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(₅₂Cr)=732.8886(6) MHz and B(⁵³Cr)=732.8966(3) MHz. Asymmetric top spectra were observed for a single ¹³C substitution on the benzene ring giving B( ¹³C-bz)=729.9606(3) and C(¹³C-bz)=727.9024(2) MHz. For a single ¹³C substitution on one of the carbonyl carbons B( ¹³CO)=731.9036(8) and C(¹³CO)=729.1657(8) MHz. Since no effects of possible internal rotation were observed on the ¹³C asymmetric top spectra, we can place a lower limit on the V₆ potential for internal rotation of V⁶>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 ⁵³Cr quadrupole coupling strength is low, eqQ(2(⁵³Cr)=-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) Å, the chromium-carbonyl bond length r(Cr-CO)=1.86(1) Å and the OC-Cr-CO interbond angle α=88(1)°.