Diffraction Measurements and Equilibrium Parameters

Structural studies are largely performed without taking into account vibrational effects or with incorrectly taking them into account. The paper presents a first-order perturbation theory analysis of the problem. It is shown that vibrational effects introduce errors on the order of 0.02 Å or larger (sometimes, up to 0.1-0.2 Å) into the results of diffraction measurements. Methods for calculating the mean rotational constants, mean-square vibrational amplitudes, vibrational corrections to internuclear distances, and asymmetry parameters are described. Problems related to low-frequency motions, including torsional motions that transform into free rotation at low excitation levels, are discussed. The algorithms described are implemented in the program available from the author (free)

An alternative gas-phase electron diffraction and quantum chemical study of nitroethane

Shishkov IF, Sipachev VA, Dem'yanov PI, et al. An alternative gas-phase electron diffraction and quantum chemical study of nitroethane. Journal of Molecular Structure. 2010;978(1-3):41-47.The results of alternative gas-phase electron diffraction (GED) study of nitroethane are compared with those reported in the previous study by Tarasov et al. [1]. A fairly large reliability factor (R = 6.1%) obtained in that work, even though the intensity data over the range s > 27 Å-1 were excluded from the analysis, was one of the reasons for undertaking this investigation. Our structural analysis was performed with electron diffraction patterns used by Tarasov et al. In this work, various quantum chemical models were used as initial approximations, and two sets of GED data equally well fitted to theory were obtained. For the experimental intensity curve in the range of s = 3.8-32.6 Å-1, the disagreement between the theoretical and experimental data reached R = 4.3%. The natural bond orbital (NBO) analysis and the quantum theory of atoms in molecules (QTAIM) were used to reveal the reasons of a low barrier to rotation of nitro group