Microwave spectrum and methyl internal rotation of tricarbonyl(trans-pentadiene-1,3) iron

Abstract

The rotational spectrum of tricarbonyl(pentadiene-1,3) iron was recorded in the frequency range from 3 to 23 GHz using a Balle–Flygare-type Fourier transform microwave spectrometer. Two isomers of this molecule exist, differing by the position of the methyl-group: tricarbonyl(cis-pentadiene-1,3) iron and tricarbonyl(trans-pentadiene-1,3) iron. In this experiment only one of them, the trans-compound, could be detected in the molecular beam. This could be proofed unambiguously by the rotational constants resulting from the molecular geometry. All three dipole components of the rotational spectrum could be measured for this asymmetric top, giving 105 a-type, 90 b-type and 124 c-type transitions, with a maximum J-quantum number of J=10. Precise centrifugal distortion constants were determined and the fine structure of the rotational transitions caused by the internal rotation of the methyl group enabled us to derive a barrier for this internal motion. The rotational constants were used to determine the most uncertain structural parameters, using the rotational constants of the ⁵⁴Fe-isotopomer (5.9% natural abundance), too.