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|Title:||N.M.R. Studies of the Bonding in Arene Chromium Tricarbonyl Complexes|
|Authors:||Fletcher, Leslie John|
|Advisor:||McGlinchey, M. J.|
|Abstract:||<p>The effects on the chemical and spectroscopic properties of an arene brought about by complexation to a chromium tricarbonyl moiety have been investigated by comparing the NMR chemical shifts of the free and complexed arenes.</p> <p>The ¹⁹F n.m.r. chemical shifts of a series of free fluoroarenes, FC₆H₄X = H, F, Cl, CH₃, CH₃O, NH₂ and CF₃, have been correlated with the Swain-Lupton field and resonance parameters. Comparison of these data with the corresponding Cr(CO)₃ complexes shows little change in the transmission of mesomeric effects by para-substituents. Meta-substituents which interact primarily by a field effect have very little influence on the fluorine chemical shift in the complexes suggesting that the σ framework of the ring interacts with the chromium. The constancy in the transmission of mesomeric effects implicates an unperturbed π system in the complexes. Conversely, participation of the chromium atom in this transmission would negate this latter conclusion. An investigation of the fate of the aromatic ring current upon complexation has been used to resolve these two possible conclusions.</p> <p>-paracyclophane has been used to reevaluate present ring current theories. The ¹H n.m.r. spectrum of -paracyclophane has been recorded at 220 M Hz and the methylene protons assigned on the basis of homonuclear decoupled spectra. Local anisotropic contributions to chemical shifts for protons sited above or near the aromatic ring of -paracyclophane have been calculated using a classical model previously proposed by Grant. The residual incremental shift was shown to follow the Waugh-Fessenden-Johnson-Bovey classical ring current model, but the loop separation originally invoked was shown to be unnecessary. These corrected ring current contributions also correlate very well with the quantum-mechanical approach of Haigh and Mallion. This model has been tested with a series of cyclophanes. The ¹³C n.m.r. spectrum of -paracyclophane has also been recorded.</p> <p>The ¹H n.m.r. of the chromium tricarbonyl complex of -paracyclophane has also been recorded at 220 M Hz. Local anisotropic contributions were derived using ¹³C shielding tensors from benzene Cr(CO)₃ and hexaethylbenzene Cr(CO)₃. The ring current was found to be reduced to approximately 50% of its former value upon complexation.</p> <p>The ¹³C n.m.r. spectrum of hexaethylbenzene chromium tricarbonyl is shown to exhibit two resonances for ring carbons, as well as for methylene and methyl carbons. Cessation of ring rotation is postulated. Activation energies of 53.1 ± 2.5, 44.8 ± 2.1 and 46.9 ± 1.7 kJ mol⁻¹ for ring, methylene and methyl carbons are found.</p> <p>The overall conclusions drawn are that there is a significant decrease in the aromatic ring current upon complexation and that, therefore, transmission of substituent effects between para positions of the ring may be mediated by the chromium atom.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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