Microwave spectra, density functional theory calculations and molecular structure of acetylenemethyldioxorhenium

The molecular structure for acetylenemethyldioxorhenium, (C2H2)(CH3)ReO2, w as obtained by measuring and analyzing the rotational spectra for 14 isotop omers. This appears to be the first gas-phase measurement for a rhenium met allacyclopropene, and the first structural data on this compound. This comp lex is closely related to reaction intermediates in methyltrioxorhenium and osmium tetroxide catalyzed oxidation reactions, which are important in ind ustrial chemical production and syntheses of chiral products. The microwave spectra were measured in the 4-11 GHz range using a Flygare-Balle-type pul sed-beam microwave spectrometer. The structural parameters obtained using t he 42 measured rotational constants are in very good agreement with results from the new density functional theory calculations. The structure of the acetylene ligand is modified through interaction with the metal atom, and e xhibits partial sp(2) hybridization in the complex. The C-C bond length is increased by 0.09 Angstrom to 1.29 Angstrom. The H-C-C interbond angles are reduced from 180 degrees to 146 degrees, and 147 degrees. The measured rhe nium, methyl-carbon bond length is 2.116(2) Angstrom, and rhenium-acetylene carbon bond lengths are 2.043(2) Angstrom and 2.067(2) Angstrom. Large qua drupole splitting patterns arising from the two rhenium isotopes [chi (cc)( Re-185)=812.62 MHz, and chi (cc)(Re-187)=769.023 MHz], with large off-diago nal components [\chi (ab)(Re-185)\=725.40 MHz, and (\chi (ab)((187) Re)\=68 6.384 MHz] caused some initial difficulty in assigning transitions. When th e assignments and analysis were completed, the systematic changes in the me asured quadrupole coupling tensor components due to rotations of the princi pal inertial axes on isotopic substitution were obtained, and helped to con firm assignments for the various isotopomers. (C) 2000 American Institute o f Physics. [S0021-9606(00)01342-8].