Comparison of the high-pressure and low-temperature structures of ethanol and acetic acid

Abstract

We have determined the high-pressure crystal structures of ethanol and acetic acid, including the positions of the hydrogen atoms, using a combination of single-crystal x-ray-diffraction techniques and ab initio pseudopotential calculations. We find that in the high-pressure structure of ethanol the molecules are arranged in infinite hydrogen-bonded chains that adopt a structural conformation that is distinctly different from that of the low-temperature form. The hydrogen-bond lengths and bond angles within the chains are equal by symmetry and, as the molecules also have an alternating alignment to the chains, the molecular chains are relatively unstrained. It is proposed that this uniformity and lack of strain within the chains enables ethanol to crystallize much more readily than methanol at high pressure. For acetic acid we find that the molecules are also arranged in infinite hydrogen-bonded chains that are essentially identical to those in the low-temperature structure. However, they adopt markedly different relative orientations, which leads to a more efficient molecular packing and a radically different methyl-methyl contact motif between adjacent molecular chains. The calculated enthalpies of the high-pressure and low-temperature structures show that the high-pressure phase is the most energetically favorable. We find a relatively small 0.056 eV/molecule enthalpy difference between the two structures and this is reflected in the very low freezing pressure of approximately 0.2 GPa at room temperature compared to the freezing temperature of $16\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ at ambient pressure.