On the crystal structures, residual entropy and dielectric anomaly of methanol

Abstract

The orthorhombic form of methanol has the space group D~-Cmcm. There are four molecules in a unit cell of dimensions a = 6-43, b= 7.24 and c=4.67 A. Infinite zigzag chains of hydrogen bonds exist parallel to the (100) face. The C-O distance is 1.42 A, and the O . O hydrogen-bond distance is 2.66 A, indicating strong hydrogen bonds. Surrounding a particular methyl, there are two methyls at 3.88 A, four methyls at 4.01 A, two oxygens at 3.86/~ and four oxygens at 4-09 A. The diffraction effects obtained from single crystals cooled through the ~ point indicate that the transition is a simple displacive transformation involving a puckering of the infinite hydrogenbonded chains, preserving the infinite chains of hydrogen bonds. The essential aspects of this transformation may be described by a monoclinic unit, containing two molecules, with a=4-53, b=4-69, c=4.91 /~ and fl=90 ° in the space group C~h-P2~/m, although the presence of some weak reflections suggests a superlattice. It is concluded that the residual entropy is zero, and that the dielectric anomaly is associated with the puckering of the chains discussed above.