Crystal structure of propionitrile (CH3CH2CN) determined using synchrotron powder X-ray diffraction

The structure and thermal expansion of the astronomical molecule propionitrile have been determined from 100 to 150K using synchrotron powder X-ray diffraction. This temperature range correlates with the conditions of Titan's lower stratosphere, and near surface, where propionitrile is thought to accumulate and condense into pure and mixed-nitrile phases. Propionitrile was determined to crystallize in space group, Pnma (No.62), with unit cell a = 7.56183(16)Å, b = 6.59134(14)Å, c = 7.23629(14), volume = 360.675(13)Å3 at 100K. The thermal expansion was found to be highly anisotropic with an eightfold increase in expansion between the c and b axes. These data will prove crucial in the computational modelling of propionitrile-ice systems in outer Solar System environments, allowing us to simulate and assign vibrational peaks in the infrared spectra for future use in planetary astronomy.