Knowledge Bank

In a recent study of the infrared absorption spectra of various metal carbonyls and their $PF3$ substitution products in the CO stretching region $(19002100cm-1)$, we have been able to obtain the stretching and interaction constants for CO vibrations in these molecules. The approximation of high frequency separation has been used. With these constants absorption spectra are now calculated for the $C13$-substituted carbonyls and in general very good agreement with experiment is found. The weak absorption bands observed in the carbonyls containing $C13$ in natural abundance are accounted for in this way. Metal carbonyls of varying complexity such as $Ni(CO)4,Cr(CO)6,Fe(CO)5$ and $Mn2(CO)10$ are treated. In some cases previous assignments of fundamentals as well as $C13$ vibrations are corrected, in others new absorption bands are observed at very high concentrations. Since interaction constants in metal carbonyls are always found to be positive, the $C13$ analogues of symmetric vibrations are shifted less than those of the corresponding antisymmetric vibrations. More quantitative estimates may be made through application of the sum rule.