Acetic acid under pressure: The formation below 0 °C, x-ray powder diffraction pattern, and far-infrared absorption spectrum of phase II

This paper presents the phase diagram of acetic acid below 0 °C and 2 kbar, and the x-ray powder diffraction pattern and the far-infrared absorption spectrum of acetic acid II at ∼100 °K and atmospheric pressure. The x-ray powder diffraction pattern of acetic acid I and the far-infrared absorption spectra of acetic acid I and propanoic acid have been remeasured and are presented for comparison. The transition from acetic acid I to II becomes very slow below −20 °C, so that acetic acid II can not be readily prepared below 1050 bar. The extrapolation of the equilibrium line suggests that phase II is the stable phase at atmospheric pressure below about −60 °C. Acetic acid II does not convert rapidly to acetic acid I at atmospheric pressure below −34 °C. The transition pressure at 0 °C is 1135±18 bar, compared with previous values of 1106 and 1064 bar. The x-ray powder pattern of phase II has been indexed on the monoclinic cell a = 13.30 Å, b = 10.39 Å, c = 8.59 Å, β = 86.1°, Z = 16. The x-ray data and the far-infrared spectra, which can not be fully interpreted, both suggest that phase II, like phase I, contains chains of hydrogen-bonded monomers which are packed in a slightly different way in the two phases.