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Carbon monoxide (CO) is the second most abundant molecule in the gas-phase ofthe interstellar medium. In dense molecular clouds, it is also present in thesolid-phase as a constituent of the mixed water-dominated ices covering dustgrains. Its presence in the solid-phase is inferred from its infrared (IR)signals. In experimental observations of solid CO/water mixed samples, its IRfrequency splits into two components, giving rise to a blue- and a redshiftedband. However, in astronomical observations, the former has never beenobserved. Several attempts have been carried out to explain this peculiarbehaviour, but the question still remains open. In this work, we resorted topure quantum mechanical simulations in order to shed some light on thisproblem. We adopted different periodic models simulating the CO/H$_2$O icesystem, such as single and multiple CO adsorption on water ice surfaces, COentrapped into water cages and proper CO:H$_2$O mixed ices. We also simulatedpure solid CO. The detailed analysis of our data revealed how the quadrupolarcharacter of CO and the dispersive forces with water ice determine theenergetic of the CO/H$_2$O ice interaction, as well as the CO spectroscopicbehaviour. Our data suggest that the blueshifted peak can be assigned to COinteracting {\it via} the C atom with dangling H atoms of the water ice, whilethe redshifted one can actually be the result of CO involved in differentreciprocal interactions with the water matrix. We also provide a possibleexplanation for the lack of the blueshifted peak in astronomical spectra. Ouraim is not to provide a full account of the various interstellar ices, butrather to elucidate the sensitivity of the CO spectral features to differentwater ice environments.

IR Spectral Fingerprint of Carbon Monoxide in Interstellar Water Ice Models