Measurements of the microwave spectrum and structural parameters for benzene chromium tricarbonyl

Abstract

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, DJ=0.05 kHz and DJK=−0.05 kHz, indicating a fairly rigid structure. The 53Cr quadrupole coupling strength is low, eqQ(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)°.