The complete harmonic force field of the cyclopentadienyl anion, LiCp and ferrocene has been calculated by local (LDA) and non-local (LDA/NL) density functional methods at the geometry optimized at LDA/NL level. The calculated LDA force field reproduced the harmonic vibrational frequencies of ferrocene with an average deviation of 35 cm-1 after revising the empirical assignments of six observed frequencies.
Consideration has been given to the proper representation of the skeletal internal coordinates that facilitate the comparison of the internal force constants between the complexed and the free cyclopentadienyl ring. The valence force constants of the three compounds are compared and the differences are interpreted. The complete force field of ferrocene is expressed in both valence and symmetry coordinate representation.
A comparison of the force fields of the free Cp- and that of ferrocene shows that the out-of-plane force constants are remarkably different for the two compounds. Furthermore, the differences in the frequencies are mainly due to the different force fields and not to the kinematic coupling effect as previously thought.