HyDRA-II: spectroscopic results and BEsT guesses for the mono- and dihydrate blind challenge

HyDRA blind challenges aim at unbiased forecasts of water vibrations in vacuum-isolated hydrate complexes of organic molecules by different computational protocols. In this work, directly after the completion of the HyDRA II challenge and ignorant of its computational results, the experimental findings for the 9 new solvates with a total of 12 hydrogen-bonded water OH stretching wavenumbers are disclosed and discussed. We further summarise and analyse the 70 entries of the training database, which were made available at the start of the challenge. Monohydrates of 4 other previously uncharacterised solvates are added to support the assignment of the blind test entries and to widen the future training database. The compounds for which gas phase monohydrate OH stretching fundamentals are reported for the first time comprise a difluorobenzonitrile, a difluorobenzoic acid, eucalyptol, an iodo-methyl-pyrazole, a methoxypyridine, Hünig's base, a pyrrolidine ethanol, TEMPONE, diethylhydroxylamine, pyrazine, pyrimidine, a triazine and a chiral chloropropionic acid. An experimental bridging approach (Bridging Experiments by Theory, or in short BEsT guess) using inexpensive harmonic density functional theory predictions from the training database for educated guesses of unknown vibrational wavenumbers is proposed to assist the future evaluation of the blind challenge. Proposals on how to compare computational predictions to experiment depending on the employed level of anharmonic perturbation are made. It will be interesting to see how different computational protocols perform in the wide range between outstanding predictions (root-mean-square deviation <10 cm^-1) and placebo-quality deviations (>100 cm^-1) for the 12 disclosed wavenumbers.