Microwave measurements of rhenium quadrupole coupling in cyclopentadienyl rhenium tricarbonyl

Microwave measurements of rotational transitions for cyclopentadienyl rhenium tricarbonyl were performed in the gas phase using pulsed beam Fourier transform microwave spectroscopy. One hundred and sixty five transitions were assigned to two isotopomers of rhenium. The location of the rhenium atom near the center of mass produced significant overlap between the spectra of the two isotopomers. The data were accurately fit using a symmetric top, rigid rotor Hamiltonian that included nuclear quadrupole coupling and centrifugal distortion constants. The rotational constants obtained were $B(187Re$⁠) = 724.9794(2), $B(185Re$⁠) = 724.9795(2). The quadrupole coupling constants obtained were eQq$(187Re$⁠) = 614.464(12) and eQq$(185Re$⁠) = 649.273(14) MHz. The successful fitting of the measured spectra to a symmetric top Hamiltonian indicates that the cyclopentadienyl group retains $C5v$ symmetry, and the $Re(CO)3$ group $C3v$ symmetry in the gas phase. The measured rotational constants are in reasonable agreement with those calculated from the structural parameters obtained in the earlier x-ray work. The Re quadrupole coupling constants obtained are compared with values for other complexes.