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

S. G. Kukolich, S. M. Breckenridge-Estes, S. M. Sickafoose

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Abstract

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 ΔJ = 0.0146(5), ΔJK = 0.015(3), ΔK = 0.049(4), δJ = 0.0034(3), and δK = -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.

Original languageEnglish (US)
Pages (from-to)4916-4918
Number of pages3
JournalInorganic Chemistry
Volume36
Issue number22
StatePublished - Dec 1 1997

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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