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

Abstract

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 ⁽ⁱ⁾_e  − B ⁽ⁱ⁾₀ ) 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) Å, r(C–N) = 1.468(4) Å, r(N–O) = 1.223(2) Å, r(C–H)_av = 1.071(3) Å, ${\angle}$C2–C1–C6 = 123.5(6)°, ${\angle}$C1–C2–C3 = 117.8(3)°, ${\angle}$C2–C3–C4 = 120.3(3)°, ${\angle}$C3–C4–C5 = 120.5(6)°, ${\angle}$C–C–N = 118.2(3)°, ${\angle}$C–N–O = 117.9(2)°, ${\angle}$O–N–O = 124.2(4)°, ${\angle}$(C–C–H)_av = 120.6(20)°. These structural parameters reproduce the experimental B ⁽ⁱ⁾₀ 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.