| Abstract
| - The microwave rotational spectrum of(η4-C8H8)Fe(CO)3 was measured in the 4−12 GHz range using apulsed-beam, Fourier transform spectrometer system. Nearly all strongtransitions can be fit using a near-rigid-rotormodel with very small centrifugal distortion terms. These resultsindicate that this complex has a fairly rigidstructure in the gas phase, with no observed evidence for fluxionalbehavior or internal rotation on the microwavetime scale (τ < 10-4 s). Thesegas-phase observations initially appeared to be in sharp contrast tothe solutionNMR studies, where this complex is observed to be a “ring whizzer”with cyclic 1,2 carbon shifts. Furtheranalysis indicates these two sets of measurements are compatible since,with an intermediate level barrier tointernal motion (V> 500 cm-1),the structure would appear rigid on the microwave time scale, yet couldappearquite fluxional on the much longer NMR time scale. The rotationalconstants are A = 825.2559(2), B =510.5149(1), and C = 462.5246(1) MHz. Centrifugaldistortion constants are ΔJ =0.0146(5), ΔJK = 0.015(3),ΔK =0.049(4), δJ = 0.0034(3), andδK = −0.092(6) kHz. The measuredrotational constants all agree with thosecalculated from the X-ray structure to within 0.3%. The excellentagreement between the rotational constantsmakes it very likely that the solid-state and gas-phase structures arenearly identical.
- Themicrowave spectrum of iron tricarbonyl cyclooctatetraene was measuredusing a pulsed-beam, Fourier-transform spectrometer. A total of105 measured transitions can be accurately fit using a rigid-rotormodel. This complex has an intermediate-level barrier to internalrotation so it appears rigid on the microwave time scale(10-4 s), yet on the much longer NMR timescale, it appears quite fluxional. The gas-phase rotationalconstants are in excellent agreement with those calculated from thesolid-state structuralparameters.
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