Solids modeled by ab initio crystal field methods. Part 19. Structure of yellow and light yellow form of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate

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Abstract

Dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate is known for its color polymorphism in the solid state. For the yellow and light yellow form of this compound the geometry has been optimized using a supermolecule (SM) model. A cluster including the central molecule and the 14 nearest neighboring molecules was described by an ab initio Hartree–Fock wave function. This cluster was surrounded by other molecules represented by point charges placed at the atomic positions. The radius of the complete model was approximately 20 Å. The geometry of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate in the solid state has been compared to the optimized geometry of an isolated molecule in order to identify the crystal field effects. The structural differences between both polymorphic forms are well described using the SM model. The influence of the intermolecular interactions on the electron distribution of the individual molecules has been studied using the cluster deformation electron density distribution.

Introduction

In the literature several examples of color polymorphism have been described [1]. In some of these compounds the color changes have been assigned to conformational changes of the molecule [2], [3], in others the color is accounted for by charge transfer caused by the stacking of molecules with overlapping π-systems [4], [5]. Recently the color polymorphism of a bis(quinoxaline) compound has been studied by combined experimental and theoretical methods [6]. The authors used the SCMP-NDDO method [7] to calculate lattice energies and divide them into their components. They also compared experimental and theoretical excitation energies for the isolated molecules and the crystals. The difference in light absorption for the two polymorphs was found to be partly caused by conformational differences.

One other case that has been extensively studied from an experimental point of view is the color polymorphism in dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate [8]. For this compound three polymorphic forms with different color have been observed in the solid state. At room temperature the yellow form (Y) is the most stable, followed by the light yellow form (LY) and the white form (W). At temperatures above 360 K, W is more stable than Y and LY. In solution also a white and yellow form have been observed, depending on the solvent used. Methanol solutions are white whereas chloroform solutions are yellow [9]. This means that the color differences in dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate are probably due to the conformational differences between the polymorphs and cannot be explained by crystal field effects. In a subsequent paper on the white form of this compound an attempt will be made to link the observed color differences between Y and W to some theoretical properties.

The X-ray structures of the three polymorphic forms of the title compound have been measured at several temperatures in the range between 97 and 353 K [10], [11]. All polymorphic forms crystallize in the same triclinic space group P1̄. The conformation and the arrangement of the molecules, however, are significantly different. The ester group is approximately in the plane of the benzene ring for Y, nearly perpendicular to it for W and in an intermediate position for LY. In all forms the molecules are stacked along the b-axis. For Y and LY the molecules are related by simple translation. For Y, the center of the benzene ring is lying above the hydroxyl group of the underlying molecule. For LY the molecules are approximately one bond length displaced along their long directions. In the white crystal structure two different molecules are found in the unit cell at two centers of symmetry: (0,0,0) and (0,1/2,0).

Quantum chemical studies of molecular crystals can be performed using either an infinite model with periodic boundary conditions [12] or a large finite cluster of molecules within a supermolecule (SM) model [13], [14]. For this paper ab initio Hartree–Fock geometry optimizations have been performed using a SM model to compare the structures of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate in the yellow and light yellow form, with special emphasis on the position of the hydrogen atoms that could not be determined accurately by the X-ray measurements. The solid state structures have also been compared to the geometry in the gas phase.

For both polymorphs the effect of the intermolecular interaction on the electron distribution of isolated non-interacting molecules has been studied using the cluster deformation electron density distribution.

Section snippets

Computational procedure

In this study, the structure of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate in the gas phase and in two polymorphic forms (Y and LY) was completely optimized using the ab initio Hartree–Fock MIA method [15]. This combination of the Multiplicative Integral Approach and the direct SCF method [16] is implemented in the program BRABO [17]. The standard gradient procedure of Pulay [18], [19] was used for the evaluation of the forces on the atoms.

To simulate dimethyl

Results and discussion

The geometry of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate has been optimized in the gas phase and in two polymorphic crystal forms (Y and LY). Optimization of both isolated crystal structures led to the same gas phase conformation. In this conformation the angle between the ester groups and the benzene ring is approximately 30° and intramolecular hydrogen bonds are formed between the hydroxyl groups and the adjacent carbonyl groups.

In Table 1 a selection of the experimental and

Conclusions

The geometry of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate in the gas phase and in two polymorphic crystal forms (Y and LY) has been studied using quantum chemical ab initio methods. A SM model has been used to optimize the structure of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate in the yellow and light yellow form. In general, the refined geometry is in very good agreement with the experimental structure. The hydrogen bonds have been studied in detail. The cluster deformation

Acknowledgements

C.V.A. and A.P. thank the Belgian National Fund for Scientific Research for an appointment as Research Director and Senior Research Assistant respectively. This paper presents results obtained under GOA-BOF-UA-23.

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