Trends in Structure and IR Spectra of Mixed H₂O/CO₂ Ices of Astrochemical Relevance

Molecular clouds in the interstellar medium harbor ices formed on the surfaces of dust grains, consisting primarily not only of H₂O but also, for example, CO, CO₂, and CH₃OH. The dangling O–H bands at frequencies around 3630–3720 cm^–1, slightly higher than the high-frequency shoulder of the bulk O–H stretch band, provide information on the structural properties of amorphous water ices. The presence of the dangling mode indicates that an interface is present: a vacuum interface is often interpreted as porosity, as opposed to an interface with a nonwater neighboring molecule. We computationally study the role played by mixing ratio, segregation level, and the type of interface present in the ice for the IR peak positions of the dangling O–H band and the CO₂ bend and stretch bands in mixed H₂O/CO₂ ices. Molecular interactions are described using the MB-nrg force field, and we show that it can accurately predict experimentally observed shifts in IR peaks, although absolute peak positions are generally overestimated. Overall, our study reveals a significant influence of the local environment of a molecule on the dangling O–H and the CO₂ stretch band, where in particular the mixing ratio, type of interface, and to a lesser extent segregation level come out as influential, while the density and temperature have negligible effects on peak positions.