Measurements of the structure of methyltrioxorhenium using microwave spectroscopy

The rotational spectra for six isotopomers of methyltrioxorhenium (MTO) were measured in the 6-14 GHz range using a Flygare-Balle-type pulsed-beam microwave spectrometer. The measured transition frequencies were analyzed to obtain rotational constants and quadrupole coupling strengths. Rotational constants from the new measurements were used, along with previous results for the normal and ¹³C isotopomers, to determine the complete, three-dimensional structure for MTO. The bond lengths obtained are r(Re-C)=2.074(4) Å, r(Re-O)=1.703(2) Å, and r(C-H)=1.088(7) Å. The interbond angles are 〈Re-C-H=108.9(2)° and 〈C-Re-O =106.4(4)°. This complex appears to have C_3v symmetry in the gas phase, unlike the solid-state structure. Structural parameters are compared with neutron-diffraction, electron-diffraction, and density functional theory results. Spectra for the asymmetric-top isotopomers were much more difficult to analyze due to effects of off-diagonal quadrupole coupling terms and possible internal rotation. The Re quadrupole coupling strength is increased by 1.4% for CD₃ReO₃ compared with CH₃ReO₃, illustrating a secondary isotope effect.