ИСТИНА

Comparison of the structure and internal rotation in methyl substituted nitromethanes 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 nitroethane (I) 1, 2-nitropropane (II) 2 and 2-methyl-2-nitropropane (III) 3 and the internal rotation of the nitro group in these molecules have been studied recently in gas phase using electron diffraction data accompanied with quantum chemical calculations and experimental rotational constants (if available) in the framework of the large-amplitude motion model for internal rotation 4, 5. QC results of geometry relaxations, the effects of anharmonicity and interactions between the large amplitude motion and “rigid” vibrations were also included. Table shows the increase of C – C, C – N and N=O distances, decrease of O=N=O and N-C-C angles in the row I – III. The lowest rotational barrier is equal to 70 cm-1 in (III). In (I) and especially in (II) barrier heights were found of significant values in opposite to the estimates of earlier investigations. I II III re(N=Osyn), Å 1.213 (4) 1 1.222 (4) 1 1.226 (5) 1 re(N-O) , Å 1.215 1 1.221 1 1.226 1 re(С-N), Å 1.503 (11) 2 1.501 (5) 2 1.520 (3) 2 re(C-Csyn) , Å 1.509 2 ------ 1.515 2 re(C-C) , Å ------ 1.516 2 1.521 2 e O=N=O, ° 125.9 (0.6) 124.8 (0.4) 124.3 (0.4) e C-C-Csyn, ° ------ ------ 111.5 (0.4) 3 e C-C-C, ° ------ 112.8 (1.1) 111.3 3 e N-C-C, ° ------ 108.7 (1.0) 106.4 (0.4) 4 e N-C-Csyn, ° 112.3 (1.1) ------ 109.4 4 conformation syn-C syn-H syn-C Barrier height, cm-1 120 (50 – 200) 380 (220 – 560) 70 (0 – 200) Numbers 1 ... 4 in columns I – III denote parameters optimized in groups This work was supported by RFBR grant 08-03-01104-a. 1. Yu.I. Tarasov, I.V. Kochikov, N. Vogt, A.V. Stepanova, D.M. Kovtun, A.A. Ivanov, A.N. Rykov, R.Z. Deyanov, B.K. Novosadov, J. Vogt. J. Mol. Struct., 872(2-3) (2008) 150 – 165. 2. Yu.I. Tarasov, I.V. Kochikov, D.M. Kovtun, A.A. Ivanov, J. Mol. Struct., 921 (2009) 255. 3. Yu.I. Tarasov, I.V. Kochikov, D.M. Kovtun, A.A. Ivanov, J. Mol. Struct., 2010, in press. 4. I.V. Kochikov, Yu.I. Tarasov, N. Vogt, V.P. Spiridonov, J. Mol. Struct., 607 (2002) 163. 5. I.V. Kochikov, Yu.I. Tarasov, Struct. Chem., 14 (2003), 227.