Gas-Phase Iron Tricarbonyl Cyclooctatetraene Exhibits a "Rigid-Rotor" Microwave Spectrum

The microwave rotational spectrum of (μ⁴-C₈H₈) Fe(CO)₃ was measured in the 4-12 GHz range using a pulsed- beam, Fourier transform spectrometer system. Nearly all strong transitions can be fit using a near-rigid-rotor model with very small centrifugal distortion terms. These results indicate that this complex has a fairly rigid structure in the gas phase, with no observed evidence for fluxional behavior or internal rotation on the microwave time scale (τ < 10^-4 s). These gas-phase observations initially appeared to be in sharp contrast to the solution NMR studies, where this complex is observed to be a "ring whizzer" with cyclic 1,2 carbon shifts. Further analysis indicates these two sets of measurements are compatible since, with an intermediate level barrier to internal motion (V > 500 cm^-1), the structure would appear rigid on the microwave time scale, yet could appear quite fluxional on the much longer NMR time scale. The rotational constants are A = 825.2559(2), B = 510.5149- (1), and C = 462.5246(1) MHz. Centrifugal distortion constants are Δ_J = 0.0146(5), Δ_JK = 0.015(3), Δ_K = 0.049(4), δ_J = 0.0034(3), and δ_K = -0.092(6) kHz. The measured rotational constants all agree with those calculated from the X-ray structure to within 0.3%. The excellent agreement between the rotational constants makes it very likely that the solid-state and gas-phase structures are nearly identical.