GAS-PHASE IRON TRICARBONYL CYCLOOCTATETRAENE EXHIBITS A RIGID-ROTOR MICROWAVE-SPECTRUM

The microwave rotational spectrum of (eta(4)-C8H8) Fe(CO)(3) was measu red in the 4-12 GHz range using a pulsed-beam, Fourier transform spect rometer 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 intern al rotation on the microwave time scale (tau < 10(-4) s). These gas-ph ase observations initially appeared to be in sharp contrast to the sol ution NMR studies, where this complex is observed to be a ''ring whizz er'' with cyclic 1,2 carbon shifts. Further analysis indicates these t wo sets of measurements are compatible since, with an intermediate lev el barrier to internal motion (V > 500 cm(-1)), the structure would ap pear rigid on the microwave time scale, yet could appear quite fluxion al 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 di stortion constants are Delta(J) = 0.0146(5), Delta(JK) = 0.015(3), Del ta(K) = 0.049(4), delta(J) = 0.0034(3), and delta(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 t he rotational constants makes it very likely that the solid-state and gas-phase structures are nearly identical.