image: The methanol monohydrate structure contains chains of oxygen molecules (red spheres joined by red rods), bridged by the methyl groups of the methanol molecules. These bridges are formed by weak C-H...O hydrogen bonds (dashed rods). Upon warming at constant pressure, atoms experience increasing thermal vibrations; the motion of the methyl groups is characterized by increasingly vigorous rotational libration about the axis of the methanol molecule, illustrated schematically by the blurred methyl hydrogens. The effect of this is to shorten the C-H...O bonds, 'pulling' on the oxygen atoms, and thereby causing the O-O chains to shorten along their length. This explains the thermal contraction in this direction of the crystal structure as it is heated. On compression, however, the balance of compressive forces instead pushes the oxygen atoms further apart, thereby extending the O-O chains. This is the likely origin of the negative compressibility observed when methanol monohydrate is subjected to a hydrostatic pressure. This image relates to an article that appeared in the Feb. 11, 2011, issue of Science, published by AAAS. The study, by Dr. A. Dominic Fortes at University College London in London, UK, and colleagues was titled, "Negative Linear Compressibility and Massive Anisotropic Thermal Expansion in Methanol Monohydrate." view more
Credit: Image © <i>Science/</i>AAAS