The gas phase structure of ethynylferrocene using microwave spectroscopy

Gas phase structural parameters for ethynylferrocene have been determined using microwave spectroscopy, Rotational transitions due to a- and b-type dipole moments were measured. Twenty four rotational constants have been determined by fitting the measured transitions of various isotopomers using a rigid rotor Hamiltonian with centrifugal distortion constants. Least-squares fits to determine structural parameters and Kraitchman analyses have been used to determine the gas phase structural parameters and the atomic coordinates from the rotational constants. The distance between the Fe atom and the C atoms of the cyclopentadienyl rings is r(Fe-C₁) = 2,049(5) Å, and the distance between the carbon atoms of the cyclopentadienyl ring is r(C-C) = 1.432(2) A. The ethynyl group is bent away from the Fe atom and out of the plane of the carbon atoms in the adjacent cyclopentadienyl ring by 2.75(6)°, Structural parameters were also obtained from DFT calculations and Kraitchman analyses, and the results are compared. Analysis of fit results for ¹³C isotopic substitution data indicates that the carbon atoms of the two cyclopentadienyl rings are in an eclipsed conformation in the ground vibrational state. Trends in microwave experimental values for the distance from the Fe atom to the center of the cyclopentadienyl ring for a series of substituted ferrocenes have been analyzed. This analysis provides an estimate of the gas phase distance from the Fe atom to the centers of the cyclopentadienyl rings for ferrocene of 1.65(1) Å.