"The Nuclear Magnetic Resonance spectra of five complete series of para-substituted benzenes have been investigated: the benzoic acids, benzonitriles, benzoyl chlorides, methyl benzoates and nitrobenzenes. Precise values of aromatic proton chemical shifts and coupling constants were obtained from LAOCOON3 computer analyses of the spectra. Using the relative internal chemical shift technique of Beachell and Beistel, excellent linear correlations among all five series were found. All substituents gave proton shifts which lay on the least-squares line, so it is concluded that all substituents are well behaved. Using the correlation plots the chemical shifts of the aromatic ring protons can be predicted to 0.01 ppm.
Highly resolved, first-order spectra were observed at 100 MHz for the 4-alkyl-nitrobenzenes. The group designations A₂B₂X, A₂B₂XX' and A₂B₂XX'X" were assigned to 4-nitro-cumene, 4-ethyl-nitrobenzene and 4-nitro-toluene respectively. Basic spin wave functions were constructed for both the A₂B₂X and A₂B₂XX' systems and both diagonal and off-diagonal matrix elements were evaluted. No methyl-ring proton coupling was found for 4-t-butylnitrobenzene and ring proton spectrum was simply A₂B₂.
The perturbed valence bond model of Beistel has been used to rationalize substituent effects on the p.m.r. spectra of the para-substituted benzenes. This model assumes that the hybridization of the skeletal atoms can be perturbed from sp² to (sp²+Δp) or (sp²-Δp), depending on the directive abilities of the substituent. The perturbation influences the proton shift upfield relative to benzene when the proton is bonded to a carbon hybridized (sp²+Δp), and downfield for a carbon hybridized (sp²-Δp). Available carbon-13 shift data has proved consistent with the proposed model.
The directive abilities of the substituents decrease in the sequencer N(CH₃)₂, NH₂, OCH₃, OH, F, CH₃, C₂H₅ , H, C₃H₇, Cl, C₄H₉, Br, CN, COOCH₃, COCl, I and N0₂. That is according to the hybridization of the central atom, its relative nuclear charge and the principal quantum number of the valence electrons. The groups COH, CONH₂ and COCH₃ have not been included in the sequence since complete series were not studied. Files of spectral data and selected measurements suggest that these groups lie between CN and COOCH₃ in the order indicated"--Abstract, pages iii-iv.
Beistel, Donald W.
Stoffer, James O.
Biolsi, Louis, Jr.
Zung, Joseph T.
Ho, C. Y. (Chung You), 1933-1988
Ph. D. in Chemistry
University of Missouri--Rolla
xiii, 159 pages
© 1972 Holger Er-Chah Chen, All rights reserved.
Dissertation - Open Access
Benzene -- Spectra
Nuclear magnetic resonance spectroscopy
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Chen, Holger Er-Chah, "Substituent effects on the magnetic resonance spectra of 1, 4-disubstituted benzenes" (1972). Doctoral Dissertations. 210.