Molecular structure and conformation of 1,3,5-tris(trifluoromethyl)-benzene as studied by gas-phase electron diffraction and quantum chemical calculations

The molecular structure of 1,3,5-tris(trifluoromethyl)benzene (1,3,5-TTFB) was studied by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP method with 6-31G(d,p) basis set and MP2 method with cc-pVTZ basis set). The best fit of the experimental scattering intensities (R-G = 4.0%) was obtained for the structure of C-s symmetry. The differences between some geometric parameters were constrained at the values calculated at the MP2/cc-pVTZ level. The principal structural parameters rh(1)(< h(1)) determined by GED are (bond lengths in Angstroms and bond angles in degrees with 3 sigma in parentheses): r(C=C) = 1.392(4), r(C-C)=1.512(4), r(C-F)(av) = 1346(2), < C-C(CF3) C = 120.9(2), < C-C(H)-C = 119.1(2), <(C-C-F)(av) = 111.6(2). The structure of the carbon ring deviates from a regular hexagon due to the sigma-electronegative effect of the CF3 groups. The geometric parameters of the trifluoromethyl groups deviate considerably from regular tetrahedron arrangement. The experimental structural parameters agree well with the results of B3LYP/6-31G(d,p) and MP2/cc-pVTZ calculations. The electron diffraction data are in agreement with nearly free rotation of the CF3 groups around C-methyl-C-phenyl axis. (C) 2014 Elsevier B.V. All rights reserved.