Electron diffraction study of meta- and para-fluoronitrobenzene using results from quantum chemical calculations.: Trends in the carbon-halogen bond length found in ortho-, meta- and para-halonitrobenzenes (Halo = F, Cl, Br, I)

The molecular geometry of meta- and para-fluoronitrobenzene has been determined by gas electron diffraction and quantum chemical calculations using HF, MP2 and DFT, and 6-311G**, 6-311++G** and cc-pVTZ basis sets. The obtained geometrical parameters (r(g), in Angstrom, anglealpha in degrees) for meta-fluoronitrobenzene are: r(C-C)(av) = 1.397(4), r(N-O)(av) = 1.227(3), r(C-N) = 1.484(3), r(C-F) = 1.333(8), r(C-H)(av) = 1.107(16), angle-C1C2N = 118.3(6), angleC5C6F = 120.6(24), angleONO = 125.3(37), angleC1C2C3 = 122.9(24), angleC3C4C5 = 119.2(18) and phi(C-N) = 0 (fixed) and for para-fluoronitrobenzene: r(C-C)(av) = 1.393(2), r(N-O)(av) 1.232(3), r(C-N) = 1.479 (dependent), r(C-F) = 1.338(12), r(C-H)(av) = 1.124(27), angleC6C4F = 118.7 (fixed), angleONO = 124.2(23), angleC4C5C6 = 123.8(17), angleC3C2N = 119.1 (fixed) and phi(C-N) = 0 (fixed). Estimated error limits are three standard deviations from least-squares refinement using a diagonal weight matrix. B3LYP/6-311G** has been used to calculate the molecular structure for all sixteen different molecules XC6H5 (X = F, Cl, Br, I) and ortho-, meta- and para-XC6H4NO2 (X = F, Cl, Br, I). These general trends are predicted by the calculations: the C-X bond in XC6H5 is longer than the C-X bond in ortho-XC6H4NO2, the C-X bond in ortho-XC6H4NO2, is shorter than the C-X bond in meta-XC6H4NO2, the C-X bond in meta-XC6H4NO2 is slightly longer than the C-X bond in para-XC6H4NO2, the variation in the C-X bond length is approximately the same for X = F and Cl, smaller for X = Br and smallest for X = I. These trends are essentially also found experimentally. The C-X bond lengths are all calculated too long compared to experimental values. (C) 2004 Elsevier B.V. All rights reserved.