MICROWAVE MEASUREMENTS AND CALCULATIONS ON CYCLOPENTADIENYLRHODIUM DICARBONYL, A V-10 HINDERED ROTOR

The microwave rotational spectrum of (eta(5)-C5H5)Rh(CO)(2) was measur ed in the 5-15 GHz range using a Flygare-Balle type spectrometer syste m. Attempts to fit the spectrum using a rigid-rotor model resulted in large deviations between measured and calculated frequencies and an un reasonably large A rotational constant. A better fit, including more t ransitions, was obtained using a V-10-barrier hindered rotor Hamiltoni an, and a much more reasonable A rotational constant was obtained. The barrier height obtained is V-10 = 347(13) GHz (0.14 kJ mol(-1)). The rotational constants are A = 1613(10) MHz, B = 1046.7(7) MHz, and C = 759.7(10) MHz. Quantum-mechanical calculations using the DFT options i n Gaussian 94 provided structural parameters in good agreement with me asurements and a barrier height V-10 = 272 GHz (0.11 kJ mol(-1)). The calculated structure showed C-2-symmetry distortions of the cyclopenta dienyl moiety, in agreement with previous X-ray data for CpRh(CO)(2). The calculated rotational constants and V-10 barrier height are in go od agreement with the experimental results. The calculations indicate that there is coupling between the internal rotation and distortions o f the Cp ligand, and this could very likely provide the major contribu tion to deviations between the hindered-rotor calculated frequencies a nd the measured frequencies.