ИСТИНА

Quantum chemical and electron diffraction study of the structure and internal rotation in 2-methyl-2-nitropropane Yury I. Tarasov, Igor V. Kochikov, Dmitry M. Kovtun, Arkady A. Ivanov Moscow State University, 119991, Moscow, Russia; E-mail: tarasov@phys.chem.msu.ru The equilibrium structure of the 2-methyl-2-nitropropane molecule and the internal rotation of the nitro group have been studied 1 in gas phase using electron diffraction data accompanied with quantum chemical calculations and experimental rotational constants in the framework of the large-amplitude motion model for internal rotation 2, 3. The effects of geometry relaxation, anharmonicity, interactions between the large amplitude motion and “rigid” vibrations were also included. Quantum chemistry calculations at the MP2/6-311G(d), MP2/cc-pVTZ, MP2/aug-cc-pVTZ, B3LYP/6-311G(d), B3LYP/cc-pVTZ, B3LYP/6-311++G(d), B3LYP/6-311++G(3df,2p) levels of theory using Gaussian 03 software unambiguously predict syn-C minimum energy molecular conformation when one of the oxygen atoms eclipses one of carbon atoms (dihedral angle C-C-N-O is zero), with relatively close values of internal rotation barrier (65 to 85 cm-1). The experimental data are consistent with dynamic model governed by sixfold cosine potential energy function with the minimum located in the same syn-C position. It has also been found that internal rotation is very slightly hindered, with the barrier height in the range of 0 to 200 cm-1 (0 to 0.6 kcal/mole) the most probable value being about 70 cm-1. The main equilibrium structure parameters in syn-C configuration are as follows (values in parentheses correspond to 3 times standard deviations): re(C–C)/re(C–C)/re(C–N) = 1.515/1.521/1.520(3) Å, re (N=O) = 1.226(5) Å, C–C–N = 106.4/109.4 (0.4) °, O=N=O = 124.3 (0.4)°. Thermally averaged parameters for comparison with the results of traditional studies are also provided 1. This work was supported by RFBR grant 08-03-01104-a. 1. Yu.I. Tarasov, I.V. Kochikov, D.M. Kovtun, A.A. Ivanov, J. Mol. Struct., 2010, in press. 2. I.V. Kochikov, Yu.I. Tarasov, N. Vogt, V.P. Spiridonov, J. Mol. Struct., 607 (2002) 163. 3. I.V. Kochikov, Yu.I. Tarasov, Struct. Chem., 14 (2003), 227.