The molecular structure and conformation of nitrobenzene has been reinvestigated by gas-phase electron diffraction (GED), combined analysis of GED and microwave (MW) spectroscopic data, and quantum chemical calculations. The equilibrium r e structure of nitrobenzene was determined by a joint analysis of the GED data and rotational constants taken from the literature. The necessary anharmonic vibrational corrections to the internuclear distances (r e -r a ) and to rotational constants (B e (i) -B 0 (i) ) were calculated from the B3LYP/ccpVTZ quadratic and cubic force fields. A combined analysis of GED and MW data led to following structural parameters (r e ) of planar nitrobenzene (the total estimated uncertainties are in parentheses): r(C-C) av = 1.391 (3) Å , r(C-N) = 1.468(4) Å , r(N-O) = 1.223(2) Å , r(C-H) av = 1.071(3) Å , \C2-C1-C6 = 123.5(6)°, \C1-C2-C3 = 117.8(3)°, \C2-C3-C4 = 120.3(3)°, \C3-C4-C5 = 120.5(6)°, \C-C-N = 118.2(3)°, \C-N-O = 117.9(2)°, \O-N-O = 124.2(4)°, \(C-C-H) av = 120.6(20)°. These structural parameters reproduce the experimental B 0 (i) values within 0.05 MHz. The experimental results are in good agreement with the theoretical calculations.The barrier height to internal rotation of nitro group, 4.1±1.0 kcal/mol, was estimated from the GED analysis using a dynamic model. The equilibrium structure was also calculated using the experimental rotational constants for nitrobenzene isotopomers and theoretical rotation-vibration interaction constants.

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