Mikrowellenspektrum, Hinderungspotential der internen Rotation und Dipolmoment des meta-Fluortoluols

Abstract

The rotational spectrum of meta-fluorotoluene has been measured in the four lowest states of internal rotation of the methyl top m = 0, 1, 2, 3. The rotational constants obtained for m = 0 are A = 3715,2, B = 1766,51, C = 1197,58 MHz. The barrier hindering internal rotation is very low. For the first time threefold and sixfold-symmetrical potential components V 3 and V 6 of comparable magnitude have been found. This appears to be plausible in view of the fact that for this molecule the bond geometrical asymmetry responsible for V 3 (meta substituent) is much more distant from the CH 3 -group than the local C 2v -symmetry of the benzene ring causing V 6 . Fitting V 3 and V 6 to the spectral data one cannot discriminate between two possible sets of potential parameters which differ by the sign of V 6 and, to a lesser extent, by the magnitude of V 3 and V 6 : set I: V 3 = 45,3, V 6 V 6 = + 22,8, set II: V 3 = 48,4, V 6 = — 15,1 cal/mole. The reason for the sign remaining effectively indeterminate although physically relevant in connection with the shape of the barrier is discussed. — The dipole moment components derived from Stark-effect measurements are μ α = 1,71, μ b = 0,62 D.