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 C-C-C-C 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 C(2). C(2h). C(1) and C, point groups, respectively, with relative energies Delta E(ZPE) 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 Delta(r(e) - r(a)) to the experimental r(a) bond lengths were calculated from the MP2/cc-pVTZ quadratic and cubic force constants. The obtained equilibrium structural parameters of the dominant conformer 1 are the following (bond lengths in angstrom, angles in degrees): r(e)(C(sp3)-C(sp3)) = 1.508(3), r(e)(C(sp3)-C(sp2)) = 1.499(2), r(e)(C-O) = 1.343(2), r(e)(C=O) = 1.202(1), (e)(C-C-C) = 111.8(4), (e)(C-C-O) = 112.0(4), (e)(O-C-O) = 123.0(1), tau(C-C-C-C) = 69.9(11). Yielding the best agreement with the GED structure, the MP2/cc-pVQZ approximation overestimates the C-O and C=O bond lengths by ca. 0.005(2) angstrom. (C) 2011 Elsevier B.V. All rights reserved.