Benchmark study of molecules with large-amplitude ring-twisting motion: accurate equilibrium structure of succinic anhydride from gas electron diffraction data and coupled-cluster computations

Accurate Determination of Equilibrium Structure of 3-Aminophthalonitrile by Gas Electron Diffraction and Coupled-Cluster Computations: Structural Effects Due to Intramolecular Charge Transfer

“…Vibrational corrections to the experimental bond lengths calculated with B3LYP/cc-pVTZ anharmonic force constants, Δ r anh , are estimated to be in the range 0.007–0.010 Å for the single and aromatic C–C bonds and up to 0.016 Å for the C–H bonds (see Table ). They seem to be accurate given that they are of the same order of magnitude as those calculated previously for other organic molecules from MP2/cc-pVTZ force field (see, for example, refs ,, ). For the C–N amino bond, Δ r anh is 0.007 Å and has an opposite sign pointing to the shortening of this bond during inversion.…”

“…For the molecules consisting of first-row elements, the accuracy of such composite structure was estimated previously to be very high, namely, on the order of a few thousandths of Å units for the bond lengths and a few tenths of degree for the bond angles, − i.e., it can be of the same order of magnitude or even higher than that of the GED structure. The accuracy of the dihedral angles computed in such a way is noticeably lower (on the order of a few degrees) being dependent on the less accurate MP2 corrections …”

“…The accuracy of the computed dihedral angles could not be estimated by such a way because of large uncertainties of the GED values. It is probably a few degrees as shown in the previous study . In general, the accuracy of the best estimated ab initio structure seems to be higher than the accuracy of the GED method.…”

Axial–equatorial isomerism and semiexperimental equilibrium structures of fluorocyclohexane

“…11 It is easy to obtain reliable predicates for the CH bond lengths, the single CC bond lengths and related bond angles. However, as observed in fructose, 12 deoxyribose, 12 proline, 13 succinic anhydrid, 14 pseudopelletierine 15 and several other molecules the prediction of accurate dihedral angles is a problem, the error being sometimes as large as several degrees. Indeed, a provisional criticism of the ME method is the difficulty to obtain reliable estimates for a full set of internal coordinates and to simultaneously estimate their accuracy.…”

“…Slightly later, the microwave spectra of both forms were analyzed by Pierce et al 17,18 In 1971 Scharpen measured the relative intensities of rotational transitions of both conformers and determined an energy difference of 259(28) cal mol À1 . 19 Empirical structures for both conformers were independently determined more recently by Bialkowska-Jaworska et al 20 and Durig et al 21 The latter authors also determined an accurate value of the enthalpy difference, DH = 137 (14) cal mol À1 from the analysis of the infrared spectra in gas and xenon solution. The intensities of several well-isolated and well-shaped conformational bands were measured as a function of temperature (at 5.0 1C intervals between À60 and À100 1C) and DH was determined by application of the van't Hoff equation assuming that the conformational enthalpy differences are not a function of temperature in the range studied.…”