Thermochemistry of (E)- and (Z)-disubstituted alkene species: a combined experimental and theoretical investigation of isomeric dimethyl fumarate and dimethyl maleate.

The first unequivocal thermochemical/calorimetric determination of the enthalpies of combustion, phase change, and formation of a pair of (E)- and (Z)-disubstituted olefins of the type XCH = CHX, where X is an electron withdrawing group, is reported in this paper for the isomeric species dimethyl fumarate and dimethyl maleate (X = COOCH3). The corresponding density functional calculations, corrected for vibrational and thermal effects, confirm that the former isomer is more stable by some 30 kJ mol-1.