low sensitivity to the hydrogen bond, leaving OHb as the single most promising vibrational probe for hydration in vacuum-isolated 1:1 complexes, which are particularly attractive from the perspective of theoretical accessibility. Low temperature is mandatory to remove spectral complexity and is traditionally realized by supersonic expansion in rare gas mixtures. Size- and conformationally selective spectroscopic techniques are helpful in discriminating different 1:1 complexes from each other and from larger assemblies. However, they are mostly restricted to aromatic systems, despite occasional claims of more universal applicability. Because it would be too narrow to sample only aromatic binding partners, we contribute results from an alternative jet FTIR technique, which has no such restrictions and - once critically checked by more rigorously selective techniques in accessible cases - offers a broader and spectrally definitely linear view on the hydration effects in organic matter.
The role of theory in understanding hydration is multifacetted. Initially, it helps to assign the experimental spectra. In the future, it may even replace experiment to some extent. However, before the latter becomes realistic, there is a need for unbiased tests of different predictive theoretical approaches by experiment.
On the basis of a growing experimental vibrational database on 1:1 hydrate complexes between organic molecules and water monomers, we propose a blind challenge for different theoretical approaches to predict the OHb stretching fundamental vibration. We call it HyDRA, for Hydrate Donor Redshift Anticipation. Tongue-in-cheek, the acronym also calls to mind greek mythology and some properties of freshwater polyps. All kinds of theoretical aproaches from scaled harmonic models over explicit anharmonic treatments (variational, perturbational, ...) to machine learning algorithms are invited. Recent experimental evidence indicates that different kinds of anharmonic effects may be involved (suggesting other interpretations of the acronym HyDRA like "Hydrogen bond Donor Resonance Assignment" or "Hydrate Docking: Resonances and other Anharmonicities") but their regularity indicates that one can also model the experimental data by implicit inclusion of such anharmonic effects, thus opening the field of competing models.
In Phase I, the experimental vibrational spectroscopy community was invited to make suggestions for experimentally well-secured database entries which could serve as a training pool for the models in the blind challenge. These suggestions were carefully curated with respect to diversity, complexity, experimental suitability and rigor of assignment and a compact final training data set (perhaps around 10 systems) was then put together. You can see the selected systems here.
In Phase II, the experimental structural spectroscopy community was invited to make suggestions for systems which have not yet been investigated vibrationally but appear attractive for the blind challenge based on structure determination. These suggestions were again curated, and suitability for the local jet-FTIR approach carefully considered. They are complemented by our own suggestions for unexplored systems.
In Phase III, a final test data set (probably less than 10 systems) was presented together with the call for theory groups to contribute to the blind challenge in a 6-month time window. Some of the experimental results for the test data set were recorded before the start of the blind challenge, but will be kept confidential. Others will only be recorded during the 6-month window and also be kept secret afterwards (double-blind component). Therefore, it is possible that the initial test data set will shrink somewhat, due to unexpected experimental difficulties which may prevent a unique assignment in particular cases or due to the unexpected discovery or disclosure of experimental data before the deadline for theory contributions.
In Phase IV, the standardized submissions received until the pre-announced deadline will be analyzed together with the experimental data and published jointly, discussing the strengths and weaknesses of the different models for the training and the test dataset, but perhaps also suggesting experimental misassignments.
To get an idea about the planned blind challenge, we recommend the supporting information published under https://doi.org/10.1021/acs.jpclett.0c03197, which describes part of the intended training set and introduces a robust anharmonic resonance which may somewhat limit the performance of purely harmonic models for some systems. Furthermore, an earlier blind challenge for methanol solvation of furans with a much wider scope and focus on relative energies is described under https://doi.org/10.1063/5.0004465. Note that relative energy is not a direct performance measure for the present challenge, but methods which predict the correct lowest (and optionally second-lowest) conformation may have an advantage in predicting their spectroscopic parameters.
Feel free to contact Martin Suhm if you have questions about the planned blind challenge at this early stage or want to join the mailing list for updates on the project. Currently, Phase I started in February 2021 and Phase II in July 2021. Phase III was kicked off in 24.08.2021 and Phase IV will start 6 months later, with the aim of submitting the manuscript for publication in 2022.
Opportunities to discuss aspects of this blind challenge will be a CECAM workshop in Paris planned for November 2021 and an international Bunsen discussion meeting in Göttingen planned from 29-31st of March 2022.