Predicting accurate binding energies and vibrational spectroscopic features of interstellar icy species. A quantum mechanical study

Abstract

In the coldest, densest regions of the interstellar medium (ISM), dust grains are covered by thick ice mantles dominated mainly by water. Although more than 300 species have been detected in the gas phase of the ISM by their rotational emission lines within the radio frequency range, only a few were found in interstellar ices, e.g. CO, CO₂, NH₃, CH₃OH, CH₄ and OCS, by means of infrared (IR) spectroscopy. Observations of ices require a background-illuminating source for absorption, constraining the available sight lines for investigation. Further challenges arise when comparing with laboratory spectra due to the influence of temperature, ice structure and the presence of other species. In the era of IR observations provided by the James Webb Space Telescope (JWST), it is crucial to provide reference spectral data confirming JWST's assigned features. For this purpose, this study addresses the adsorption of the aforementioned species on water ice surfaces and their IR features by means of quantum chemical computations grounded on the density functional theory (DFT) hybrid B3LYP-D3(BJ) functional, known to give reliable results for binding energy and vibrational frequency calculations, including IR spectra simulation. The calculated binding energies and IR spectral data are presented in the context of experimental spectra of ices and the new findings from the JWST, which have already proven to be insightful thanks to its unmatched sensitivity. We show that quantum chemistry is a powerful tool for accurate frequency calculations of ISM ice interfaces, providing unprecedented insights into their IR signatures.