Determination of molecular structure of succinic acid in a very complex conformational landscape: Gas-phase electron diffraction (GED) and ab initio studies

The molecular structure of succinic acid has been investigated by the gas-phase electron diffraction (GED) method for the first time. According to predictions of MP2/cc-pVTZ calculations, the molecule has 18 stable conformers with the CCCC chain in the gauche (G) or anti (A) configuration, and four of them, I (G), II (A), III (G) and IV (A) belonging to the C2, C2h, C1 and C1 point groups, respectively, with relative energies ΔEZPE within 2.2 kcal/mol can be present at the experimental temperature of 445 K in noticeable amounts. The ratio of the conformers I:II:III:IV = 45(15):20(15):10(assumed):25(15) (in %) has been determined in the GED analysis guided by theoretical predictions. To take into account vibrational effects, the corrections Δ(re − ra) to the experimental ra bond lengths were calculated from the MP2/cc-pVTZ quadratic and cubic force constants. The obtained equilibrium structural parameters of the dominant conformer I are the following (bond lengths in Å, angles in degrees): re(Csp3Csp3) = 1.508(3), re(Csp3Csp2) = 1.499(2), re(CO) = 1.343(2), re(CO) = 1.202(1), e(CCC) = 111.8(4), e(CCO) = 112.0(4), e(OCO) = 123.0(1), τ(CCCC) = 69.9(11). Yielding the best agreement with the GED structure, the MP2/cc-pVQZ approximation overestimates the CO and CO bond lengths by ca. 0.005(2) Å.