Characterization of CF Bonds with Multiple-Bond Character: Bond Lengths, Stretching Force Constants, and Bond Dissociation Energies

Isoelectronic C=F+ and C=O bonds contained in fluoro-substituted carbenium ions, aldehydes, and ketones are investigated with regard to their bond properties by utilizing the vibrational spectra of these molecules. It is demonstrated that bond dissociation energies (BDEs), bond lengths, vibrational stretching frequencies, and bond densities are not reliable descriptors of the bond strength. The latter is related to the intrinsic BDE, which corresponds to nonrelaxed dissociation products retaining the electronic structure and geometry they have in the molecule. It is shown that the harmonic stretching force constants k(a) of the localized internal coordinate vibrations (adiabatic vibrational modes) reflect trends in the intrinsic BDEs. The k(a) values of both CO and CF bonds are related to the bond lengths through a single exponential function. This observation is used to derive a common bond order n for 46 CO- and CF-containing molecules that reliably describes differences in bonding. CF bonds in fluorinated corbenium ions possess bond orders between 1.3 and 1.7 as a result of significant pi back-bonding from F to C, which is sensitive to electronic effects caused by substituents at the carbenium center. Therefore, the strength of the C=F+ bond can be used as a sensor for (hyper)conjugation and other electronic effects influencing the stability of the carbenium ion. The diatomic C=F+ ion has a true double bond due to pi donation from the F atom. The characterization of CF bonds with the help of adiabatic stretching modes is also applied to fluoronium ions (n = 0.3-0.6) and transition states involving CF cleavage and HF elimination (n=0.7-0.8).