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 pulse d-beam microwave spectrometer. The measured transition frequencies wer e analysed to obtain rotational constants and quadrupole coupling stre ngths. Rotational constants from the new measurements were used, along with previous results for the normal and C-13 isotopomers, to determi ne the complete, three-dimensional structure for MTO. The bond lengths obtained are r(Re-C)=2.074(4) Angstrom, r(Re-O)=1.703(2) Angstrom, an d r(C-H)=1.088(7) Angstrom. The interbond angles are angle Re-C-H=108. 9(2) Angstrom and angle C-Re-O=106.4(4)degrees. This complex appears t o have C-3v Symmetry in the gas phase, unlike the solid-state structur e. Structural parameters are compared with neutron-diffraction, electr on-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 intern al rotation. The Re quadrupole coupling strength is increased by 1.4% for CD3ReO3 compared with CH3ReO3, illustrating a secondary isotope ef fect. (C) 1997 American Institute of Physics.