Color polymorphism: understanding the diverse solid-state packing and color in dimethyl-3,6-dichloro-2,5-dihydroxyterephthalate.

Dimethyl-3,6-dichloro-2,5-dihydroxyterephthalate (MCHT) is known to exist in three differently packed crystals having three different colors, namely yellow (Y), light yellow (LY), and white (W). Apart from the difference in their color, the molecules in the crystals also differ in their intramolecular O-H⋅⋅⋅O and O-H⋅⋅⋅Cl hydrogen bonds. Time-dependent DFT calculations reveal the role of the various types of hydrogen bonds in controlling the color of the polymorphs. Mechanistic pathways that lead to such transformations in the crystal are elucidated by solid-state dispersion-corrected DFT studies. Relative stabilities of the various polymorphs rationalize the experimentally observed transformations between them. Calculations reveal that the minimum-energy pathway for the conversion of the Y form to a W form is through stepwise disrotatory motion of the two -OH groups through a hybrid intermediate having one intramolecular OH⋅⋅⋅O and one O-H⋅⋅⋅Cl bond. The LY form is shown to exist on the higher-energy pathway involving a concerted Y→W transformation.