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 V 2 and V4 were determined using assumptions about V 6 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.
Journal of Molecular Structure, 2014, Vol 1071, p. 111-122