The microwave spectrum of cyclopentadienyl vanadium tetracarbonyl, a fluxional molecule

The microwave spectrum of the symmetric-top, transition metal complex n⁵-C₅H₅V(CO)₄ was measured in the 3-11 GHz range using a Flygare-Balle-type spectrometer. The molecular parameters obtained from the spectra are B = 640.429(5) MHz, eqQ(⁵¹V) = -4.77(6) MHz, and D_J = 0.17(4) kHz. Approximately one-half of the measured transition frequencies did not fit the pattern expected for a symmetric-top, rigid rotor with quadrupole coupling and are believed to be due to the effects of a low-frequency vibrational mode. In addition, the line centers for the rotational transitions which were fit to the symmetric-top model showed systematic deviations of approximately ± 20 kHz. It is proposed that these two anomalies are due to low-frequency CO bending modes which result in deviations from the C_4ν symmetry of the V(CO)₄ group.