Determination of structural parameters for ferrocenecarboxaldehyde using Fourier transform microwave spectroscopy

Gas-phase structural parameters for ferrocenecarboxaldehyde have been determined using Fourier transform microwave spectroscopy. Rotational transitions due to $a$-, $b$-, and $c$-type dipole moments were measured. Eighteen rotational constants were determined by fitting the measured transitions of various isotopomers using a rigid rotor Hamiltonian with centrifugal distortion constants. Least-squares fit and Kraitchman analyses have been used to determine the gas-phase structural parameters and the atomic coordinates of the molecule using the rotational constants for various isotopomers. Structural parameters determined from the least-squares fit are the Fe–C bond lengths to the cyclopentadienyl rings, $r(Fe–C)=2.047(4)Å$⁠, and the distance between the carbon atoms of the cyclopentadienyl rings, $r(C–C)=1.430(2)Å$ and $r(C1–C1′)=1.46(1)Å$ of ring carbon and aldehyde carbon atom. Structural parameters were also obtained using density-functional theory calculations, and these were quite helpful in resolving ambiguities in the structural fit analysis, and providing some fixed parameters for the structural analysis. The results of the least squares and the calculations indicate that the carbon atoms of the Cp groups for ferrocenecarboxaldehyde are in an eclipsed conformation in the ground vibrational state.