Molecular Structure, Conformation, and Potential to Internal Rotation of 2,6- and 3,5-Difluoronitrobenzene Studied by Gas-Phase Electron Diffraction and Quantum Chemical Calculations

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• 作者：Olga V. Dorofeeva ; Anna V. Ferenets ; Nikolai M. Karasev ; Lev V. Vilkov ; Heinz Oberhammer
• 刊名：Journal of Physical Chemistry A
• 出版年：2008
• 出版时间：June 5, 2008
• 年：2008
• 卷：112
• 期：22
• 页码：5002 - 5009
• 全文大小：225K
• 年卷期：v.112,no.22(June 5, 2008)
• ISSN：1520-5215

文摘

3,5-Difluoronitrobenzene (3,5-DFNB) and 2,6-difluoronitrobenzene (2,6-DFNB) have been studied by gas-phase electron diffraction (GED), MP2 ab initio, and by B3LYP density functional calculations. Refinements of r_h1 and r_e static and r_h1 dynamic GED models were carried out for both molecules. Equilibrium r_e structures were determined using anharmonic vibrational corrections to the internuclear distances (r_e − r_a) calculated from B3LYP/cc-pVTZ cubic force fields. 3,5-DFNB possesses a planar structure of C_2v symmetry with the following r_e values for bond lengths and bond angles: r(C−C)_av = 1.378(4) Å, r(C−N) = 1.489(6) Å, r(N−O) = 1.217(2) Å, r(C−F) = 1.347(5) Å, ∠C6−C1−C2 = 122.6(6)°, ∠C1−C2−C3 = 117.3(3)°, ∠C2−C3−C4 = 123.0(3)°, ∠C3−C4−C5 = 116.9(6)°, ∠C−C−N = 118.7(3)°, ∠C−N−O = 117.3(4)°, ∠O−N−O = 125.5(7)°, ∠C−C−F = 118.6(7)°. The uncertainties in parentheses are three times the standard deviations. As in the case of nitrobenzene, the barrier to internal rotation of the nitro group in 3,5-DFNB, V₉₀ = 10 4 kJ/mol, is substantially lower than that predicted by quantum chemical calculations. The presence of substituents in the ortho positions force the nitro group to rotate about the C−N bond, out of the plane of the benzene ring. For 2,6-DFNB, a nonplanar structure of C₂ symmetry with a torsional angle of φ(C−N) = 53.8(14)° and the following r_e values for structural parameters was determined by the GED analysis: r(C−C)_av = 1.383(5) Å, r(C−N) = 1.469(7) Å, r(N−O) = 1.212(2) Å, r(C−F) = 1.344(4) Å, ∠C6−C1−C2 = 118.7(5)°, ∠C1−C2−C3 = 121.2(2)°, ∠C2−C3−C4 = 119.0(2)°, ∠C3−C4−C5 = 121.1(4)°, ∠C−C−N = 120.6(2)°, ∠C−N−O = 115.7(4)°, ∠O−N−O = 128.6(7)°, ∠C−C−F = 118.7(5)°. The refinement of a dynamic model led to barriers V₀ = 16.5 1.5 kJ/mol and V₉₀ = 2.2 0.5 kJ/mol, which are in good agreement with values predicted by B3LYP/6-311++G(d,p) and MP2/ cc-pVTZ calculations. The values of C−F bond lengths are similar in both molecules. This is in contrast to the drastic shortening of the C−F bond in the ortho position in 2-fluoronitrobenzene compared to the C−F bond length in the meta and para position in 3- and 4-fluoronitrobenzene observed in an earlier GED study.