Internal rotation potential and structure of six fluorine substituted nitrobenzenes studied by microwave spectroscopy supported by quantum chemical calculations

Microwave spectra of the vibrational ground state and several torsionally excited states were used to investigate the internal rotation potential and the structure of six fluorine substituted nitrobenzenes: 3-fluoro- and 4-fluoronitrobenzene were planar molecules just as nitrobenzene whereas 2-fluoro-, 2,4-difluoro- and 2,5-difluoronitrobenzene were found to be non-planar with a dihedral angle, γ0, between the benzene ring and the nitro group of 31.8°, 27.1°, and 30.0° respectively and 2,4,6-trifluoronitrobenzene was non-planar with γ0 = 55.0°. The lower of the two barriers separating the potential minima in the non-planar molecules were 125.5, 74.9, 98.4 and 163 cm−1 respectively. Parameters for structural relaxation during the internal rotation were calculated by the B3LYP method using aug-cc-pVDZ basis and by the MP2(full) method using aug-cc-pVTZ basis. Using these relaxation parameters clearly improved the fit by the internal rotation model, SAF, to observed rotational constants as compared with fits without relaxation of structure. For 2-fluoro-, 2,4-difluoro- and 2,5-difluoronitrobenzene the coefficients V2, V4 and V6 in the Fourier expansion of the potential were determined. For the planar molecules and for 2,4,6-trifluoronitrobenzene V2 and V4 were determined using assumptions about V6 based on the quantum chemical calculations. For all non-planar molecules tunnelling through the lower of the two barriers was observed as a splitting of the torsional energy levels.