Molecular structure and conformation of nitrobenzene reinvestigated by combined analysis of gas-phase electron diffraction, rotational constants, and theoretical calculations

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/cc-pVTZ 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)angstrom, r(C-N) = 1.468(4)angstrom, r(N-O) = 1.223(2) angstrom, r(C-H)(av) = 1.071(3) angstrom, angle C2-C1-C6 = 1.23.5(6)degrees, angle C1-C2-C3 = 117.8(3)degrees, angle C2-C3-C4 = 120.3(3)degrees, angle C3-C4-C5 = 120.5(6)degrees, angle C-C-N = 118.2(3)degrees, angle C-N-O = 117.9(2)degrees, angle O-N-O = 124.2(4)degrees, angle(C-C-H)(av) = 120.6(20)degrees. 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.