Molecular Dynamics DOI: 10.1002/anie.200701127 Asymmetric Crystal Growth of a-Resorcinol from the Vapor Phase: Surface Reconstruction and Conformational Change Are the Culprits** Jamshed Anwar,* Jittima Chatchawalsaisin, and John Kendrick A fundamental issue in crystal growth is the challenge of disentangling the relative contributions of the intrinsic crystal structure and the various external factors, such as the effects of solvent, to the resulting morphology of the crystal. Crystals of polar molecules in noncentrosymmetric space groups often exhibit asymmetric growth along the polar axis. The cause of this asymmetric growth is a mystery but has commonly been attributed to solvent effects, the inherent contribution of the crystal forces being relegated to mere modulation of the morphology. [1–3] The archetypical example in this context is probably the unidirectional crystal growth of a-resorcinol (space group Pna2 1 ) along the polar axis in aqueous solvents. [4,5] Recently, asymmetric crystal growth from the vapor phase has been observed for a number of polar crystals, including a-resorcinol. [6,7] In view of this finding, it has been proposed that asymmetric crystal growth along the polar axis may be an intrinsic feature of polar crystals, which may be modulated by solvent effects. For the particular case of a- resorcinol, “self-poisoning” has been suggested as the under- lying cause for the observed asymmetric growth. [8] We show herein, by means of molecular-dynamics simu- lation, that for the case of a-resorcinol the surfaces bounding the [011] polar axis undergo reconstruction when equilibrated and exhibit marked asymmetry in terms of their crystalline order. The slower-growing (011) surface shows extensive disorder akin to melting, whilst the (01 ¯ 1 ¯ ) surface remains essentially crystalline. Also, we observe that molecules at the disordered surface can adopt different conformations arising from rotation of the hydroxy groups. The presence of disordered layers and molecules with “rogue” conformations at the (011) surface are expected to significantly hinder crystal growth at this surface relative to the (01 ¯ 1 ¯ ) surface, which would be entirely consistent with experimental observations. The asymmetry in the surface reconstruction of the polar faces of a-resorcinol may be a general feature of polar surfaces and possibly the root cause of the asymmetric growth of polar crystals from the vapor phase. The resorcinol molecule can exist in three possible conformations, which are shown in Scheme 1. The molecular conformation in the a form [9,10] of resorcinol is the symmetric structure A. In the b, [10,11] the other polymorph of resorcinol, the molecular conformation is the asymmetric structure B. The a form is the stable phase at room temperature and pressure. The a !b transition occurs at a relatively low temperature; the reported temperature, which varies from 337 [12] to 369 K, [13] is probably dependent on the quality of the crystals, which would influence the extent of superheating/ supercooling. The transition can also be induced by pressure at about 0.5 GPa. [14] The (011) and (01 ¯ 1 ¯ ) faces of a-resorcinol that limit the [011] polar axis are shown in Figure 1. The (011) face mostly exposes the phenyl rings, whilst the (01 ¯ 1 ¯ ) face is rich in hydroxy groups. An important aspect concerning the growth kinetics of polar crystal faces is the inherent stability of the faces. The energy of the surfaces that terminate the polar axis diverges when they are in their structurally pristine state. Such surfaces Scheme 1. The three possible conformations of resorcinol and the molecular axis used to define the dipole moment. Figure 1. Molecular packing at the (011) and (01 ¯ 1 ¯ ) faces of a-resorcinol. [*] Prof. J. Anwar, Dr. J. Kendrick Computational Laboratory Institution of Pharmaceutical Innovation University of Bradford Bradford, BD71DP (UK) Fax: (+ 44)1274-234-679 E-mail: j.anwar@bradford.ac.uk Dr. J. Chatchawalsaisin Faculty of Pharmaceutical Sciences Chulalongkorn University Bangkok 10330 (Thailand) [**] We gratefully acknowledge discussions with Neil L. Allan and Steve C. Parker on the nature of polar surfaces in ionic materials. J.C. thanks the Institute of Pharmaceutical Innovation (IPI) and the Faculty of Pharmaceutical Sciences, Chulalongkorn University, for supporting her stay at the IPI. Angewandte Chemie 5537 Angew. Chem. Int. Ed. 2007, 46, 5537 –5540 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim