Microwave measurements of the rotational spectrum of butadiene iron tricarbonyl

The rotational spectrum of butadiene iron tricarbonyl was measured using a Flygare-Balle type microwave spectrometer. A total of 71 a-dipole and c-dipole transitions in the 5-17 GHz range were obtained for this asymmetric top transition metal complex. The "best fit" rotational constants are A = 1005.4201(3) MHz, B = 958.0408(2) MHz, and C = 933.6865(3) MHz. Centrifugal distortion constants were also obtained in the least-squares fit to the measured transitions. The present measurements indicate an upper limit of 40 kHz for splittings due to internal rotation, which places a lower limit on the barrier height of V₃ ≥ 1.2 THz. The measured rotational constants are used to determine CO bond angles and the distances between Fe and the butadiene C atoms.