Vibrational spectra and molecular conformation of diethyl fumarate and especially the maleate

Infrared and Raman spectra of diethyl maleate and diethyl fumarate were recorded in the liquid and solid states. From the spectral changes on phase transition the known conformational equilibrium s-trans/s-cis ? s-trans/s-trans in the case of dialkyl fumarates is confirmed, and the view that dialkyl maleates exist only in a single conformation is corrected. The observation of several bands in the vCC region of IR and Raman spectra of diethyl maleate, and significant intensity shifts in this region with phase transition prove the existence of at least two conformers in liquid dialkyl maleates.