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 V3 and V6 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 V3 (meta substituent) is much more distant from the CH3-group than the local C2v-symmetry of the benzene ring causing V6. Fitting V3 and V6 to the spectral data one cannot discriminate between two possible sets of potential parameters which differ by the sign of V6 and, to a lesser extent, by the magnitude of V3 and V6: set I: V3 = 45,3, V6V6 = + 22,8, set II: V3 = 48,4, V6 = — 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 μα = 1,71, μb = 0,62 D.