Structure and ordering of methyl acetate.

Solid methyl acetate is known to occur in interstellar and cometary ices, but details of its crystal structure at low temperature remain ambiguous. In the only published diffraction study, by Barrow et al. (1981), the structure at 145 K was determined from a single crystal, revealing evidence for disorder of one of the methyl groups. The same authors also identified what they believed to be a plastic phase using X-ray powder diffraction. Calorimetry subsequently showed that there is a broad region over which there are gradual changes in the positional disorder of the methyl hydrogens. Since the structural details are important in both interpreting experimental vibrational spectra and computing spectra from first principles, and since spectroscopy is the principal means of characterizing the state of ices detected remotely in the cosmos, it is essential to determine the complete structure of methyl acetate as a function of temperature using neutron diffraction.