Microwave Spectra for (C₆H₅D)Cr(CO)₃, (C₆D₆)Cr(CO)₃, and (C₆H₅F)Cr(CO)₃ and the Structure of (Benzene)chromium Tricarbonyl

Microwave spectra for the D₁ and D₆ isotopomers of (benzene)chromium tricarbonyl and (fluorobenzene)chromium tricarbonyl were measured using a Flygare-Balle type microwave spectrometer system. The new rotational constants for the deuterium-substituted isotopomers are used with previous data which include ¹³C isotopomers to obtain a gas-phase structure for (benzene)chromium tricarbonyl. The new rotational constant for the C₆D₆, symmetrictop, isotopomer is B(bz-d₆) = 698.6934(1) MHz. Asymmetric-top structure were observed for a single deuterium substitution on benzene, giving B(bz-d₁) = 730.0491(4) MHz and C(bz-d₁) = 723.6641(4) MHz. Rotational constants obtained for (fluorobenzene)chromium tricarbonyl are A = 814.1491(2) MHz, B = 641.1594(2) MHz, and C = 586.5866(1) MHz. The centrifugal distortion constants are small, with D_J ≅ 0.05 kHz. Structural parameters determined are center of the benzene carbon plane to chromium distance r(Cr—bz) = 1.66(2) Å, the chromium to carbonyl carbon bond length r(Cr—CO) = 1.876(7) Å, the benzene carbon to hydrogen bond length r(C—H) = 1.080(3) Å, and the carbonyl carbon to oxygen bond length r(C—O) = 1.148(7) Å. The OC—Cr—CO interbond angle is α = 87.4(6)°. The average benzene C—C bond length is 1.413(4) Å. A small isotopic shift in the ₅₃Cr quadrupole coupling strength was observed for the C₆D₆ isotopomer relative to the normal isotopomer. The hydrogen atoms are displaced out of the benzene carbon plane toward the Cr atom with the C—H bond axis at an angle γ= 2.8(2)° with respect to the benzene carbon plane.