INTERNAL ROTATION AND EQUILIBRIUM STRUCTURE OF THE 2-METHYL-2-NITROPROPANE MOLECULE from JOINT PROCESSING OF GAS PHASE ELECTRON DIFFRACTION DATA, VIBRATIONAL AND MICROWAVE SPECTROSCOPY DATA, AND QUANTUM CHEMICAL CALCULATION RESULTS
Yu. I. Tarasov1,2, I. V. Kochikov2,3, D. M. Kovtun1, E. A. Polenov2, A. A. Ivanov4
1Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow, Russia
2Moscow Technological University, Moscow, Russia
3Moscow State University
4Moscow State University, Moscow, Russia
Keywords: 2-метил-2-нитропропан, нежесткие молекулы, внутреннее вращение, газовая электронография, квантово-химические расчеты, равновесная структура, ангармоничность, микроволновая и колебательная спектроскопия, 2-methyl-2-nitropropane, nonrigid molecules, internal rotation, gas phase electron diffraction, quantum chemical calculations, equilibrium structure, anharmonicity, microwave and vibrational spectroscopy
Abstract
The structure and internal rotation of the 2-methyl-2-nitropropane molecule is studied by electron diffraction and quantum chemical calculations with the use of microwave and vibrational spectroscopy data. The electron diffraction data are analyzed within the general intramolecular anharmonic force field model and the quantum chemical pseudoconformer model, considering the adiabatic separation of the degree of freedom of large amplitude motion, i.e., the internal rotation of the NO2 group. The equilibrium eclipsed configuration of the Cs symmetry molecule has the following experimental bond lengths and valence angles: re(N=O) = 1.226//1.226(8) Å, re(C-N)// re(C-C) = 1.520//1.515/1,521(4) Å, ∠еC-C-N = 109.1/106,1(8)°, ∠еO=N=O = 124.2(6)°, ∠eC-C-Havg = 110(3)°. The equilibrium geometry parameters are well consistent with MP2/cc-pVTZ quantum chemical calculations and microwave spectroscopy data. The thermally average parameters previously obtained within the small vibration model show a satisfactory agreement with the new results. The electron diffraction data used in this work do not allow a reliable determination of the barrier to internal rotation. However, at a barrier of 203(2) cal/mol, which is derived from the microwave study, it follows from the electron diffraction data that the equilibrium configuration must correspond to an eclipsed arrangement of C-C and N=O bonds, which is also consistent with the results of quantum chemical calculations of various levels.
|
