Molecular structure and stabilities of fumaric acid conformers: Gas phase electron diffraction (GED) and quantum-chemical studies

The molecular structure of fumaric acid was reinvestigated by the gas phase electron diffraction (GED) method because it was determined several years ago assuming the existence of one conformer only. According to MP2/cc-pVTZ predictions, the molecule has six stable conformations, and three of them, sp,sp,sp,sp (I), ap,sp,sp,sp (II) and ap,ap,sp,sp (III), with relative energies within ca. 3 kJ mol−1 should be present in detectable amounts at the experimental temperature of ca. 480 K. The best fit to the GED intensities was achieved at the approximately equal amounts of the conformers I–III. To take into account vibrational effects, the corrections to the experimental ra bond lengths (re − ra) were calculated using quadratic and cubic force constants from high-level ab initio calculations (MP2/cc-pVTZ). The deduced equilibrium bond lengths of the conformer II with the estimated total errors (in Å) are the following: re(CC) = 1.331(3), re(CC)s-trans = 1.473(3), re(CC)s-cis = 1.479(3), re(CO)s-trans = 1.345(3), re(CO)s-cis = 1.344(3), re(CO)s-trans = 1.205(3), re(CO)s-cis = 1.204(3). The structural effects arising due to presence of the CC double bond were analysed. In comparison to the experiment, the MP2/cc-pVQZ approximation overestimates some bond lengths in fumaric acid and similar molecules by up to 0.005 Å. The rotational constants of the conformers I–III were derived from the GED equilibrium geometries.