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

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 ahharmonic 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-2 upsilon symmetry with the following r(e) values for bond lengths and bond angles: r(C-C)(av) = 1.378(4) angstrom, r(C-N) = 1.489(6) angstrom, r(N-O) = 1.217(2) angstrom, r(C-F) = 1.347(5) angstrom, angle C6-C1-C2 = 122.6(6)degrees, angle C1-C2-C3 = 117.3(3)degrees, angle C2-C3-C4 = 123.0(3)degrees, angle C3-C4-C5 = 116.9(6)degrees, angle C-C-N = 118.7(3)degrees, angle C-N-O = 117.3(4)degrees, angle O-N-O = 125.5(7)degrees, angle C-C-F = 118.6(7)degrees. 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, V-90 = 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 C-2 symmetry with a torsional angle of phi(C-N) = 53.8(14)degrees and the following r(e) values for structural parameters was determined by the GED analysis: r(C-C)(av) = 1.383(5) angstrom, r(C-N) = 1.469(7) angstrom, r(N-O) = 1.212(2) degrees, r(C-F) = 1.344(4) angstrom, angle C6-C1-C2 = 118.7(5)degrees, angle C1-C2-C3 = 121.2(2)degrees, angle C2-C3-C4 = 119.0(2)degrees, angle C3-C4-C5 = 121.1(4)degrees, angle C-C-N = 120.6(2)degrees, angle C-N-O = 115.7(4)degrees, angle O-N-O = 128.6(7)degrees, angle C-C-F = 118.7(5)degrees. The refinement of a dynamic model led to barriers V-0 = 16.5 +/- 1.5 kJ/mol and V-90 = 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.