Structural aspects of phase transition in pyrrole-2,5-dithioamide single crystals

The new pyrrole thioamide derivative 1H-pyrrole-2,5-dithiocarboxylic acid bis(n-butylamide) exists in two different phases with the first-order phase transition at similar to 195.8 K on heating up. We describe details of the lower and higher temperature phases and the nature of the phase transition. The high-temperature phase is centrosymmetric, with the molecule having one statically disordered aliphatic chain, whereas the low-temperature crystals are characterized by chiral P2(1) space group symmetry with the unit cell volume six times bigger than the volume of the high-temperature phase. Although the unit cell volume changes continuously with temperature, all other geometric parameters of the unit cell (a, b, c, beta) change in a discrete manner during the phase transition. We propose a model explaining the structural disorder observed at high temperature and discuss the variation of properties of weak interactions on decrease of symmetry. Copyright (c) 2005 John Wiley & Sons, Ltd.