Vibrational Overtone Spectroscopy and Overtone Intensities of Cyclohexadiene Iron Tricarbonyl and 1,3-Cyclohexadiene

The CH stretching overtone spectra of liquid cyclohexadiene iron tricarbonyl (CHDIT) have been recorded for the first, second, and third overtone regions (Δv = 2−4). The gaseous spectrum of CHDIT has also been obtained for the first overtone region (Δv = 2). Observed transitions are assigned to the overtones of different nonequivalent CH bonds and to the local mode−normal mode combination bands. Oscillator strengths have been calculated with the harmonically coupled anharmonic oscillator (HCAO) local mode model and ab initio dipole moment functions. The theoretical values have been compared to the experimental oscillator strengths and agree well with experiment. The comparison of the overtone spectrum of CHDIT to the corresponding spectrum of uncomplexed 1,3-cyclohexadiene reveal changes that occur in the ligand upon complexation with iron tricarbonyl. The presence of the metal lengthens the terminal olefinic CH bond by 0.002 Å and shortens the nonterminal olefinic CH by 0.001 Å. The axial and equatorial CH bonds of the free ligand become similar in length in the complex. The implications of the observed spectral features to the structure and bonding in the complex are discussed.