Polymorphism in cis-trans Muconic Acid Crystals and the Role of C-H•••O Hydrogen Bonds

Cis-trans isomerization of muconic acid is an important component of various chemical syntheses. Despite the existence of three geometric muconic acid isomers, little atomically precise information is actually known of their structures. In this work, the complete crystal structures of cis-cis, cis-trans, and trans trans muconic acid were determined via single-crystal X-ray diffraction and then further analyzed using terahertz vibrational spectroscopy and solid-state density functional theory. In addition to being the first reported crystal structure for cis trans muconic acid, two polymorphs of this species were discovered, with one exhibiting a two-dimensional hydrogen-bonding pattern stabilized through C-H center dot center dot center dot O interactions. Terahertz spectroscopy aided in polymorph characterization and also provided clear evidence of a hydrated crystalline species that grows only from one of the cis-trans muconic acid polymorphs. Ab initio simulations showed that while the typical end-to-end hydrogen-bonding motif found in dicarboxylic acid crystals dominates at room temperature for cis-trans muconic acid, the two-dimensional hydrogen-bonding network with C-H center dot center dot center dot O hydrogen bonding is energetically favored at very low temperatures.