Testing Densities and Refractive Indices of Extraterrestrial Ice Components Using Molecular Structures - Organic Compounds and Molar Refractions

The use of infrared spectra to determine molecular abundances of icy astronomical objects and to study their chemistry requires laboratory measurements of reference spectra and related quantities, such as the index of refraction (n) and density (ρ) of candidate ices. Here we present new n and ρ measurements on ices involving over 30 C-, H-, and O-containing compounds, both acyclic and cyclic, representing seven chemical families. We examine the results in a way that is rare in the astrochemical literature, namely one in which data from an ice formed from molecules of a particular chemical family are compared to measurements on another member of the same family, such as of a homologous series or a pair of isomers. Apart from the intrinsic usefulness of the n and ρ data, a structure-based comparison can help establish trends and identify possibly spurious results. As liquid-phase data sometimes are used in low-temperature astrochemical work in the absence of solid-phase measurements, we compare our new ice results to those for the corresponding room-temperature liquids. We emphasize the use of our n and ρ data to compute the molar refraction (R _M) for each of our ices, and how the resulting R _M values compare to those expected from molecular structures. The use of calculated R _M values and measured n values to calculate ice densities, in the absence of direct measurements, is also addressed.