The first HyDRA challenge for computational vibrational spectroscopy.

Vibrational spectroscopy in supersonic jet expansions is a powerful tool to assess molecular aggregates in close to ideal conditions for the benchmarking of quantum chemical approaches. The low temperatures achieved as well as the absence of environment effects allow for a direct comparison between computed and experimental spectra. This provides potential benchmarking data which can be revisited to hone different computational techniques, and it allows for the critical analysis of procedures under the setting of a blind challenge. In the latter case, the final result is unknown to modellers, providing an unbiased testing opportunity for quantum chemical models. In this work, we present the spectroscopic and computational results for the first HyDRA blind challenge. The latter deals with the prediction of water donor stretching vibrations in monohydrates of organic molecules. This edition features a test set of 10 systems. Experimental water donor OH vibrational wavenumbers for the vacuum-isolated monohydrates of formaldehyde, tetrahydrofuran, pyridine, tetrahydrothiophene, trifluoroethanol, methyl lactate, dimethylimidazolidinone, cyclooctanone, trifluoroacetophenone and 1-phenylcyclohexane-cis-1,2-diol are provided. The results of the challenge show promising predictive properties in both purely quantum mechanical approaches as well as regression and other machine learning strategies.

Doubly ionized OCS bond rearrangement upon fragmentation - experiment and theory.

The dissociation of OCS2+ ions formed by photoionization of the neutral molecule at 40.81 eV is examined using threefold and fourfold electron-ion coincidence spectroscopy combined with high level quantum chemical calculations on isomeric structures and their potential energy surfaces. The dominant dissociation channel of [OCS]2+ is charge separation forming CO+ + S+ ion pairs, found here to be formed with low intensity at a lower-energy onset and with a correspondingly smaller kinetic energy release than in the more intense higher energy channel previously reported. We explain the formation of CO+ + S+ ion pairs at low as well as higher ionization energies by the existence of two predissociation channels, one involving a newly identified COS2+ metastable state. We conclude that the dominant CO+ + S+ channel with 5.2 eV kinetic energy release is reached upon OCS2+ → COS2+ isomerization, whereas the smaller kinetic energy release (of ∼4 eV) results from the direct fragmentation of OCS2+ (X3Σ-) ions. Dissociation of the COS2+ isomer also explains the existence of the minor C+ + SO+ ion pair channel. We suggest that isomerization prior to dissociation may be a widespread mechanism in dications and more generally in multiply charged ion dissociations.

Abiotic molecular oxygen production-Ionic pathway from sulfur dioxide.

Molecular oxygen, O2, is vital to life on Earth and possibly also on exoplanets. Although the biogenic processes leading to its accumulation in Earth's atmosphere are well understood, its abiotic origin is still not fully established. Here, we report combined experimental and theoretical evidence for electronic state-selective production of O2 from SO2, a chemical constituent of many planetary atmospheres and one that played an important part on Earth in the Great Oxidation Event. The O2 production involves dissociative double ionization of SO2 leading to efficient formation of the [Formula: see text] ion, which can be converted to abiotic O2 by electron neutralization or by charge exchange. This formation process may contribute substantially to the abundance of O2 and related ions in planetary atmospheres, such as the Jovian moons Io, Europa, and Ganymede. We suggest that this sort of ionic pathway for the formation of abiotic O2 involving multiply charged molecular ion decomposition may also exist for other atmospheric and planetary molecules.