Microwave measurements and calculations on cyclopentadienylrhodium dicarbonyl, a V₁₀ hindered rotor

The microwave rotational spectrum of (η⁵-C₅H₅)Rh(CO)₂ was measured in the 5-15 GHz range using a Flygare-Balle type spectrometer system. Attempts to fit the spectrum using a rigid-rotor model resulted in large deviations between measured and calculated frequencies and an unreasonably large A rotational constant. A better fit, including more transitions, was obtained using a V₁₀-barrier hindered rotor Hamiltonian, and a much more reasonable A rotational constant was obtained. The barrier height obtained is V₁₀ = 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 in Gaussian 94 provided structural parameters in good agreement with measurements and a barrier height V₁₀ = 272 GHz (0.11 kJ mol^-1). The calculated structure showed C₂-symmetry distortions of the cyclopentadienyl moiety, in agreement with previous X-ray data for Cp*Rh(CO)₂. The calculated rotational constants and V₁₀ barrier height are in good agreement with the experimental results. The calculations indicate that there is coupling between the internal rotation and distortions of the Cp ligand, and this could very likely provide the major contribution to deviations between the hindered-rotor calculated frequencies and the measured frequencies.