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 (RG = 4.0%) was obtained for the structure of CS symmetry. The differences between some geometric parameters were constrained at the values calculated at the MP2/cc-pVTZ level. The principal structural parameters rh1(∠h1) determined by GED are (bond lengths in Angstroms and bond angles in degrees with 3σ in parentheses): r(CC) = 1.392(4), r(CC)=1.512(4), r(CF)av = 1.346(2), ∠CC(CF3)C = 120.9(2), ∠CC(H)C = 119.1(2), ∠(CCF)av = 111.6(2). The structure of the carbon ring deviates from a regular hexagon due to the σ-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 CmethylCphenyl axis.