Electron diffraction and quantum chemical study of the structure and internal rotation in nitroethane

Abstract

Studies of the molecular structure and spectra of the lower nitroalkanes are complicated due to hindered internal rotation of the nitro group around the C N bond. In the present work, the structure of nitroethane molecule is determined from electron diffraction (ED) data supplemented by results of quantum chemical calculations within the model accounting for the large-amplitude motions and anharmonicity of vibrations in the rigid fragments. Quantum chemical calculations at the MP2, QCISD and B3LYP levels of theory result in contradictory conclusions regarding the location of the minimum of internal rotation potential energy function and potential barrier height. In the ED analysis, parameters of equilibrium geometry were refined simultaneously with scaling of quantum chemical force matrix aimed at fitting experimental vibrational frequencies. The data processing has shown the minimum of the potential energy function of internal rotation of NO 2 group corresponding to the syn-C molecular configuration with torsional angle τ(C C N O) = 0°. Rotation of the nitro group was found to be slightly hindered, with potential energy barrier height in the range of 50–200 cm −1 (with the most probable value near 120 cm −1). The refined parameters of equilibrium molecular configuration are the following: r e(C C) = 1.509(3) Å, r e(C N) = 1.503(3) Å, r e(N O) = 1.213(3) and 1.215(3) Å, ∠ e(C C N) = 112.3(9)°, ∠ e(O N O) = 125.9(5)°.