Electron diffraction study of meta- and para-fluoronitrobenzene using results from quantum chemical calculations. Trends in the carbon–halogen bond length found in ortho-, metaand para-halonitrobenzenes (Halo ¼ 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**, 6-311þþG** and cc-pVTZ basis sets. The obtained geometrical parameters (rg; inA° , /a in degrees) for meta-fluoronitrobenzene are: rðC–CÞav ¼ 1:397ð4Þ; rðN–OÞav ¼ 1:227ð3Þ; rðC–NÞ ¼ 1:484ð3Þ; rðC–FÞ ¼ 1:333ð8Þ; rðC–HÞav ¼ 1:107ð16Þ; /C1C2N ¼ 118.3(6), /C5C6F ¼ 120.6(24), /ONO ¼ 125.3(37), /C1C2C3 ¼ 122.9(24), /C3C4C5 ¼ 119.2(18) and w(C–N) ¼ 0 (fixed) and for para-fluoronitrobenzene: rðC–CÞav ¼ 1:393ð2Þ; rðN–OÞav ¼ 1:232ð3Þ; rðC–NÞ ¼ 1:479 (dependent), rðC–FÞ ¼ 1:338ð12Þ; rðC–HÞav ¼ 1:124ð27Þ; /C6C4F ¼ 118.7 (fixed), /ONO ¼ 124.2(23), /C4C5C6 ¼ 123.8(17), /C3C2N ¼ 119.1 (fixed) and w(C–N) ¼ 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 ¼ F, Cl, Br, I) and ortho-, meta- and para- XC6H4NO2 (X ¼ 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 ¼ F and Cl, smaller for X ¼ Br and smallest for X ¼ I. These trends are essentially also found experimentally. The C–X bond lengths are all calculated too long compared to experimental values. q 2004 Elsevier B.V. All rights reserved.