Ferrocene frequency assignment

We have reproduced the vibrational frequencies of benzene, transition metal carbonyls and transition metal complexes of benzene and cyclopentadiene fairly accurately. Our calculations indicate some ambiguities in the original empirical frequency assignments for ferrocene and dibenzene-chromium, for which we suggest alternatives. Our calculations confirm the frequency assignments for BzCr(CO)3. 

Based on these principles it is possible to make a frequency assignment for ferrocene. Both practically and historically this problem has fulfilled a special role in this field. For simplicity, the abbreviation Cp will be used for C5H5, in conformity with international usage. Thus, for example ferrocene will be written as Cp2Fe. 

The comparison between experimental frequencies and our calculated values is the only direct way in which we can obtain information about the accuracy of the computational method. Also, the unusually large deviations between calculated and observed frequencies for some vibrational modes suggest uncertainties in the empirical assignments. We begin our discussion with ferrocene. 

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