MEASUREMENTS OF THE MICROWAVE-SPECTRUM AND STRUCTURAL PARAMETERS FOR BENZENE CHROMIUM TRICARBONYL

Microwave spectra for four isotopomers of benzene chromium tricarbonyl were measured in the 4-17 GHz range using a Flygare-Balle type pulsed beam spectrometer. Rotational constants obtained are B(Cr-52)=732.8886(6) MHz and B(Cr-53)=732.8966(3) MHz. Asymmetric top spectra were observed for a single C-13 substitution on the benzene ring giving B(C-13-bz)=729.9606(3) and C(C-13-bz)=727.9024(2) MHz. For a single C-13 substitution on one of the carbonyl carbons B((CO)-C-13)=731.9036(8) and C((CO)-C-13)=729.1657(8) MHz. Since no effects of possible internal rotation were observed on the C-13 asymmetric top spectra, we can place a lower limit on the V-6 potential for internal rotation of V-6>4.0 THz (=1.6 kJ/mole). The centrifugal distortion constants are small, D-J=0.05 kHz and D-JK=-0.05 kHz, indicating a fairly rigid structure. The Cr-53 quadrupole coupling strength is low, eqQ(Cr-53)=-12.11(1) MHz, indicating a near octahedral charge distribution around the Cr atom. Structural parameters obtained are the center of the benzene chromium distance r(Cr-bz)=1.67(2) Angstrom, the chromium-carbonyl bond length r(Cr-CO)=1.86(1) Angstrom and the OC-Cr-CO interbond angle alpha=88(1)degrees.