Unraveling the\nInterface Chemistry between HCN and\nCosmic Silicates by the Interplay of Infrared Spectroscopy and Quantum\nChemical Modeling

Understanding the interaction between hydrogen cyanide\n(HCN) and\nsilicate surfaces is crucial for elucidating the prebiotic processes\noccurring on interstellar grain cores as well as in cometary and meteoritic\nmatrices. In this study, we characterized the adsorption features\nof HCN on crystalline forsterite (Mg<sub>2</sub>SiO<sub>4</sub>) surfaces,\none of the most abundant cosmic silicates, by combining experimental\ninfrared spectra at low temperatures (100–150 K) with periodic\nDFT simulations. Results showed the coexistence of both molecular\nand dissociative HCN adsorption complexes as a function of the considered\nforsterite crystalline face. Molecular adsorptions dominate on the\nmost stable surfaces, while dissociative adsorptions occur predominantly\non surfaces of lower stability, catalyzed by the enhanced Lewis acid–base\nbehavior of surface-exposed Mg<sup>2+</sup>–O<sup>2–</sup> ion pairs. On the whole set of adsorption cases, harmonic frequency\ncalculations were carried out and compared with the experimental infrared\nbands. To disentangle each vibrational mode contributing to the experimental\nbroad bands, we run the best nonlinear fit between the predicted set\nof frequencies and the experimental bands. The outcome of this procedure\nallowed us to (i) deconvolute the experimental IR spectrum by assigning\ncomputed normal modes of vibrations to the main features of each band\nand (ii) reveal which crystal faces are responsible for the largest\ncontribution to the adsorbate vibrational bands, giving information\nabout the morphology of the samples. The present straightforward procedure\nis quite general and of broad interest in the fine characterization\nof the infrared spectra of adsorbates on complex inorganic material\nsurfaces.