Equilibrium structure of maleic anhydride from gas-phase electron diffraction (GED) and quantum-chemical studies

The molecular structure of maleic anhydride was reinvestigated by the gas-phase electron diffraction (GED) method because it was determined with large uncertainties in a previous study. To take into account vibrational effects, the corrections to the experimental bond lengths (ra) were calculated using quadratic and cubic force constants from high-level ab initio calculations (MP2/aug-cc-pVTZ). The determined equilibrium structural parameters (re(C-C) = 1.485(1) Å, re(CC) = 1.332(1) Å, re(C-O) = 1.386(1) Å, re(CO) = 1.192(1) Å, ∠e(C-CC) = 107.8(1)°, ∠e(OC-C) = 129.2(2)°) are very close to the parameters of the rs structure from a previous microwave spectroscopic (MW) study. Some aspects of the combined analysis of GED and MW data are discussed. The agreement of the experimental rotational constants, B0(i) (MW), corrected for anharmonic effects, Be(i) (MW), with the rotational constants derived from the determined equilibrium geometry, is better than with Be(i) values from quantum-chemical calculations.