HARMONIC FORCE-FIELDS AND VIBRATIONAL FREQUENCIES OF BENZENE, DIBENZENE-CHROMIUM, BENZENE-CHROMIUM TRICARBONYL, AND CHROMIUM-HEXACARBONYL - A DENSITY-FUNCTIONAL STUDY

The geometries of dibenzene-chromium [Bz(2)Cr], benzene-chromium tricarbonyl [BzCr(CO)(3)], and chromium-hexacarbony1 [Cr(CO)(6)] have been optimized using the local density approximation (LDA) augmented with self-consistent nonlocal corrections (LDA/NL). The optimized structures were used as reference geometries in LDA calculations of the harmonic force constants. The calculated nonstationary LDA force fields have been properly transformed into a valence coordinate representation and applied in a normal-coordinate analysis. This approach resulted in an improved agreement with the empirical vibrational frequencies compared to results from previous density functional studies of similar systems. The internal force constants of the organometallic molecules and that of benzene were compared. We found significant changes in the CC stretching and the out-of-plane deformation force constants of benzene as this molecule is coordinated to a metal center. The wavenumber shifts of benzene vibrations upon complexation can be explained by the shifts in the force constants and not by kinematic effects as previously suggested by some studies. The CO force constants of Cr(CO)(6) and BzCr(CO)(3) are also significantly different. The differences are explained based on a qualitative orbital analysis. The skeletal force constants of Bz(2)Cr and BzCr(CO)(3) are also compared. The complete force fields of Bz(2)Cr and BzCr(CO)(3) are reported here for the first time.