ИСТИНА

Internal rotation in nitroethane molecule: the unusual relation between LAM dependent amplitudes Yury I. Tarasov, Igor V. Kochikov, Dmitry M. Kovtun Moscow State University, 119991, Moscow, Russia; E-mail: tarasov@phys.chem.msu.ru The equilibrium structure of the nitroethane molecule and the internal rotation of nitro group have been studied recently1 in gas phase using electron diffraction data accompanied with quantum chemical calculations in the framework of the large-amplitude motion model for internal rotation 2, 3. The effects of anharmonicity and interactions between the large amplitude motion and “rigid” vibrations were also included as well as QCISD/cc-pVT(D)Z relaxations for the geometry parameters the latter being of principal interest. Fig. 1 shows the comparison of two calculated rg(C…O) distances (not optimized by ED data) with account for relaxations (lines with markers) and ignoring the relaxations (solid lines). Fig. 2 shows the resulting contributions of the C…O terms to F(r). In these two cases C…O the ratio of amplitudes for shorter and longer distances shows opposite behavior (see table below). The effects of geometry relaxation result in l(C…O)long > l(C…O)short , contrary to the more symmetric trimethylnitromethane, nitrobenzene and a lot of other nitro compounds. In Fig. 2, radial distributions for the mentioned C…O distances are shown for the best-fit potential function of internal rotation. It is evident from Fig. 2 that radial distribution function peaks are strongly asymmetric and can hardly be described by the approximation of distorted Gaussian functions. The difference of amplitudes is explained by the difference in peak shapes. In the mentioned work 1, thermally averaged parameters (including amplitudes) were not used in structural analysis directly and were calculated with illustrative purpose. Fig. 1. rg(C…O) distances against CCNO torsion. Fig. 2. Summarized C…O contributions to F(r). l(C…O)short, Å l(C…O)long, Å Relax. ignored 0.184 0.150 Relax. included 0.153 0.168 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. 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.