The vibrational force field of halogenated aromatic molecules: In-plane vibrations of benzene and fluorobenzenes

The force fields for the in-plane vibrations of benzene and its fluoro derivatives are determined using the overlay technique. Between 36 and 39 parameters were allowed to vary in different fields and in all cases convergence on the experimental frequencies was smooth and rapid. Only one solution could be found. This solution agrees extremely well with the benzene field of Duinker and Mills. The variation of diagonal force constants in an internal valence coordinate basis set with fluorine substitution in the coordinate environment is determined. The variation in certain force constants is considerable. Thus for the CF stretching constant it is 12% for replacement of two hydrogens by two fluorines. Attempts are made to interpret these variations. Various aspects of the force constant problem for benzene are discussed.