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

摘要

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) 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 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.