Equilibrium configuration and barriers of four fluorine substituted nitrobenzenes,obtained by microwave spectroscopy

MW Spectra of 3- and 4-fluoronitrobenzene in several torsional states and of 2- and 2,4,6-trifluoronitrobenzene in the ground state have been obtained. The inertial defects have been used to show that the 3- and 4-substituted molecules are planar and to find their twofold barriers (1428 cm^?1 and 1213 cm^?1 respectively). The 2- and 2,4,6-substituted molecules have very large negative inertial defects indicating non-planarity. The out-of-plane angles have been calculated (32° and 57° respectively).